Refractor collimation using toucam.

asimov (John) · #1 08-12-2005, 02:48 PM

Hi all.

I know most of you don't use refractors, especially...achro refractors for astrophotography like I do but I found this pretty interesting.

I've spent a great deal of time collimating my 150mm refractor until it's as close to perfect as I can get it.

The other night after setup I decided to do a few experiments & one of them was to inspect my collimation by way of toucam & laptop on a star.

After getting canopus on the laptop I defocused to see if I could see any diffraction rings, & after adjusting the toucams brightness/gain etc etc they were as plain as day. No central obstruction with a refractor as you know but there is a central round 'blob' of light in the middle of the diffraction rings to go by to see if collimation is out. (Airy disc?) This central blob was not quite in the middle I noticed. It was perhaps 2mm from central.

I took the dew shield off to get to the objective cell push/pull screws & after doing the 'stick finger over front of scope to work out which screw to tweak' got a screw-driver and did so.

After turning the appropriate screw about a 16th of a turn I noticed no (laptop) visual difference....I turned the screw a 1/2 turn....no difference......

turned the screw THREE full turns

........NO difference! took avi of saturn & stacked/processed/reviewed.....looked normal to me!

Conclusion?....

SURELY by turning that screw as far as I did would have thrown collimation way out?

I should have taken photos of the collimation while I was at it & next time I will.

This has me thinking I know LESS about refractor collimation than when I first started out.

I'm thinking this has everything to do with the draw-tube not being 'square on' & less to do with the objective being 'square on'..

Interesting..

Merlin66 (Ken) · #2 08-12-2005, 03:03 PM

I'm VERY surprised. I find even a touch on the screws shows up!! Something wrong somewhere!! I'm assuming its f8 or so, then throwing the objective off by 1/20 x 3 = 0.15 " ( over 3mm!) assuming the screws are threaded 20 threads per inch; is excessive.

asimov (John) · #3 08-12-2005, 03:12 PM

Yes, VERY suprised about this also. Definitely something wrong here.. F/8 AOE 152mm (meade)

ving (David) · #4 08-12-2005, 03:15 PM

very wierd asi, very weird

asimov (John) · #5 08-12-2005, 03:29 PM

Just to relate a bit more of the story: I now use a cheshire (with cross-hairs) for the refractor. The 2 donuts one sees were perfectly lined up, with the cross-hairs central on top of those. (with diagonal in place).

I have not checked to see how far out it is now by way of cheshire....it's a safe bet to assume those 2 donuts will be seperated a long way! Actually I'll go & check it right now.

asimov (John) · #6 08-12-2005, 03:36 PM

Yep! The 2 donuts are quite separate to each other, separated at least half a donut diameter. I think I'll just line them up again & forget about experimenting!

ving (David) · #7 08-12-2005, 03:42 PM

your making me hungry!

[1ponders] (Paul) · #8 08-12-2005, 04:33 PM

Asimov have you checked the collimation without a diagonal. I remember doing something similar to you with the diagonal in and seeing quite a difference without. If you are considering shooting avies through it then it might be worth your while to check without the diagonal. Are you leaving the diagonal in when your imaging?

asimov (John) · #9 08-12-2005, 09:40 PM

Hi Paul. Yes I leave the diagonal in while shooting because it supposedly gives you a bit more image scale.

When I first got the the refractor, I collimated without the diagonal. Then I thought: well if I'm going to use the diagonal on it full time, why not collimate with the diagonal on there....There is a difference in collimation between having it on & not so I found out.

IE: collimate without the diagonal then put the diagonal on & check....it will be out a fair bit!

Would it be better to lose the diagonal while I'm shooting perhaps?

[1ponders] (Paul) · #10 08-12-2005, 11:48 PM

Maybe, maybe not. I'm wondering if your diagonal can be adjusted as well. I remember reading somewhere (I'll try to find it) about collimating refractors that if you're going to use a diagonal then collimate without and then adjust the diagonal if possible (by adjusting the screws underneath?) I'll dig the article up tomorrow night, I know its on one of the other computers.

The other option (and it removes one level of glass from the train) might be to get an extension tube for increasing the image scale and shot prime with the ToUcam from there. That way you know your collimation will be spot on. Just a thought.

asimov (John) · #11 09-12-2005, 01:37 AM

Yep, thanks for that. I already use an extension tube but it won't hurt to lose the diagonal, collimate & bung the toucam & extension on & see what happens. Yes, I have tried adjusting those 4 screws on the diagonal before, with mixed results.

asimov (John) · #12 09-12-2005, 01:46 AM

Actually, another thing I might mention is that there are several baffles in the OTA & the draw tube. One of them in the draw tube is not round, and it juts out into the light cone slightly, giving a slightly oval view (as seen in the cheshire). It doesn't appear to effect the views when using the scope though..I'm thinking about removing it none the less.