My verdict on the Duncan Mask for SCT Collimation

[RobertI]

I find collimating my 8" SCT a bit of a chore so I was interested to read about the Duncan Mask  which promised a much easier method. I could not find many reports of people actually using one, so I constructed my own and used it for the first time last night.

Although it appeared to work as described, I concluded that it didn't really make things after all and eventually I took the mask off and reverted to the standard method .

The main reasons it didn't really work for me were:

• The 'Y' shape was tiny and was quite tricky to see whether the three bars were in the correct configuration.
• The image was significantly dimmed making it less easy to see, especially if using Polaris - the bright star image without the mask was much easier
• It was not easy to know which screw to adjust - with the standard method you can place your hand on the edge of the tube to create a shadow to see which screw to adjust, you can't seem to do this with a Duncan mask (the 'Y' should tell you which screw to adjust but there is no initial frame of reference unless you start twiddling screws to see the effect)

After that the standard method did not seem like such a chore after all and was quite quick, so I guess the whole exercise was was of some use after all!!

I hope this was of interest to others thinking of trying a Duncan Mask. I'd be interested to know if there are people using it successfully?

Rob

[swag72]

I tried a Duncan mask a while ago with my SCT - You found exactly the same issues as I did. It never came out again! I'm sure that in principle it's a great idea, but that 'Y' shape was TINY

[Peter Drew]

With just one optical component and 3 adjustment screws, the SCT is still the easiest telescope to collimate.  

[michael.h.f.wilkinson]

Interesting report. I haven't tried yet, but thought of using the following tweaks (not original, others have suggested the same): use my ASI174MM camera with most powerful tele-centric (Baader TZ4, perhaps)  or barlow available and image the star on a laptop screen. Apart from increasing brightness due to the sensitivity of the camera, and increasing the scale of the Y, this means I can approach the problem from the front of the scope, which is so much easier than reaching round. Might try using the laptop anyway, first with the standard method before making a Duncan mask.

[paul mc c]

Found the exact problems my self,and resorted back to the usual method.

[RobertI]

It's interesting to hear other people experienced the same thing, thanks for the responses.

Interesting report. I haven't tried yet, but thought of using the following tweaks (not original, others have suggested the same): use my ASI174MM camera with most powerful tele-centric (Baader TZ4, perhaps)  or barlow available and image the star on a laptop screen. Apart from increasing brightness due to the sensitivity of the camera, and increasing the scale of the Y, this means I can approach the problem from the front of the scope, which is so much easier than reaching round. Might try using the laptop anyway, first with the standard method before making a Duncan mask.

I had not considered that a Duncan mask might work better for collimating using a camera. I use a Lodestar for VA/EAA and I need to collimate carefully as I am planning to use an F3.3 reducer with the camera. I will also try both collimation methods using the camera (and Lodestar Live's zoom feature to increase the scale of the 'Y') and see how I get on. Need to retrieve the mask from the bin.......  

[michael.h.f.wilkinson]

It's interesting to hear other people experienced the same thing, thanks for the responses.

I had not considered that a Duncan mask might work better for collimating using a camera. I use a Lodestar for VA/EAA and I need to collimate carefully as I am planning to use an F3.3 reducer with the camera. I will also try both collimation methods using the camera (and Lodestar Live's zoom feature to increase the scale of the 'Y') and see how I get on. Need to retrieve the mask from the bin.......  

Sounds good. Let me know how you get on