Tuning your optics - using a DSLR

[g__day]

An Idea occurred to me last night whilst I was tuning my SCTs collimination. Simply de-focus the star (I used Antares) take a shoy and measure on the screen how far the central black circle was off and adjust it until it is perfect.

So I am taking shots (see below) until with the image about 200 mm across and measuring the centre to within 0.5mm - anyone else tried this or have comments? It seemed to go quite well (to inexperienced me)!

Matt

[anthony2302749]

If you have a webcam the little program from http://www.astrogeeks.com/Bliss/MetaGuide/ will do an even a better job with collimation and in real time.

[Dennis]

Hi Matthew

I think you may be too defocused with the example image?

AFAIK, only small inside-outside adjustments are made with the focus knob, at high magnification; it looks like you have turned the focus knob several turns? All the stuff I have read seem to write about small distances inside and outside of focus, to optimise the sensitivity of the test.

Cheers

Dennis

[jase]

You'll need the little airy disc to validate your collimation. I agree with Dennis, you're too far out of focus for quantitative results. Metaguide is ok. If you are serious about reaching collimation for imaging purposes only, then its hard go past CCDInspector (http://www.ccdware.com/products/ccdinspector/)

It will provide you with the following info;

FWHM
Aspect Ratio
Background Brightness
Signal to Noise Ratio
Altitude of the image
Curvature amount
Collimation error
Tilt in X
Tilt in Y
Date and time of the image
Image Scale
Focal Length
Pixel Size
Camera Gain
Total stars used in analysis
The blurb - http://www.ccdware.com/products/ccdinspector/features.cfm

[Ian Robinson]

I don't have a SCT , but imagine collimation would be a similar process to collimating a cassegrain and I have a copy of Kenneth Novak's Cassegrain Notes that might be helpful and I may be willing scan some of that book as a PDF if someone asks nicely.

[Dennis]

Dear Mr Robinson

It has come to my attention that you are an awfully nice bloke; someone who is always willing to be of assistance to the Australian Amateur Astronomy community.

I have read extensively about your selfless, philanthropic and altruistic efforts in helping others who do not have a copy of the said tome.

Therefore, would you please scan the relevant pages of the aforementioned document as I would dearly love to have a copy…ingratiate, ingratiate, ingratiate …

Cheers

Dennis

PS – provided that copyright infringements do not cause you grief, otherwise I would be happy to send the cover price to the author.

[gbeal]

Holy Heck Den, you have legs all up your underside mate, LOL.

[Ian Robinson]

Cripes .... I am blushing .... how can I refuse after that ..... :D I'll dig out the book and crank up the scanner and scan in the appropriate pages later tonight .... wts

Scanned individual pages.(4 pages) , might be helpful to you .

I think the old chap who owned the copyright has gone to great observatory in the sky ....

[g__day]

Thanks all,

Not surprised I did it wrong (or only partially right) - I just massively de-focused a star and tried to centralise the obstruction - it was a fair way out of whack.

To fine tune it I presume you have to act on a just slightly out of focus star - either side of focus - as there are only 3 screws I can alter at the front. Still if the central obstruction is centred I presume this is pretty close to ideal?


Will look at CCD Inspector too Jase!

[Dennis]

Hi Matthew

I reckon Jase might have hit the nail on the head...

He mentioned Airy Disc and I'm just beginning to realise that the Airy disc may only refer to those very close/fine diffraction rings when the star is sharply focused, whereas diffraction rings occur when you grossly de-focus the star. So, being close to focus, the Airy disc is a more accurate indicator than the grossly sized diffraction rings?

Just guessing, but I'm off to do some investigation on Airy verses diffraction rings.

Cheers

Dennis

[jase]

Matt,
Reference - http://legault.club.fr/collim.html

You are only reaching step 2 with your assessment. You need the Airy disc should you want to seek the best collimation (defined in step 3).

[Dennis]

Top stuff. The penny has finally dropped! Great website!

Cheers

Dennis

[g__day]

Thanks guys - onto step 3 tomorrow then!