Thursday, July 9, 2009

Michael A. Phillips' Astronomy Lesson on Seeing, Collimation and Focusing

Why this subject?  There's not a lot of comprehensive basics to understanding the importance of these fundamental variables to Astronomy and Astrophotography.

I remember when I got started with my 8" SCT in 2005 for the Mars opposition.  I just could not see nor photograph the quality that I found on the Internet.  This was due to the main factors to be discussed in detail here:

1) Seeing
2) Collimation
3) Focusing


Seeing as I've said is king especially for any hi-res views or photographs of planets.

What is seeing?  - This site has a good explanation:

Understanding that seeing impacts your views and photographs is important.  Perhaps even more important are how to judge seeing and how to 'predict' it.

Damian Peach has a wonderful lesson on the Pickering Seeing scale which most folks will use to say the seeing was S: 6/10  (

Predicting seeing is like predicting any weather, it's not 100% accurate.  Basic info is obtained on the the Clear Sky Clocks:

Understand that this info is not 100% and augment with a look at the jet stream which will also greatly affect your seeing conditions:

Visible satellite for clouds - only works during the day

Geostationary Satellite - aka IR nighttime cloud cover threats!

University of Wisconsin-Madison: Space Science and Engineering Center
and more

Jetstream1 - BEST



I can't stress the importance of seeing enough!


As the owner of a Schmidt-Cassegrain I will talk mainly about it.  Thierry Legault has a great tutorial for the matter: 

The take away is once you've got the basics of collimation (aka USE BOB'S KNOBS!) and the out of focus collimation always make your adjustments in good seeing and using the airy disk described in Thierry's 3rd step and even use Damian Peach's charts as a reference.

If this guide appears complicated then try this rule of collimation...

it's a simple task... make it round!  :)

you can for the most part trial and error it, but the goal is the same, make it round by turning 3 knobs!  Do be careful to tighten one and loosen another to prevent over or under (aka mirrors falling) tightening of your secondary.

Also, over time you can refine your collimation, you may think you've got it right via your eyes or by metaguide, but stacking hundreds of frames in moderate to good seeing yields better results where you may go, humm, i though it was in, but it's off a  bit on the 4 o'clock position... I'll fix next time!  :)

Another good reference:

Once you've got the above two steps under your belt then focusing is easy.  If time and space allow, slew to a medium bright (mag 2) star and focus on that in each channel until you see the airy disk.  Without touching the camera with respect to your OTA move back to your target and you'll see something very exquisite!

Hope this tutorial helps everyone!


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