How diagnose problems preventing diffraction limitation.

[SteveBz]

Hi Guys,

I regularly check and collimate my scope. I work hard on focus and polar alignment, yet I have never seen an Airy disk through my c8n.

This was brought home to me at a stargazing event last night where I looked through a friend's refractor telescope only to see the image was clear and almost pixelated. I realised it was an Airy disk.

How should I go about diagnosing the problem here?

Regards

Steve.

[Peter Drew]

I don't think there is very much you can do about it. The seeing conditions will be rarely be good enough to see an Airy disk in a 8" telescope, even a 8" refractor. Small refractors and even good small long focus reflectors will usually show an Airy disc at high magnification, hence their good reputation. 

[SteveBz]

Thanks for your reply, Peter. 

So that's a bit disappointing.  Are you really saying that a C8-N can never really be called diffraction limited (as per their advertising blurb)?

I find it really, really hard to reach the focal sweet point, so that stacking improves the image.

I'm in the process of attaching my Skywatcher focuser via an Arduino to my laptop, so that I can get a full screen image, together with keyboard assisted focusing (and maybe, eventually, software focusing).  But, if for instance, my focuser tube is not precisely aligned with the optical axis then I should focus on that, or if my little ring on the main mirror is not precisely centred, then that is my problem.  I find it hard to decide how to prioritise my time.  I was thinking if there were a sort of diagnostic decision tree I could eliminate potential causes to concentrate on the real cause of the problem.

Your thoughts would be welcome..

Regards

Steve.   

[Peter Drew]

I would focus your attention on keeping the telescope in good collimation rather than worry about the visibility of an Airy disc as the circumstances that affect this are pretty much beyond your control. The thermal problems associated with a Newtonian telescope and the slight reduction in contrast due to a secondary obstruction  are a small price to pay for an affordable 8" apochromat.  

[SteveBz]

I tend to collimate every 2 or 3 weeks, but each time I try to improve the process. 

I wear glasses, so that I'm never sure that the sort of "flares" that I see from a star are a function of my glasses or collimation, or something else.

Steve.

[John]

The scope probably diffraction limited in terms of it's optical quality but that does not itself guarantee that you will get refractor-like airy disks. As I understand it, there are a number of other factors, including the secondary obstruction and it's supports, air currents within the tube, plus external factors such as the atmosphere that disrupt the optics ability to deliver to the dawes or (slightly) less stringent rayleigh limits.

 

 

 

 

 

[SteveBz]

15 hours ago, John said:

The scope probably diffraction limited in terms of it's optical quality but that does not itself guarantee that you will get refractor-like airy disks. As I understand it, there are a number of other factors, including the secondary obstruction and it's supports, air currents within the tube, plus external factors such as the atmosphere that disrupt the optics ability to deliver to the dawes or (slightly) less stringent rayleigh limits.

 

 

 

 

 

So is it that you mainly get Airy disks in refractors and not reflectors?

[John]

29 minutes ago, SteveBz said:

So is it that you mainly get Airy disks in refractors and not reflectors?

They tend to be more clearly defined, less "cluttered" if you like, with refractors, maksutov-cassegrains and maksutov-newtonians.

 

[YKSE]

22 hours ago, SteveBz said:

Hi Guys,

I regularly check and collimate my scope. I work hard on focus and polar alignment, yet I have never seen an Airy disk through my c8n.

This was brought home to me at a stargazing event last night where I looked through a friend's refractor telescope only to see the image was clear and almost pixelated. I realised it was an Airy disk.

How should I go about diagnosing the problem here?

Regards

Steve.

What was the eyepiece focal length you used to see airy disk in C8N?

My understanding is that you need to use EP of 5mm (1mm exit pupil for any scope) or higher mag to see airy disk, scopes with central obstruction wiill have less distinct rings than refractors, nonetheless, 0.5mm exit pupil will definite show it. the completeness of the rings though will be more seeing dependant in this high mag.

[SteveBz]

46 minutes ago, YKSE said:

What was the eyepiece focal length you used to see airy disk in C8N?

20mm.  And I don't know what EP was used on Saturday by my friend, so you're right it might have been that.

Steve.