[Aavso-photometry] NEw or used CCD's

[David Higgins]

I do lots of photometry and 3 of my 4 CCD's are old second hand models
(ST-8E, 9E are both parallel).

Your choices are for CCD's with very small chips (these days they are being
used as guiders rather than main imagers).  The problem with a small chip is
getting an image resolution that will also allow the image to contain
comparison stars!

The SX cameras have built in ABG's so if you do get one take care to
measures it's linearity.  My MX516 (I don't remember there being an MX512
though Terry may have re-badged them as 516's when he started sending them
out with the parallel to USB 1 interface) was linear over 95% of its range
and although it was parallel, you could purchase a USB1 (and now an SXV USB2
interface) for it.  Another issue with this camera is that the pixels were
rectangular and not square.

I'd recommend at least an ST-7 sized chip (SBIG ST-7E, ME402, SX MX716) and
even then you are likely to need to use a focal reducer to get a large
enough field to get Comparison Stars.  Remember not to reduce image
resolution below ~2"/px (that is, not 3", 4"/px etc) otherwise you end up
under sampling and reducing the quality of your photometry.

Not sure if any tests have been done with DSLR images but in theory, if you
can get RAW images and convert them to FITS format without any loss then
that should work (note you can only convert one channel so you need to take
mono images I believe).  Again - you need to take care of image resolution
as commercial DSLR chips generally have very small pixels.  Then there are
limitations on how faint you can image as they have no cooling so faint
stuff gets lost in the noise and I couldn't tell you how linear their
response is.

Cheers

David


Date: Thu, 29 Nov 2007 12:35:27 -0800
From: cspratt <cspratt at islandnet.com>
Subject: [Aavso-photometry] Used CCDs
To: aavso-photometry at mira.aavso.org
Message-ID: <474F228F.4090709 at islandnet.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Thanks for the replies on my previous question about new or used CCDs 
for photometry. I was looking at a used  SBIG 237A CCD which is light in 
weight but doesn't have a guiding chip (do I need one?) . My old 
computer has a parallel port so that isn't a problem. I have an AP600E 
GOTO mount which can handle up to a 9.25" SCT. But this is a portable 
setup and takes time to calibrate the guiding chip, thus I wouldn't use 
the guiding chip in any case. The are aslo some older Starlight Xpress 
CCDs on the used market at a reasonable price. I'm looking at the HX516 
or MX512 as an alternative to the SBIG 237A. Both of these use a 
parallel port interface and are small and lightweight. Again, I don't 
know if either of these are suitable for photometry.

I could also mount a Canon DSLR on the scope and use a telephoto lens or 
small refractor instead of a regular CCD through the main scope, Is this 
the way to go?

I also have a Schuler green 1.25" filter marked "1.25"V bvri VISIBLE". 
Is this the correct V one for photometry?

Thanks again,

Chris. Spratt
Victoria, BC 

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