Collimation issue?

[kev100]

Hi all,

I guess I should know this already, but I can't find an answer on the web anywhere. I've collimated my 250PX as best I can. The secondary looks circular and central when viewed through the collimation cap, the primary mirror clips are all visible, and the laser collimator beam runs central. All seems well. The view is great, with stars defocussing to circular discs, with a central spot. However, when defocussing, the objects seem to stretch slightly in one direction (let's say 'horizontally') when defocussing outward, before becoming a circular disc, and 'vertically, when defocussing inwards (hope this makes sense).

Does this indicate a slight collimation error?

Cheers,

Kev

[mikeyj1]

focus tube out of square?

[Peter Drew]

Seems like astigmatism to me, is the primary loose or tight ?. If tight, try backing the clips off so that they barely touch the mirror.   

[kev100]

Thanks guys. I'll check the focus tube (as best I can). I did remove it when I flocked the OTA, and it might not be exactly square. As for the primary mirror, it also was out when I flocked the OTA, but I didn't touch the clips, so I'd assume they're still at factory settings ...

Kev

[Dave In Vermont]

'Pinched-Optics' is the name for the condition of the clips be too tight.This happens all-the-time coming out of the 'factory.' I just back them all off a bit. Then, in a cross-pattern, I tighten-down the retaining-bolts - slowly - until they're all 'snug' - but not tight or loose. Many things can cause this too - such as temperature variations changing the tension of all the bolts & screws from expansion/contraction of the metal.

It works for me. And I'm one of those nuts who actually enjoys collimating telescopes!

Fear not -

Dave

[Peter Drew]

The "factory" have a tendency to overtighten primary mirror clips, possibly as an added safety measure during transit. 

[kev100]

Thanks all. I was thinking about taking out the mirror to give it a clean anyway ... I'll check the mirror clips while I'm at it.

Cheers,

Kev

[John]

These web pages are a sort of "Observers Book of Telescope Aberrations" and can be quite useful for identifying what you are seeing:

https://starizona.com/acb/basics/equip_optics101_aberrations.aspx

[kev100]

Hi all, thought I'd post an update. I took out the mirror, and checked on the clips ... Of the 6 pairs of screws (2 per clip), some were very tight, some were totally loose! I slackened off the tight ones, screwed in the loose ones, popped the mirror back in and recollimated, and yes! A quick check showed a definite improvement. Still not 100%, but near enough.

I was reluctant to slacken off all the clip screws, and balance them exactly, as some screws were so tight, but I guess I got lucky and did just enough.

Cheers,

Kev

[kev100]

Hi again,

I thought I'd update this thread. I was growing increasing dissatisfied with having broken primary mirror retaining clip screws in my scope, so took the plunge and had the remaining ones drilled out and replaced with some nice stainless steel ones.

I replaced the mirror, with all 12 screws set to the same tension, checked the collimation and tried out the scope on a star test. No more vertical/horizontal distortion at all. Very happy.

However, I then had a look at Venus, and was a little surprised to see this odd view when defocusing ... the crescent shape split into several versions before becoming a circular, out-of-focus disc ... looks a bit like this.

I've never noticed this phenomenon before. It's very odd that on a star test the scope performed perfectly (Aldebaran defocussed to a circular disc without splitting into several versions), but Venus behaved like I've shown here in the image. 

I rechecked collimation using a collimation cap and a laser, and it's definitely spot on.

 

[Peter Drew]

Almost certainly the effects of seeing, either atmospheric or instrumental during cool down. 

[kev100]

Hi Peter, Thank you. It's true that I wasn't able to leave the scope to cool completely. Will try again when (if) we ever have another clear night.

Cheers!

Kev.

[K4PDM]

My guess is a combination of seeing and reflections.

Venus is extremely bright!

[Stu]

I'm sure Peter is correct on this one, it is an affect I have seen many times before.

Apart from making sure the scope is cooled, try getting Venus as early as possible whilst it is still higher up. You can see it easily against a twilight sky and this sometimes reduces the glare you get of it in full darkness.

[kev100]

Thanks guys. Still waiting for a chance to check it again.