Airy disc and diffraction rings in reflectors?

[PaulR]

In another thread I asked if my scope might need collimating as my star images are not terribly sharp. The manual I downloaded describes the collimation process and it seems pretty straightforward (ha!). It does however say that stars should present an Airy disc and diffraction rings subject to atmospheric conditions but one of my old astronomy books says that Newtonians are unlikely to show disc/rings due to short focal ratio (?).

If you are a reflector user what do you see or have you any advice?

(PS I think my secondary mirror needs adjustment either way as the reflection of the focusser appears off-centre but I'm not sure if that will have any effect on the above)

TIA

Paul

[James]

I've never seen an Airy disc nor diffraction rings in my Newt and I know beyond doubt that its been collimated very well. Usually the problem is seeing which seems to be rubbish most of the time I get to go out. I have an f/4.7 scope.

James

[Astroman]

I see airy disks most nights in both my 10" F/7 Newt and C8 SCT. Nice and dry in the desert.

[James]

[barkis]

Seeing conditions will dictate the ease with which the diffraction rings can be seen. Any turbulence, whether low altitude, or low will destroy them. You will see them James, but only if the conditions to do so prevail. Here is a list for you in piture form. Some of the patterns you will not wish to see.

Ron.

(click to enlarge)

[GazOC]

Nice 'wallchart' Ron. I've D/L'd, hope you don't mind?

[barkis]

Nice 'wallchart' Ron. I've D/L'd, hope you don't mind?

Not at all Gaz.

[themos]

http://stargazrs.com/ext/video_player.swf?config={videoFile: 'http://stargazrs.com/themos/video/31.flv', splashImageFile: 'http://stargazrs.com/images/video_player.jpg', autoBuffering: false, initialScale: 'scale', autoPlay: false, }

There you go, the view of Polaris through defocused 4.5" f/8 reflector.

http://stargazrs.com/#themos/action/videos/show/31/ajax/true

[PaulR]

Thank you all for your replies! It looks like mileage varies depending upon seeing conditions more than anything else.

My situation seems to match the 'coma' image on Ron's chart although I haven't had chance to study an out of focus image yet. Hopefully that will change tonight so I'll report back - I'm sure this thread will be usefull for relative newbs like me who aren't sure if their gear is set up properly!

Thanks again

[John]

Thank you all for your replies! It looks like mileage varies depending upon seeing conditions more than anything else.

My situation seems to match the 'coma' image on Ron's chart although I haven't had chance to study an out of focus image yet. Hopefully that will change tonight so I'll report back - I'm sure this thread will be usefull for relative newbs like me who aren't sure if their gear is set up properly!

Thanks again

The trouble with startesting is that, unless you really know what to look for and have startested a number of scopes, it is easy to jump to the conclusion that your scope is at fault when much of the time it's the prevailing conditions that are causing the "defects" that you see.

Speaking from personal experience you then embark an a major collimation / adjustment excercise which does nothing to build your confidence in either your ability or the scope's. At best you end up with the performance much as it was when you started, at worst you loose confidence in the scope completely and sell it !.

Although I do startest from time to time, I now have some "test objects" (double stars mostly) which I am familliar with and have observed with different scopes and in varying conditions. I find these the best barometer to go on. My favourite test doubles include the "Double Double" Epsilon in Lyra, Castor in Gemini, Polaris, Sigma in Orion, Epsilon in Bootes and Iota in Casseopea.

John

[themos]

[ My favourite test doubles include the "Double Double" Epsilon in Lyra, Castor in Gemini, Polaris, Sigma in Orion, Epsilon in Bootes and Iota in Casseopea.

Very good idea. Should write down what is resolvable for each scope/eyepiece.

[barkis]

The secondary mirror has to be offset Paul. It is offset away from your drawtube to enable it to intercept the whole of the returning cone of light from the main mirror. This is especially true in a fast reflector. IE short focal length mirror.

If you draw out your lightpath to scale will do, then put in your diagonal in the correct place, it will become obvious as to why it will need to be displaced away from the drawtube, in order to collect all the light beam from your objective.

Sometimes an oversized secondary would be supplied for astrophotography, and therefore what I said previously might not apply, but if the secondary is not for photography, then it most probably is offset away from the drawtube.

I hope this makes sense to you.

Ron.

[John]

[ My favourite test doubles include the "Double Double" Epsilon in Lyra, Castor in Gemini, Polaris, Sigma in Orion, Epsilon in Bootes and Iota in Casseopea.

Very good idea. Should write down what is resolvable for each scope/eyepiece.

This chap has done some useful notes for a few good test doubles:

http://homepage.mac.com/joebergeron/doubles.htm

John

[PaulR]

Thanks again all.

I'm probably just being a bit picky as I've worked in optics for 30 years I had about 4 or 5 years of regular observing from a town garden in my teens so I'm not put off by poor seeing.

I'll be having a good squint down the tube tonight....

Thanks once more.