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!