Re: [APML] ABG vs NABG

From: Tony Hallas

...I am almost through Ron Wodowski's nice book on CCD imaging, and he spends some time discussing ABG vs NABG cameras... why this ties into the APML is because Ron is of the opinion that for shooting really deep, clean, artistic (there's that word again) images with a CCD camera, the ABG camera is the way to go. This assumes that one has a good mount and dark skies... allowing long exposures with great and faint detail in them. He states that one can emulate the ABG camera with a NABG camera by making numerous shorter exposures, but nothing really compares to taking several really long exposures with an ABG camera under dark skies and combining those images.

I have not read Ron's book. Does he quantify his arguments, or just resort to hand waving? ;-) (I'd love to see quantified arguments, if they are presented.)

Up until now I assumed that ABG cameras were to be avoided... but he makes it seem that given the right circumstances, they are more fun to work with and can produce ultimately deeper images, not to mention being able to image in the proximity of bright stars.

I'll buy the 'bright star and blooming problem' that ABG cameras help reduce.

However, with a well working, 16 bit CCD...I don't think you need ABG very often to avoid blooming stars. You can shoot really short exposures of the trapezium to avoid blooming, and longer exposures for fainter/outlying portions...then composite the images in something like Photoshop.

My main interest is having fun and creating hopefully artistic images, not doing science...

Unless you are shooting in really dark skies, with slow f/ratios...unfiltered CCD images of 1-4 minutes duration are 'sky limited' and can be short enough to help you avoid blooming problems in many cases. (It's those narrow band filters that need longer exposures...photon starved H Alpha shots.)

Also, depending on how irregular your seeing is...you may be surprised at how variable seeing is from minute to minute if you take a series of (60-120 second) CCD images. If seeing is variable, that's one reason for stacking shorter exposures...find and throw away the bad seeing images to make a higher res final stack.

You may want to contact Bill McLaughlin about this ABG/NABG debate...I think he has lots of experience and some good recommendations. (Rats, I don't have his email/website handy.)

NABG cameras are a bit more sensitive (roughly 30%?), but you avoid most blooming problems with ABG cameras.

Me...I'm a linear/NABG amateur science kind of guy, but my cheesy, 12 bit, NABG Cookbook camera does a decent job if I work within its limitations. You would be less hindered with a good 16 bit NABG CCD, so getting a NABG CCD won't be any death knell to your imaging efforts. (See http://overton.tamu.edu/aset/krajci/ for some pretty picture examples with my 'limited' NABG camera...decent results are possible.)

Either way you go, in my opinion, is not a terrible fate...just a slightly different set of relative strengths and weaknesses.

One last thought...if you shoot bright objects with an ABG CCD...and then try your hand at deconvolution to squeeze a bit more resolution...the non-linear behavior of an ABG CCD may make for more problems with deconvolution, such as 'black donuts' around stars, etc. (Most deconvolution routines assume a linear behavior of the detector.)

Tom Krajci