Mechanical Vs Optical collimation

[matt]

Is it possible to have a telescope perfectly collimated from an optical point of view (checked using a laser collimator and cheshire eyepiece) and still be out of collimation mechanically???

I'm getting eveything nicely lined up in the cheshire and the concentric rings look good in star tests outside focus ... but inside focus is a different story - can hardly see any rings and the white area of the unfocused star is dimmer and less defined???

I'm concerned about the positioning of the secondary.

Thanks

That sounds more like primary mirror errors, of which there a several. Have a read of this site and see if your tests are similar to any of these.

http://www.willbell.com/tm/tm5.htm

regards,CS

[matt]

Thanks Rob

I'm not sure how concerned I should be. The scope delivers pretty nice views of most objects, but then I'm not sure of what to point it at to put it to the test, optically???

Omega Centauri and Tuc47 look great, but you'd expect that given they're among the easiest targets among the fainter objects.

Jupiter looked OK lat Sept ... but was not at its viewing best. I've also cught glimpses of surface features on Mars at around 200x.

Any suggestions for failsafe testers?

Thanks

Originally Posted by matt

Thanks Rob

I'm not sure how concerned I should be. The scope delivers pretty nice views of most objects, but then I'm not sure of what to point it at to put it to the test, optically???

Omega Centauri and Tuc47 look great, but you'd expect that given they're among the easiest targets among the fainter objects.

Single point stars are probably the best target to test with.

Originally Posted by matt

Jupiter looked OK lat Sept ... but was not at its viewing best. I've also cught glimpses of surface features on Mars at around 200x.

If it is mirror errors you probably wont notice the defects until you do a
side by side with a similar scope. Or you are experienced enough to detect
the errors when using it visually.

Sorry I cant be of more help, I'm sure someone else will pipe in.

regards,CS

[matt]

Rob

I've pointed it at single stars. That's what I star test with

I guess I'm after some suggestions of visual objects which will prove a challenge and perhaps show up any probs which may exist???

Thanks for your help though

[Starkler (Geoff)]

Originally Posted by matt

I'm getting eveything nicely lined up in the cheshire and the concentric rings look good in star tests outside focus ... but inside focus is a different story - can hardly see any rings and the white area of the unfocused star is dimmer and less defined???

This is very likely just related to seeing conditions and nothing to worry about.
My mirror star tests very well in great seeing, but on 90% of nights the rings inside focus look mushy and ill defined, whilst outside focus they look ok.

Meaningful star testing for optical quality in larger scopes demands excellent seeing conditions and a well cooled scope.

A few years ago I owned a Saxon 8 inch f5 dob, which I suspect has the same optics as the Skywatcher. Star testing showed obvious spherical abberations but even that being the case, my best ever views of Jupiter were through that scope, even though my present scope has superior optics.
The point being that with larger scopes your limiting factor is likely to be seeing conditions 95% of the time.

If objects look good in focus thats the most important thing

[rumples riot]

A good book to loan or buy is Star Testing Astronomical Telescopes: a maual for optical evaluation and adjustment, Suiter, published by Willmann-Bell Inc 1994-2001.

It is a comprehesive manual and will solve this issue for you. I have found it to be most interesting.

Another consideration, and one that is covered by the book; is the possibility that you may have pinched optics. That said though there is no way of knowing without comparing your images with that in the book.

Best of Luck.

[matt]

Once again ... many thanks.

rumples: I've been eyeing that book off for a few weeks now. May make a purchase, even if there's nothing wrong with my Skywatcher. I agree it would make for interesting reading and be a real handy resource up on the bookshelf.

starkler: you put my mind a little more at ease. It never dawned on me anything less than great seeing could result in different star test results when comparing inside and outside focus? I just assumed that if I was getting nice diffraction rings outside focus I'd get the same image inside, assuming there was nothing wrong with my primary? If you're right, it's a great way to get a quick gauge of what the seeing's like? It's a seeing "tester"!!! lol

Originally Posted by matt

Once again ... many thanks.

rumples: I've been eyeing that book off for a few weeks now. May make a purchase, even if there's nothing wrong with my Skywatcher. I agree it would make for interesting reading and be a real handy resource up on the bookshelf.

It's the same book I posted a link to earlier, there is an extract there that
could be handy to you.

Originally Posted by matt

starkler: you put my mind a little more at ease. It never dawned on me anything less than great seeing could result in different star test results when comparing inside and outside focus? I just assumed that if I was getting nice diffraction rings outside focus I'd get the same image inside, assuming there was nothing wrong with my primary? If you're right, it's a great way to get a quick gauge of what the seeing's like? It's a seeing "tester"!!! lol

Not entirely, when I do the same thing in my scope the pattern inside or
outside of focus is pretty much the same, even when all mushy from seeing
conditions. Maybe my mirror has errors but then again I've manage to pull
off few dandy images through it so it cant be too bad. Anyways good luck
with it. I have a Synta mirror in my scope too, and I have heard Synta still
throw out the odd dodgy one. I think this is why maybe GSO are lil more
popular as they seem more consistant.

regards,CS

[ving (David)]

how does the inside and outside focis differ?

[rumples riot]

Matt, I got the book several months back and while there is a lot of technical jargon on optical theory it is a good reference. When ever something is troubling me about my scopes I just go back to the book.

[bird (Anthony Wesley)]

Originally Posted by ving

how does the inside and outside focis differ?

As I had it explained to me... when you go inside infinity then you are focussing more on turbulent layers in the upper atmosphere so the detail can be mushed, whereas outside infinity is focussing somewhere a looong way away :-0

Bird