Duncan Collimating mask - useful?

[michaelmorris]

I'm looking at getting better at collimating my Meade 8" SCT.  Yesterday someone told me about the Duncan Collimation Mask.  http://alpha-lyrae.co.uk/2013/12/31/schmitt-cassegrain-collimation-made-easy-using-a-duncan-mask/

Who here uses them?  How do they compare with other methods in terms of easy of use and accuracy?

Thanks

[swag72]

When I had my SCT I got one of these as searching around the net they seemed like a great idea.

In reality I found it far from useful. The view in the eyepiece was so small to get it clear that I couldn't make out any fine adjustment requirements at all. Using it with the camera or larger magnification eyepiece just gave such blurred lines that there was again little room for accuracy.

[michaelmorris]

When I had my SCT I got one of these as searching around the net they seemed like a great idea.

In reality I found it far from useful. The view in the eyepiece was so small to get it clear that I couldn't make out any fine adjustment requirements at all. Using it with the camera or larger magnification eyepiece just gave such blurred lines that there was again little room for accuracy.

Thanks.  What size SCT were you using?

[swag72]

I was using a C9.25.

[rowan46]

I used one on my c5 and found it useful it took a lot of the guesswork out for me as using the mask I knew exactly which knob to twiddle

[niallk]

Very interesting & ingenius! Quick Q: do you know if the a Duncan mask useful on other types of scope... Say newts? Thanks.

[russ.will]

I know this is a bit hopeful, but: How similar is this to the Astro Engineering Focus Mask? That used three sub-aperture circular holes precisely spaced at 120 degrees. The claim was that it worked broadly as per a Bahtinov mask, but would also work on non-point sources like planets. I'm guessing that in practice, it's not so different?

Russell

[Toxophilus]

I found the same link as you and decided to have a go. I used a cardboard template initially and then got a sheet of Fab Foam and cut out the final version from that. I also added some dots with a silver marker to remind me how to align the mask with the collimation screws. This is what I ended up with:

and this is how it looks when fitted:

When it came to collumation I found the usual trick of using very high magnification did not work and I found that the best eyepiece for the job was a 15mm in my 8" LX90. The image is very small and you need to be patient and careful adjusting the screws but it worked a treat. I have always used the "align the hole in the donut" method but never been really satisfied with the result and the banding on Jupiter has always been a little fuzzy at higher magnifications. This seemed far more precise. Afterwards I slewed to Jupiter and was treated to the best views I have ever had through my LX90. It was so quick and easy to do I think its worth using it almost every time for a quick check.

I can't remember how much the Fab Foam cost but if I recall correctly it was about 75p so for less than £1 what have you got to loose.

Mark

[Toxophilus]

BTW: the 15mm eye piece gave me around 133x magnification

[bumm]

I'm LATE to this thread, but dealing with astigmatism, less than steady seeing, etc, I found a Duncan Mask to remove a lot of the guesswork from collimating my old C8.  Tweaking the specifically indicated screws to form the little "Y" was much easier for me than trying to visualize a clean diffraction ring around a star.  At 400 power, the little lines were a bit blurry, but clear enough to judge their position.  On the other hand, Polaris looked like a campfire.  For the fiirst time in 38 years, I feel I'm hitting my collimation on the button. 

     This is my Duncan Mask...  plain old cardboard with a couple pieces of electrical tape to make little handles to pull it off easily and a few blood stains from a disagreement with my circle cutter.

                                                                                                  Marty