Star testing after colimmation

[NQLD_Newby]

Hello everyone,

I recently removed my primary mirror to check the centre spot positioning etc as I wasn't sure how accurately it was placed to begin with. Anyway, as it turns out it was off alignment by a few mm. After repositioning it accordingly, and reassembling the scope, I proceded to colimmate it. Everything seemed to go as per normal with the colimmation.

Last night I took the scope out to do some viewing and decided to do a star test (first time for this), and something doesn't seem right. When I defocus one way, i get a white ring around a large black disc, (presumably this is the airy disc i have read all about), but when i defocus the otherway, the star errupts in a huge commet tail and stays that way until i have defocussed enough to actually see the secondary mirror and spider veins. Further more when a star is located off centre of the FOV, it looks more like a comet. When i move the scope to make the star rotate around the centre, the tail rotates to always point towards the edges of the FOV. I know my ep's are very dodgy, as they came with the scope, and the tails were always evident (towards the edges), but not to this extreme. Only the very centre of the FOV is without this effect now. I also get what appears to be a reflection of some kind when a bright object is not in the centre. As I move the scope to put the object in the centre, it and the reflection converge to become one, and the FOV fills with background light. Its hard to explain this bit, but its kind of like when someone shines a torch around, you can see the beam, but when it actually passes over you it shines right in your eyes.

I put the colimation tool back in to check it and it seems ok. Centre Spot on the cross and reflection cross behind original coss. I have no idea what is wrong but viewing like this is not good..

Any suggestions would be greatly appreciated.

[Space Dog (Ric)]

Hi Rex,
From what you have descibed I would suggest that your primary mirror is out of alignment.
Perhaps the original position of the centre spot was correct and the optical centre of your primary mirror is not the measured centre.
Cheers,
Ric.

[Satchmo]

Originally Posted by NQLD_Newby

When I defocus one way, i get a white ring around a large black disc, (presumably this is the airy disc i have read all about), but when i defocus the otherway, the star errupts in a huge commet tail and stays that way until i have defocussed enough to actually see the secondary mirror and spider veins.

Sounds like severe spherical aberration.

[astro_nutt]

Hi Rex,
Just a question..how were your images before you repositioned the centre spot on your primary?..the reason I ask is that sometimes a combination of factors can cause the misalignment you've described.
Some of these are...
Is the EP secured in the focuser?
Is the focuser aligned and working in proper motion without any slack?..(this can cause the EP to work one way..then drop out of alignment the other in which case you have to check and adjust by turning the tiny hex head grub screws on the top portion of the focuser next to the lock screw)
Is the EP being used in good order with no loose components? (lenses..base..etc.)
Check to see how much drift (if any) by turning the focuser in and out while viewing the centre spot via the collimation tool.
I hope it works out for you.
Cheers!

[NQLD_Newby]

First of all folks, thanks for listening and your comments.

I found out last night that my focuser must shift alignment depending on how far in or out it is. The way I proved this was, by colimating with the tube all the way in, and then winding it all the way out with the tool still in place. Well the colimation was way off when I wound the focuser out. When wound back in it realigned itself nicely, (atleast thats something, it doesnt wobble it just changes alignment). So as a test I recolimated with the tube all the way out, and likewise when wound back in the colimation moved out of alignment. OK i tested my ep's and found that they all came to focus more or less with the focuser about half out position. When barlowed slightly further out but nowhere near all the way out. With this information, I decided to put the focuser in the middle position and recolimate. When wound in and out, slight missalignment occured, but my ep's will never focus in these positions so hopefully this won't be a problem.

I took the scope outside and tested it against a star again. I have no idea what I'm doing with this star test thing, and I cannot get the rings that I have seen pictures of to appear at all. All I get is a dark disc surrounded by one white ring. So I decided to use the views as an indication of whether things were ok. That being said I could focus very crisply, and the views went back to the way they were before I moved the centre spot. Comet tails at the edge of the field, but a conciderably larger FOV without this effect. I decided to have a look at the left hand pointer (a double that I like). Normally I have to use my 10MM ep 50X to split this double, but to my surprise tonight I could very clearly split them with the 20MM ep 25X. Not sure whether perhaps, the seeing was acceptional or if it is a direct result from now being colimated better than before I moved the spot, I suppose time will tell. I also looked at the moon, extensively with my 4MM ep with 2X barlow = 250X, something I have never been able to do. Usually even the 4MM without barlow gives slightly fuzzy views. Last night they were crisp and very clear. Once again, could have been the seeing.


Astro_nut, I will check to see if my focuser has these adjustments tonight, I'm not sure it does. My ep's are in good order, just not good quality. Will be upgrading to a 12" after Chrissy, so will have slightly better ep's at that stage. It would appear that "Drift" as you call it is the culpret.

Satchmo, How do I fix SA? and what causes it?

Space Dog, it would appear that you were correct, at certain focuser positions.

Thanks again for your help people.

Ok things seem to be back to a level where I am happy to use my scope again. If anyone has any ideas about how to fix this problem, or even what causes it, I would be only too pleased to hear from you.

[Starkler (Geoff)]

The fact that the collimation changes with focuser position indicates that you havent lined up the focuser axis to the optical axis.

This is step two in collimation and involves using your collimation tool to align the secondary to the centre of the mirror. With a laser this is intuitive, and using a cheshire its a matter of centering the cross hairs over the reflection of the mirror centre spot. Once this is done, the focuser axis is aligned. Then proceed to adjust the primary.

[NQLD_Newby]

I put the colimation tool back in to check it and it seems ok. Centre Spot on the cross and reflection cross behind original coss. I have no idea what is wrong but viewing like this is not good..

As stated in my first post Geoff, I have aligned the Spot with the cross hair. The problem seems to be that when I move the focuser it unalignes itself. If i realign it in that position, when i change the focuser again it unaligns itself yet again. Why this happens I have no idea, thats why I asked for help?

As stated in my second post, the best I can come up with is to colimate it around the position of my ep focus. Although this is not ideal, it seems to have atleast made my scope usable again.

I know that I may ask dumb questions but isn't that how we learn?

[rmcpb (Rob)]

From what you have said the actual focuser may not be square to the optical axis and may need shimming. Never had to do this before but you should be able to check it with a laser collimator reflecting off a piece of white papeer inside the tube. Mark where the spot is when the focuser is centred then rack the focuser out and mark the position of the spot, then rack it in and mark the spot. If it moves either sideways or up and down then the focuser is not square to the tube and will need shimming to fix this problem otherwise you will collimate till the cows come home and never be happy with the result.

Hopefully this is not a problem but it is easily checked.

Cheers

[NQLD_Newby]

Thanks Rob, I will play with it some more and see what I can come up with.