Collimating a SCT question

[5ash (Philip)]

I know the Duncan mask has been discussed in the past on this forum and am interested to get some first hand experience of its use. Does it do anymore than centre the secondary mirror as does a very defocused star? I get confused by owners publishing collimation as simply centering the secondary mirror shadow in the defocused star and that’s it , done and dusted without going to the next stage of observing a star at high magnification and centering it in the airy disc. Something that with poor eyesight and poor conditions I find difficult . Thus the question of the Duncan mask. Artificial stars are also cited and used at distances where the artificial star can be resolved? And always the suggestion that you have to fine tune on a real star. Any comments , experience and suggestions appreciated.
Regards philip

[multiweb (Marc)]

Initial alignment can be done with a tak collimator, howie glatter with a grid pattern or a hotech laser board.

You can use an artificial star at ~25m and reach focus with extension tubes at the back so you keep the distance between mirrors close to optimal.

You can also use a webcam or planetary to view the airy disc on a laptop screen if you prefer.

Final collimation do it on a real star with a 6mm or 4mm eyepiece if seeing permits.

Got some pictures here:
http://www.astropic.net/astro/C11/article.list

If you search these forums for the CN-212 resto I also documented collimation steps with photos and images of artificial star and airy discs.

[Wavytone]

Hi 5ash,

Duncan masks, artificial stars and other gadgets will get the alignment close, but not perfect - for that you really must test at high mag on a bright star.

What I have shown several times to other SCT owners is that while they thought they had it “close enough is good enough” the scope was really underperforming. In excellent seeing an SCT that is just a tad off will appear fairly good, but when the collimation is correct a significant improvement should be noticeable. In poor seeing a star image often breaks down into a smear, yet side by side with my scope, it’s still showing tight pinpoints though they dance around a bit. But not smeared.

For the star test you need an eyepiece atound 5-7mm and choose a bright star close to the zenith, ideally mag 1 or brighter. On a nice bright star when refocused a tad, just enough to show the secondary shadow, you should see the bright Poisson dot inside the secondary shadow. This tiny bright spot in the dead centre of the OTA is caused by diffraction around the secondary shadow - and it dies not move when the secondary collimation is adjusted.

When collimated correctly you should see the Poisson spot concentric with the shadow, with concentric out of focus rings both inside and outside focus. At focus you should be able to see the diffraction disk of the star, plus one or two rings.

However quite a few SCTs do not really produce a diffraction pattern at focus - what you may see is just a jumbled mess of intense light. Over the years I’ve seen quite a few like that.

I see you are in the Hunter Valkey - if you ever come this way PM me - bring your OTA and I’ll do it for you, and maybe show what a really really nice star test looks like in my Santel.

[Troy]

Hi Philip,
You are right about the duncan mask it only roughly collimates your telescope.
A duncan mask is only needed if you don't know how far to defocus the star or have trouble seeing the amount of defocus.

Step 1:
For me roughly collimating a scope is defocusing a star so I can see 3 rings - after the collimation is done at this amount of defocus I then adjust the defocus to 2 rings and continue to adjust the collimation screws until it's as close as I can get it.

At this stage the collimation is close but for me it's not good enough for high resolution work such as planetary imaging.

Step 2:
Fine collimation - I focus on a star using a Bahtinov mask then I take a short video of the focus star. After this I process the video using autostakkert (stacking program) to determine what screw needs to be adjusted and I repeat this process until I get an image that looks like the one attached.

Only after the second step I will then start to use the scope. Hope this helps or have more questions just let me know

ps. the image below is an actual image not simulated!

[5ash (Philip)]

Hi Troy thanks very much for the info , just what I needed. Thanks Marc and Nick for your help as well
many thanks philip