collimation Collimation - An Airy Disk Realised at Last

[DeepSkyMan]

I had a moment of optical understanding this past weekend.  Our stargazing group were away at our favourite dark skies site in the Scottish Highlands for an observing weekend.  Weeks prior to this I swapped out the old phillips collimation screws on my 8" EdgeHD for a set of Bob's Knobs knowing that I would need to re-collimate after the installation.  Sure enough the optics were way out, I spent the next 3 weeks waiting for a weather break that I could take advantage of.  Sadly no such opportunities arose to get the scope collimated, the result being that I knew I had a collimating exercise to go through before I would be able to carry out any observing.  So on arriving at our observing site, I immediately set up an artificial star at about 90ft distance from the scope (being new to collimating an SCT I wanted a stationary target to work with), and set about collimation.  After about 20 mins at about 200 mag, I believed I had the collimation decently in order.  Following recomended procedure I then bumped the mag up higher (about 400), and with the typical degree of defocus things still looked good,  I then proceeded to bring things into focus.  As I adjusted the focuser closer and closer to focus I noticed that one corner of the image seemed to be coming into focus significantly sooner than the others, resulting in the typical comatic image of an uncollimated set of optics.  I compared this image from memory to what I had been experiencing prior to installing the Bob's Knobs and thought that in all honesty this was pretty close to what I was getting before installation of the Bob's Knobs (I received the scope second hand from a recognised vendor in London, that unfortunately went out of business about 6 months later.   I should also note at this point that up until this occassion I had not been able to collimate the scope due to the screws being seized in position).  Was I happy with this level of collimation accuracy?  Not really.  Unfortunately I was now running out of daylight and wanted to start prepping for the nights observing.  I hummed and hawed about what to do, live with what I believed was a tolerable image, or persist with collimation and risk compromising what I already had.  Having read up considerably on the collimation procedure I had heard people referring to '...seeing an Airy disk...'  and what I was seeing was certainly not that, so I determined to try and get things better.  After a further 20 mins or so of tinkering, I believed I had the defocused image much more concentric, and proceeded to bring things into focus.  As I wracked the focuser closer and closer to focus, I noticed that there was still a little variation in concentricity of the diffraction pattern, a little more adjustment was required.  Then proceeding to focus, I noticed each outer fresnel ring disappear until I only had a central bright spot and two unbroken rings... Eureka!!!... so thats what an airy disk looks like.  Further movement into focus and the remaining rings just disappeared simultaneously and I was left with a pinpoint image of the artificial star.  At this point I locked the primary (observing focusing is done solely with a Baader Steeltrack).

 

The moral of the story?  Have the courage of your convictions.  Proof was in the pudding that night as we experienced some excellent sky transperancy, seeing wasn't perfect but the double cluster in my newly collimated scope was just simply mind blowing, this is the first time I have seen the scope perform to the level implied by its name, pin-point stars from edge to edge across the entire field of view.

Thanks for Reading

Paul.

[Charic]

Interesting. There have been occasions in the past where upon reflection, my disk looks ok, but does not look like those portrayed in the books, showing me a clear, sharp and well defined surrounding pattern, and have recently read  an article describing that the defocusing itself is only slight in order to see the pattern, yet everything on my setup seems concentric and the images look fine, but it makes you wonder, maybe there's a little more I can squeeze of of this scope? 

[DeepSkyMan]

6 minutes ago, Charic said:

Interesting. There have been occasions in the past where upon reflection, my disk does not look like those portrayed in the books, clear, sharp, well defined, and have recently read  an article describing that the defocusing is only slight in order to see the airy disk, yet everything else on my setup seems concentric enough and the images look fine, but it makes you wonder? 

Charic, I should perhaps have made it more clear in my original post that the margin of defocus at which I saw the airy disk was miniscule, one second it was there the next it was gone, which would be in line with what you are saying.  Of course you need to factor in that I'm using an artificial star as well :-)

Paul.

[Charic]

No worries, and I have adjusted my text slightly since your reply, but yes, its  very slight, something I will observe with more interest next time round.
Its more than likely the reason that I don't  always see the surrounding pattern, or possibly to ham fisted with the focuser? but full de-focus is great too, as I can observe the reduction of warm air currents, whilst the temporary applied fan is doing its thing, especially if the scope goes out later than usual.

As for my light source, I use Polaris.

[DeepSkyMan]

2 hours ago, Charic said:

No worries, and I have adjusted my text slightly since your reply, but yes, its  very slight, something I will observe with more interest next time round.
Its more than likely the reason that I don't  always see the surrounding pattern, or possibly to ham fisted with the focuser? but full de-focus is great too, as I can observe the reduction of warm air currents, whilst the temporary applied fan is doing its thing, especially if the scope goes out later than usual.

As for my light source, I use Polaris.

Yeah, I had a go at using Polaris, unfortunately I use an Altair-Astro Sabre alt-az mount for this sort of thing and I'm not fairing too well without a set of slow motions, so resorted to a stationary target.   I found this simplifies the situation greatly :-)

Hmmm... On reflection, don't know why I've never used the GEM for collimation, I guess it would be a lot easier using the Sphinx controls to re-centre a star than trying to nudge it back into position on the alt-az...  Guess I'm just in observe mode by the time I get to that part of the evening.