Duncan Mask for SCT Collimation

[steve-fung]

Although I get satisfactory collimation from my 9.25 SCT using defocused star collimation technique, it's not always easy to get perfectly tight concentric Airy discs. I have always wondered if collimation could be done better. After searching the internet, I came across the Duncan mask, which was created by Duncan Evenden of the Orpington Astronomical Society, UK.

http://alpha-lyrae.co.uk/2013/12/31/sch ... ncan-mask/

http://www.cloudynights.com/topic/38397 ... ask/page-3

After procrastinating for several weeks (busy with work), I decided to create a Duncan mask for myself as a weekend project. For materials, I used corrugated cardboard recycled from a used but clean cardboard box. After marking a central point, 60-degree arcs and sectors were created using ruler and compass construction. Holes were then cut using #11 blade scalpel. I modified the original design by adding side notches for easy removal. The mask fits fairly snuggly around the secondary mirror housing. Unfortunately, it was a cloudy weekend, so no chance of actually testing it out.

[steve-fung]

Update – The weather cleared up enough during the week that I tried out the Duncan mask technique for SCT collimation. One problem I immediately found with using a homemade cardboard mask in Houston weather is that cardboard warps with moisture, so it did not take long during my collimation process for the mask to bend out of shape. A solution is to tape the mask down, but I am already thinking of creating a new mask using a plastic container lid, similar to what Duncan Evenden showed on CN. A second problem is significant reduction in aperture, so images are quite dim unless using a sufficiently bright star (I used Rigel after I had my EQ mount polar aligned).

Collimation is quite easy once the mask is in position and oriented such that each slot is aligned with a collimation screw (having Bob’s Knobs is of additional help). When a star is slightly defocused in the center of the field of view, the diffraction pattern forms a Y with each arm corresponding to a slot/collimation screw. To see the Y-diffraction pattern clearly, one has to use very high magnification (I was using 60-70 PPI). The objective is to adjust the collimation screws, so the three diffraction arms are of equal length. I look for the odd-length diffraction arm and adjust its corresponding collimation screw. To tell which screw to adjust, I simply cover one of the slots (I used a Post-it Note stuck on the mask) and see if the corresponding diffraction arm is masked out. When collimation is finished and the mask removed, one should see perfectly tight concentric Airy discs at very high magnification.

Two advantages I found with using a Duncan mask for collimation were (1) it is easier for me to visualize arm length than centering of concentric rings, and (2) since each collimation screw is aligned with a slot, I can easily identify which screw to tweak by finding the odd-length diffraction arm and covering its corresponding slot to test if it is masked out.

[edward-fraini]

This sounds like a great idea, you should talk to Chris Ober about making you something on his CNC system. He has made things for me our of PPE sheet that work well.

In fact I thing I will talk to him about it for myself

Ed

[steve-fung]

Hi Ed,

Yes, very good idea. Let me know how it turns out if you get one made from Chris. For right now, I was planning on going to the local hobby store and buy myself several foam boards, which are easy to cut out, lightweight, and less prone to warping.

Steve

[steve-fung]

On a side note, the Duncan mask can also be used as a focusing tool similar to the Bahtinov mask (https://en.wikipedia.org/wiki/Bahtinov_mask). When a star is precisely focused at high magnification through a Duncan mask, the individual arms of the Y-diffraction pattern cross each other through the center, forming a *. In practice, a Bahtinov mask is better in that it has easier to see large diffraction spikes. You can simulate this using Maskulator (http://www.njnoordhoek.com/?p=376).

[chris-ober]

I'll have to cut one out for my C11 and see how it compares to the Bahtinov I have. Maybe a little mini one and see if it works on an 80mm refractor

[steve-fung]

The Duncan mask is primarily for collimation. I think you will be better off using the Bahtinov mask for focusing. The diffraction spikes on the Duncan mask are very small and require a high magnification eyepiece to see clearly, but this defeats the purpose of using it for focusing unless your optics are perfectly parfocal.