Collimation - does it look right to you?

[tom33pr]

I've just had a stab at collimating my 10" DOB.

Found it pretty difficult using the baader laser collimator and ended up using a simple collimating cap.

The secondary seems in the right circular shape and reflects all 4 clips of the primary. The primary's circle point is right in the center and the offset is visible in the background (unfortunately the camera didn't manage to capture it). Does it seem right to you (see photo)?

I still have some concerns since my laser collimator shows the reflection on the primary an inch away from he center; however, I noticed that the collimator is slightly lose and even a gentle touch seriously affects the reflection on the primary - making this test somehow unreliable.

Thanks in advance for advice.

Tom.

[Peter Drew]

Looks OK to me. . A star test will confirm.

[Tim]

I'd rely on a chesire and MkI eyeball over a laser any day, but they can be useful for getting close.

How does it look on a star?

[tom33pr]

As much as cheshire was very good with my 5" I find its tube too long for the 10" - it won't let me properly see the edges of the secondary...

Haven't star tested yet, may try tonight if it stays clear.

[Adamski]

Only being relatively new to this game please correct me if i'm wrong but having spent a lot of time collimating on my scope i would say that the secondary looks like it is slightly elongated rather than round, The right side of the secondary looks like it needs to be pushed further back, using the adjusting screws. The primary is nice and round but on the right you see a thin image of the tube but on the left you can see quite a wide image of the tube. This may be ok, if so just ignore me and i'll go away.

[tom33pr]

The primary is nice and round but on the right you see a thin image of the tube but on the left you can see quite a wide image of the tube..

Yea... I was thinking the same.

Just back from my back garden though. Arcturus and Vega popped out so decided to star test it, and the secondary looked dead in the center of the defocused star. So I reckon it's ok.

[cathalferris]

Star testing is more about the appearance of the just-out-of-focus high magnification stars, and the appearance or elongation of the Airy disk rings when in focus.

Centreing of the secondary shadow will not give an accurate collimation guideline (especially if it is correctly collimated it should be offset from the focuser by a few mm anyway).

Check here:

As for the pic - it looks close, but the star test will be the defining test. I'd suggest looking at Polaris - makes it possible to startest easily without a driven mount.

[jamespels]

The primary is nice and round but on the right you see a thin image of the tube but on the left you can see quite a wide image of the tube. This may be ok, if so just ignore me and i'll go away.

I am also pretty new to this but I think what looks like a wide image of the tube to the left of the secondary is actually the body of the secondary holder (note the dark bar extending from it to the left).

I would say there is no visible image reflected in the primary and that it is pretty close to bob on as far as can be told from this image.

Hth, J.

[tom33pr]

I think what looks like a wide image of the tube to the left of the secondary is actually the body of the secondary holder (note the dark bar extending from it to the left).

You are correct - it is a part of the secondary holder.

[Jason D]

Tom,

You will be surprised how much you can read off your photo.

1- Your laser told you that the focuser axis reflection off the secondary mirror will hit the primary mirror around 1” off-center. That is consistent with what I see in the photo (cyan cross-hairs)

2- Your collimation cap told you that the primary mirror axis reflection off the secondary mirror will run through the center of the focuser (red cross-hairs)

3- Your secondary mirror is “about” at the right position – centered below the focuser (yellow circle)

4- Even the eclipse position indicated by the black arrow is consistent with the above.

What does all the above mean?

1- The focal point of the primary mirror will be very close to the focal point of the eyepiece which will keep your coma under control – this is a good thing

2- The focal plane of the primary mirror is tilted with respect to the focal plane of the eyepiece which means you can’t bring the whole view to focus simultaneously. You might or might not notice the impact.

To correct, re-iterate between the laser collimator and the collimation cap. Adjust only the secondary mirror with the laser to center the beam in the primary center spot then use the collimation cap to only adjust the primary mirror to center the reflection of the cap’s pupil inside the reflection of the primary center spot – re-iterate until both alignments are met.

Note: When done correctly, that eclipse will be pointing towards the primary mirror.

More notes:

1- If you can see the whole primary reflection, it will ALWAYS look circular. You will never see a whole primary reflection that is elliptical.

2- Star collimation can ONLY tell you how close is the primary mirror focal point to the eyepiece focal point (the most important collimation alignment). It will not tell you much about the secondary mirror position nor about the focal planar tilt.

Jason

[kniclander]

and for the colour blind among us, the cyan cross hairs are the ones that aren't yellow or red

top post, Jason - thanks

[tom33pr]

Jason,

What a thorough analysis

Ok to send you a PM with a couple of questions?

Thanks in advance.

[jamespels]

Blimey - seen my ***!

Jason, that is a fantastic analysis - other than from Astrobaby, I don't think I have ever learned so much about collimation in such concise form. It's not my post but many thanks for this.

J.

[Jason D]

Thank you everyone.

Understanding collimation gets little easier if you understand the 3 main collimation alignments and the importance of each one. You can read my post:

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4242330/page/0/view/collapsed/sb/5/o/all/vc/1

The first 3 posts in the following thread are helpful. You can skip the reminder of the thread

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Number/3532750/page/0/view/collapsed/sb/7/o/all/fpart/1

The first few pages of the following thread are also helpful. Again, you can skip the remainder of the thread

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Number/3033065/page/0/view/collapsed/sb/7/o/all/fpart/1

Tom33pr, you may PM me with questions. However, it others can benefit from your questions, you might want to post them here or on a new thread.

Attached is a photo for my collimation setup.

Jason

[dweller25]

This thread should be a sticky .....