Monday, September 22, 2014

Atlas Pro AZ/EQ-G

I managed to get several images with the CGEM DX mount over the last 2 1/2 years. Some pretty good, some not so good. It was just very unpredictable, and I started saving up some money and sold the AT8IN telescope to help fund a new mount.

There were several that I had my eye on. The new iOptron CEM60 EC seemed to be a very nice mount. The Losmandy G11 has been trusted for years and has recently been refined with the release of the Gemini 2 hand controller. Both of these mounts will carry a maximum payload of 60 lbs and looked appealing. The new Orion HDX110, which is the Skywatcher EQ8 with the Orion name, also looked very nice with its capabilities of handling up to 110 lbs of weight.

There are a few things I'd still like to buy for my astrophotography hobby. I still need to get the Astrodon OIII and SII narrowband filters. I would also like to automate my focusing with motorized Moonlite focusers on my telescopes. And I would like a nice 110-130mm apo triplet to have for visual use, as well as fill the void in my imaging line of scopes left from selling the AT8IN. Its a pretty big jump from the 480mm focal length of the 80mm triplet up to the RC's 1150mm focal length using the CCDT67 Telecompressor. So I'd really like something in the 700-800mm range.

After weighing my mount options with my wants/needs, I decided to settle for a smaller mount. The new Atlas Pro shares a few features of its big brother, the HDX110. They both use the same Altitude adjuster that is much smoother than most mounts on the market. They both use belt driven stepper motors with dual encoders. And they both use closed-loop electronics which allows you to manually move the scope, or rebalance, without it loosing its alignment. The Atlas Pro AZ/EQ-G it would be!

I ordered the new mount from Orion and it arrived in two boxes about a week later. When I set up my pier last year I removed the mount adapter from the top of my CGEM DX tripod and fabricated a way to fasten it to my pier plate. I was hoping it would work with my new mount. The metal post for adjusting azimuth was too long for the Atlas Pro mount, but it was as simple as removing the CGEM DX post and screwing in the one that came on my Atlas Pro tripod. It fits!


This picture shows the mount set up for my 80mm triplet. For larger scopes, the mount also comes with a counterweight bar extension and another 11 lb counterweight.


 The mount is so quiet when slewing! I just had to remove the cover to check out the stepper motors and drive belts. This is a really nice system and much better than gear-to-gear in my opinion. In addition to being quieter the backlash is also significantly reduced or eliminated. There is always some backlash in any gear-to-gear system.


This is a close up of the "Captain's Wheel" DEC clutch. The small t-bolt under it allows the counterweight shaft to retract up inside of the mount for easy transporting and storing.


Here is the RA clutch and just below it is the illuminated polar scope. The brightness of the illuminated scope can be adjusted with the hand controller.


This is the adjustment for altitude with the handle retracted. It is much smoother than the other equatorial mounts I have used. Below it is shown with handle out ready for adjustment.


I have not been able to really use the scope yet for imaging, but I am confident that this will be an excellent mount for astrophotography. The very first night, we did a rough polar alignment using the polar scope and did a little autoguiding and it performed great. Here is the PHD graph with no adjustment of the PHD2 settings and the rough polar alignment. I am sure with a more refined polar alignment and some fine tuning in PHD this will get even better.


One big reason I wanted to go with this mount is that it is supported by the EQMOD Project. I have read great things about EQMOD and EQASCOM and wanted this program for controlling the mount/telescope. It really is a well thought out program with excellent capabilities. I am still trying to learn more about it but everything so far has been positive. I will report in more detail about EQASCOM as soon as I feel comfortable using it and learn more about it. Please check into it yourself to see if your mount is supported. If so, I think you will really like the program. I did purchase the recommended interface cable from Shoestring Astronomy for connecting the mount to the computer. For the Atlas Pro AZ/EQ-G this cable is the USB2EQ5. For regular Atlas owners this would be the USB2EQ6 interface cable.


For aligning the mount for goto you can make a pointing model using several stars. Usually just a star to the west and one to the east will be good enough for casual goto performance. This is great for observing but with Elbrus plate-solving software its not really needed at all for imaging.


I should also mention another piece of software that is very important. With the interface cable from Shoestring Astronomy, you no longer use the hand controller. EQASCOM controls your mount but you'll need something else if you want goto capabilities. Stellarium is great for this but I found that it was too much of a memory hog for my old desktop that I have in the observatory. I found a free program that others are using called Cartes du Ciel. So far it has worked great and seems to be a nice program.



Just select a point on the sky chart to get coordinates for your plate solving software or select slew for goto. With tracking enabled it will track where your scope is pointing on the sky chart.

By this time next month I hope to have a few images under my belt using the new set up. Hopefully by then I can report a lot more on the new software and the performance of the new mount. I believe that the Atlas Pro AZ/EQ-G will be a wonderful addition at Little Piney Observatory. Clear skies everyone!

Sunday, August 17, 2014

Improving Corners and Homemade Dew Shields

This months post is a follow up to last months review of the Starlight Xpress off-axis guider. Adding the OAG in the imaging train needed some recalculating of the spacing for the Astro Physics CCDT67 Telecompressor that I use with my RC telescope. Most all scopes require some sort of corrector to make the stars in the corners of the image round. I noticed I had some serious problems but I had also experienced problems with my focuser and had blamed the aberrations in the corners on slop in the focuser. I took the focuser apart and found the problem and fixed it. I expected things to be well again but that was not the case at all.

PixInsight has a really neat tool under 'Script' in 'Image Analysis' called Aberration Inspector. It divides the image up into sections and will show in good detail how your stars look throughout the image. Here's what aberration inspector showed on my recent Bubble Nebula image.


The lower corners show extreme curvature issues and the top is so bad that the stars appear to be badly out of focus. Things are nice and tight in the center. Since the focuser was now fixed it was time to look elsewhere. I decided to careful measure my backspacing again and found that it was less than what it was previously before I added the OAG.

I mentioned in last months post that I went from M42 adapters to M48. Agena Astro has a limited selection of these spacers and I had the shortest one already with the 1/4" (6.4mm) spacer. The next step up is a 1/2" spacer. I really didnt think that I needed that much more, but at just $16.95 I decided to give it a try. It seems that this has again put things in the sweet spot for the Telecompressor. I decided to test it on a large Open Cluster and chose the Double Cluster in the constellation Cassiopeia. Here's what PixInsight's Aberration Inspector showed after increasing the spacing.



The stars in the corners are clearly much better than before. I still have work to do however as I ran across some more issues. The prism of the OAG was too far into the light path and is causing some weird diffraction patterns on the stars to the right side. I think this should be easily fixed though by simply raising the prism up a little more.

With the issue of the corner stars fixed I decided to tackle another problem I have had for a while. Dew is a huge problem here in Arkansas and a while back I had invested in a secondary dew heater for the RC scope. The problem of dew was fixed but I have not been happy with the shapes of my stars since adding the heater. The secondary dew heater was just slightly larger than the secondary holder on my 8" RC leaving the edges of the heater adding to the secondary obstruction. The edges of the heater are not particularly smooth and this, combined with the wires ran across the spider vanes left a mess of the diffraction around my stars. I put up with it as long as I could and finally peeled the heater off and looked for other solutions.

Typical Diffraction pattern with secondary dew heater

I have made a few of my own dew shields. In fact, I have never bought a dew shield before. I think I found this on the website for the DewBuster heater controllers and I must say that they work great. I went to the local Lowes hardware store and bought a roll of Reflectix insulation, a can of 3M Super 77 multipurpose adhesive, a box of the heavy duty velcro, and a can of flat black spray paint. Using these items it is quick and simple way to make a homemade dew shield.

Reflectix Insulation
Start by wrapping the Reflectix around the OTA and cut it at the desired width. Next, just notch around any dovetails you might have. For my RC, I have an ADM Losmandy style on the bottom and a Vixen style on top. Making your own dew shield likely makes for a much better fit if you have custom accessories added. The Reflectix is shiny on both sides and the inside of the shield needs to be painted flat black. After the shield has been test fit and cut as desired, the inside of the shield needs to be sprayed with the 3M glue. This acts as a primer and allows the paint to stick. Without this step the paint will not stick to the Reflectix material.




After the 3M adhesive has dried you can then spray the flat black paint. Once it has dried you can apply the velcro in the desired locations and you are finished. Presto! A dew shield! I added the soft part of the velcro along the back of the shield to protect the finish of the scope.


I also had a heater strip for my C8 telescope and decided to also add it just past the halfway point towards the back end of the OTA. I wasnt sure how this would work with my scope having a carbon fiber tube but after two nights out the mirrors have remained clear of dew. The dew shield has been soaking wet on both nights but the scope has been dry both inside and out!

I will say that I believe the secondary dew heater that I have will work fine on the 10" and larger scopes. The heater is just a little bit too big for the 8" RC's. Where the wires come out of the heater you simply can not turn them sharp enough to keep them out of the light path, so this was another source of diffraction issues.



The diffraction pattern around the stars is much improved without the secondary dew heater. Though this shows other issues with the tracking/guiding, diffraction is no longer a mess. Just last night my wife and I started trying out ASCOM pulse guiding and it seems to be doing a better job than guiding through the ST-4 port on the mount. Maybe this will be a future post from Little Piney Observatory. One thing I have learned since I started imaging, is there is always something new to learn in this hobby!

Clear Skies!

Thursday, July 24, 2014

Starlight Xpress Slimline OAG

SBIG STF-8300M, Starlight Xpress 7 position 36mm USB filter wheel, Starlight Xpress OAG w/QYH5L-II camera, M48 adapter, 6.4mm M48 spacer, Astro Physics CCDT67 Telecompressor.

The off-axis guider (OAG) is another item I have been wanting to get for a while. Since I had the Starlight Xpress (SX) filter wheel I decided to get the Starlight version of the OAG. The SX filter wheel is recessed a bit and the OAG is machined to fit perfectly onto the filter wheel. I just removed my M42 T-thread fitting from the filter wheel and installed the OAG using the supplied screws and then installed the M42 fitting onto the OAG.

I quickly noticed a problem. This configuration would be just fine with a CCD camera with a small sensor and a filter wheel using 1.25" filters, but placing the prism far enough into the light path to work properly was covering up too much of the filter and quite probably my cameras sensor. To fix this I ordered a Starlight Xpress M48 filter wheel adapter. They offer these adapters in many sizes including M54, SCT, and others.

It was tough getting a shot of the prism with the Telecompressor installed but maybe you can make out how I currently have the prism in the light path. I should have removed the Telecompressor for this image. Everything seems to be working just fine with it adjusted like this.


It was a couple weeks before I could try the new OAG out, but it wasnt because of the weather. Anyone who owns the QHY5L-II camera and is considering this OAG should know that it will NOT reach focus as it comes. The OAG comes with c-mount threads and I could not get the camera far enough in to reach focus. This QHY camera comes with a 10mm thick C-mount thread and the camera body has 1.25" filter threads. I had to order a 5mm thick CS-mount and with this I was able to get the camera to focus with the OAG. I got the CS mount from Astrofactors and I believe it was about $15. Here is a picture showing the C and CS mount camera adapters.

On the left is the c-mount that comes on the QHY5L-II and on the right is the cs-mount I ordered from Astrofactors.


Upon receiving the cs mount it was just a matter of getting things adjusted. If the prism isnt far enough into the light path the stars do not look good at all. I wouldnt call them egg shaped or even football shaped, they were much worse. With the stars looking like this PHD had a hard time calibrating. When I lowered the prism down the stars started to look much better and I was able to calibrate and start guiding. The stars on my PHD screen still are not round but it seems to be guiding and I hesitate to lower the prism any further into the light path.

Once you realize how things are adjusted its fairly simple to get things set. The long thumbscrew on the bottom is for adjusting the depth of the prism into the light path. The shorter thumbscrew on top is for adjusting the focus of the camera. For focusing the camera the c-mount slides up and down on the stalk of the OAG that the prism is attached to. The thumbscrew aides in getting the focus set and then there is also a 1.5mm allen head set screw that needs tightened once you have it set where it needs to be. Before you tighten down focus you can rotate the camera to the desired orientation.



The first night of testing went well although my PHD graph was not as smooth as what I was used to when I was using the 50mm mini guider. This is to be expected since I was guiding at around 1150mm instead of 160mm. I hope some further tweaking with the settings in PHD will smooth out the graph but as can be seen in this 100% crop of my test image (NGC 7380) it seems to be doing its job quite well. FWHM numbers were between 4.09 and 4.53 for the individual subs.

100% crop

One of things I like most about the OAG is that my stars do not drift at all. Flexure is no longer an issue! This image of NGC 7380 has not been cropped at all and you cannot see any stacking artifacts.


To show the problems I was having with flexure, here is the uncropped image of the Cocoon nebula, my most recent image done using the 50mm mini guidescope.

Stacking artifacts in luminance data seen to the left and bottom of image

One other thing I should mention is the backspacing with this setup. The SBIG STF-8300 camera has 17.5mm of backspacing and the SX filter wheel is an additional 29mm with the fittings on each end. By adding a short spacer I could achieve the recommended 55mm of backspacing for my MPCC (newt) and the Astro Tech 0.8X reducer/flattener. With the OAG added to the imaging train I am now out to about 60-61mm and can no longer use these correctors. The AP Telecompressor uses a lot of backspacing so I was good with it by adding an 6.4mm M48 spacer. Teleskop Service in Germany makes a field flattener with M48 threads that needs a ton of backspacing that I will need to purchase and use with my apo.

I would recommend the SX OAG to anyone with the SX filter wheel. I hope this post has been of some help to anyone considering this form of guiding. Feel free to contact me if you have any questions and I'll do my best to answer them.

Clear skies and happy imaging! :-)

Thursday, June 26, 2014

Summer is Back!!

Summer has to be my favorite time of year for astronomy. There's just nothing quite like seeing the Summer Milky Way stretching across the night sky and thinking about each tiny point of light being light years apart. The vastness of our own galaxy, let alone the universe, is just truly awe inspiring.

For astronomers, nothing says Summer quite like this shot from my Michigan friend, Joe Tocco, taken recently while visiting the Arizona desert and his friend Steven Aggas.

 
The image itself is stunning, but just as impressive to me is that this is a single 5 second shot at around f/3.5 with his Nikon D7100 SLR camera. This is a testament to the dark and transparent skies in the Arizona desert. The dobsonian in the foreground is Steven's monstrous 36 incher. I can only imagine what the views would be like through that telescope pointed at those pristine skies.

Here in Arkansas, the weather still seems to be in a Spring pattern. Lots of clouds, a little rain, then more clouds. There were a few clear nights where I was able to get out to the observatory. These were around the Full Moon and I was able to shoot through my Astrodon 5nm hydrogen alpha filter. I have been anxious to add color to at least a couple of these but I have to take the weather as it comes. This is only a tiny sample of some of the Summer targets and I'm looking forward to those clear nights that are sure to come. These were all shot with my 80mm Levenhuk triplet and SBIG STF-8300M camera set to -10 degrees Celsius.

North America and Pelican nebula in Cygnus
 
The Crescent nebula (NGC 6888) in Cygnus

Cygnus is a treasure trove. This is the area around Sadr, the central star in the northern cross.

To the south in Sagittarius is this beautiful pairing, the Trifid (M20) and Lagoon (M8) nebula.

 
To the north in Cepheus, you will find NGC 7380, the Wizard nebula
Just above the Wizard nebula in Cepheus is IC 1396, the Elephants Trunk nebula.

With so many things to see in the summer I sometimes feel rushed when I have so few precious clear nights. This is when I try to just slow down and try to take in the awesome beauty of what it is I am seeing. The enormity of it all is truly amazing.

As wonderful as some things are about Summer, it doesnt come without a price. Nothing does I suppose. I sure hope those two mosquitos bring back our neighbors beagle.

Have a happy and safe Summer everyone! :)

Wednesday, May 28, 2014

The QHY5L-II Planetary and Guide Camera

I recently purchased a QHY5L-II monochrome camera from AstroFactors of Plano, Texas. Bruce was a pleasure to deal with and he keeps a full stock of the QHY cameras and accessories.

The main reason I bought this camera was to replace the Orion StarShoot Autoguider (SSAG) as my primary guide camera for long exposure astrophotography. The chip in the QHY5L-II is a bit smaller than the one in the SSAG so I was a bit worried that it would not be the right choice for me, especially since I would like to someday go to an off-axis guider (OAG). The field of view would be much less shooting through an OAG compared to my current setup of guiding through the Orion 50mm finder/guider. However, so far I have been extremely pleased with this little camera....and I do mean little. Here is a shot of the camera next to an old Orion Explorer II 25mm 1.25" eyepiece.

Shown here with the focal ring from the Orion SSAG.

One of the big reasons I decided to go ahead and buy this camera is the high quantum efficiency (QE) rating. With the outstanding QE of 74% I am seeing many more stars compared to the old SSAG. With a chip this sensitive, I should have no problems finding a star to guide on when I make the move to an OAG.

My current method of guiding for long exposure astrophotography using the Orion 50mm finder/guider. Camera is inserted into the Orion extension tube and focal ring is set to easily return to focus. Also pictured is the ADM Vixen dovetail, v-clamps, and rings.
Another reason for making this purchase is that its an outstanding planetary camera. This camera, along with the ZWO ASI120MM (mono) and 120MC (color), seem to have taken over the planetary and solar system imaging forums as of late. These cameras use the same 1/3" Aptina MT9M034 CMOS sensor which has 1.2 million very small 3.75 micron pixels in a 1280 X 960 array for very high resolution imaging. The camera excels on planets, as well as lunar and solar imaging, with its high frame rate of 30 fps at full resolution. It is also capable of up to 200 fps at lower resolution. With my limited abilities with planetary imaging it will be a long time before I'll outgrow this camera. I think there might be better cameras out there, but you'd be hard pressed to find a better value. Before looking at my first attempts at planetary imaging with this camera, please take a look at this link to see what it is capable of in the hands of a more experienced user. Some have even used it for deep sky imaging of small planetary nebula! http://qhyccd.com/en/left/page3/qhy5-ii-series/

My first night using this camera was as an autoguider. It performed as expected and I was able to collect 4.8 hours of LRGB data on the Iris nebula (NGC 7023). I decided to do this with the 80mm apo at f/4.8 for a wide field shot. Main imaging camera was my SBIG STF-8300M and filters were the Astrodon Gen 2 E-series LRGB. I tried this target three different times last year with my DSLR but the heat was too much for the uncooled sensor to overcome. It turned out with fairly low noise levels this time with the new camera set to -15*C.



Since this image of the Iris nebula the weather here has been awful for astronomy. There have been a few nights though with party cloudy skies. The nice thing about planetary imaging is you can shoot about 1500 frames in about 1 minute. This, I thought, would be a perfect time to try my hand at planetary imaging!

QHY5L-II in the C8. Not shown in this picture is the 2X barlow lens that works well for solar system imaging with this setup.
For these images I used Registax 6 and had it use the best 500 frames. Jupiter and Mars were done with a 10" Meade SCT at f/10. A few nights later Saturn was done using a Celestron 8" SCT at f/20 with a 2X barlow.






Yes, more practice is needed. But it is a lot of fun and these were all done with about 1 minute each of sky time. ZWO sells a 1.25" LRGB filter set ($88) and a 5 position manual filter wheel ($88). I will likely get these to have for planetary imaging since I'd rather not take my SBIG camera off of my Starlight Xpress filter wheel. The Astrodon filters are quite expensive and I just dont want to take any chances of getting dust on them. I might even try using the QHY some evening to image some of the small planetary nebula since the Orion SSAG still works and can be used to guide.

I am hoping the clouds will part soon so I'll be able to get back to my true passion of deep sky imaging. But it sure is nice to have options, even on those mostly cloudy nights! :)

Monday, April 21, 2014

Sequence Generator Pro & Elbrus

Although I am just getting familiar with this pair, I think it will be a terrific combo for my imaging. Sequence Generator Pro (SGP) is simply an outstanding image capture program with a down to Earth price. I have just begun exploring its capabilities and already find that it is well worth the $99 price tag. Lets touch on a few of the things it can do.



The sequencing engine can control a large variety of DSLR and CCD cameras as well as several different filter wheels. I was able to download the ASCOM driver for my Starlight Xpress filter wheel from the SX website and easily install it in SGP for full control of my electronic filter wheel. You can type in any variety of targets or events. One event consist of your choice of a Light, Dark, Flat, or Bias frame as well as your selected filter, exposure time, binning, and number of frames. You can set up a large number of events and have it either finish one event before going to the next one or you can have it rotate through the events.

With the powerful Auto Focus feature I am now wanting to invest in an automated focuser to keep things focused spot-on for the entire night. SGP will measure the HFR (Half Flux Radius) of the stars and keep your scope focused without needing to slew off target to find a suitable star to re-focus when the temperture drops. Just start the evening in good focus and Auto Focus will take care of the rest. With the Flexible Focusing Patterns feature you can define patterns using time, temp, filter changes, or frame intervals to keep your images sharply focused when the temperature drops by as little as 1 degree.



SGP also has an Automated Meridian Flip feature. When used with PHD guiding and Elbrus plate-solving, SGP will automatically stop the guiding, flip the mount, re-center your target within pixels, restart the guiding and then continue your imaging sequence. I have yet to try this function but many have commented about how great it is. Once I become more familiar with Elbrus and do some wire management on my mount I will feel comfortable using this. The last thing I want now is for a wire to get caught while the mount if flipping meridian unattended. I am sure I will love this once everything is lined out, as no operator assistance is required!




SGP also has a Framing and Mosaic Wizard that is a $39 add-on. I am currently using the 45 day free trial but I will certainly be purchasing this soon. I have only used the Framing so far and it works great. Simply right click where you want the image centered and select "center here". SGP directs the mount and places your target right in the center. I have only used this a few times so far but it has worked perfectly for me every time. I'm looking forward to trying out the Mosaic Wizard sometime soon.



The Flats Calibration Wizard has been extremely useful. With my SBIG STF-8300M the target Mean Value for my flats is 30,000 ADU. I just enter this value as the target and select +/- 500 ADU as my tolerance value and it computes the exposure time needed for each filter. This has made quick work of acquiring flat frames as opposed to my previous trial and error method!



SGP supports Apogee, SBIG, QSI, FLI, Starlight Xpress, and Atik CCD cameras and filter wheels as well as Canon and Nikon DSLR cameras.

This is only a small sample of what SGP is capable of. It has excellent camera control features where you can enter the desired sensor temp and will cool down and warm up your sensor in a user defined time frame. It is also capable of controlling your mount with dithering and can control PHD, MetaGuide, and Astroart autoguiders. Frame & Focus, and many other features. I encourage anyone looking for a program like this to check out Sequence Generator Pro at http://mainsequencesoftware.com/Products/SGPro.

Elbrus is a free telescope pointing program. It uses plate solving to tell exactly where the telescope is pointed in the night sky by analyzing stars from an image and calculating the exact coordinates. The first night I used this I had the scope pointed toward M109 and told the mount to go to M51. I forgot to check "slew to" on the user interface and it took an image around the area of M109, figured out where it was after a few seconds, and then slewed the telescope to M51. Since I have started using Elbrus I have not had to hibernate my mount and I have not done anymore go-to alignments. I can now re-balance or switch out to different telescopes and not have to worry or fuss with doing a goto alignment anymore. This has made things much easier. I now just align my marks on RA and DEC at the beginning of the night and tell the mount to slew to a target. The program will then take a 5 second exposure of the stars and know exactly where it is at and move the mount to where I am wanting it to be.



It is a wonderful program that I am just beginning to explore. It has failed on me a few times but that could be due more to user error than anything else. Astrometry.net is a similar free program that I may try out sometime to use as a backup when I get things all set up. PinPoint is another plate-solver that is supposed to be very good but it cost $150. I will try Elbrus and Astrometry first because several people are reporting good success rates with these programs.

With SGP and Elbrus working together I can see myself getting a lot more imaging done in the near future. I will be able to punch in an imaging sequence on one target, and when it finishes that have it slew to, center, and start imaging another target. The best part of this is it will be working while I am indoors getting some much needed rest! I will be able to walk out to the observatory in the morning and get the data it has collected for me during the night. Well, at least this is the plan. I think with a little more time to get familiar with the programs it should become a reality sooner rather than later.

Tuesday, March 25, 2014

Lets Focus

Achieving sharp focus is an extremely important goal to every astrophotographer. All those "faint fuzzies" you have seen in your telescopes eyepiece can come to life and have depth of field when "seen" through a camera if you have proper framing and sharp focus. Even the best and most expensive telescope and camera combos wont perform to their full potential without the best possible focus. Polar aligning your mount and having good guiding also figure into the equation, but today lets talk about focus.

My first images were done with a DSLR and I used the FWHM (Full Width Half Maximum) feature in BackyardEOS for focusing. This worked well for the most part but it was still difficult on some nights getting the focus just right. FWHM is a numeric interpretation of the size of a star. This number comprises of two measurements; Full Width simply means that the full diameter of the star is used, and Half Maximum is a measure of the distance from the center of the star at which the brightness level decreases to half the level value at the center. FWHM seeks to provide an empirical method of achieving focus but because of changes in the "seeing", it is not always easy to use.

HFD (Half Flux Diameter) is a similar method of focusing. This is a feature included in the program Sequence Generator Pro (SGP). I've been using SGP since I made the switch from the DSLR to the STF-8300M CCD camera. HFD is the pixel diameter of a star in which half the energy of the star is contained. This value is similar to that of FWHM but is considered to be more reliable especially in conditions of poor seeing. Still, it has its flaws and I wanted something more precise.

I had been hearing of people using focusing mask, or Bahtinov mask. These havent been around that long but seem to have made quite a splash in the astrophotography community. In 2008, Pavel Bahtinov, a Russian amateur telescope maker and astronomer, published an ingenious new way to easily focus your DSLR camera, webcam or CCD camera for astrophotography. The method is simple, very intuitive, and the device has universally been named after the inventor: The Bahtinov Mask. They work by producing a distinctive 'diffraction pattern' in a similar manner to the way a four vane secondary mirror spider vane on a Newtonian reflector produces a four pronged star or "spikes". However, the unique design of the Bahtinov Mask produces a bright image with a diffraction pattern forming a long cross comprised of two intersecting lines and a third line that moves across the center of the cross as the telescope is adjusted in and out of focus. Correct focus is achieved when the center line fits exactly in the center of the cross. At a cost of ~$20-$30 I sure didnt have much to lose so I decided to try them.

I decided on the adjustable version from Farpoint Astro. I found what I was looking for at Agena Astro and ordered the FP400 for my 80mm Levenhuk triplet. This mask is adjustable from 2.5" to 4.5". I also placed an order for the FP404 for my 8" Levenhuk RC which is adjustable from 8.5" to 10.5" (outside diameter).

Bahtinov focus mask on 8" RC

   
Adjustable focus mask that I use for my 80mm apo


                                                                             
Example of perfect focus using the Bahtinov focus mask.

I have to say that these things simply just work. They are simple to use, cost very little, and I can now quickly achieve the best focus possible. So if you are having problems getting a sharp focus, grab one (or two) of these. I think they are something every astrophotographer should have in his or her toolbox. Makes me wonder now how I ever got by without one! I feel much more confident now that my images are focused as well as they can possibly be in any seeing conditions. Happy focusing!