From: Milton Aupperle <milton@outcastsoft.com>
Date: April 12, 2007 10:55:34 AM MDT
To: Astro_IIDC@yahoogroups.com
Subject: Re: [Astro_IIDC] Starting Out and Would Appreciate Camera Advice
Hi Doug;
On 12-Apr-07, at 8:27 AM, Doug wrote:
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In any event, my sole astro-imaging interest at the moment is H-alpha (and CaK to lesser
extent). The camera I purchase will be dedicated to this task so I'm looking for the "best"
camera for the intended application, not necessarily the most versatile. I will be using
primarily a Solarscope SF70 (70 mm) F-P etalon attached to a TV 76 refractor. I would also
prefer to stick with a non-tracking alt-az mount for the time being unless someone tells
me this is definitely not the way to go (I'm assuming image shift over a few second capture
period can be handled by registration/stacking software).
I would recommend that you pick up an inexpensive motorized EQ2/EQ3 mount instead of going with a non motorized Alt/AZ mount. No motor means it will drift quite a bit and make imaging frustrating.
With a 5.6 micron square size pixel (which is what the 640x480 DMK camera deliver) and 480 mm focal length , the slowest frame rate you can run at without blurring the image due to earth's rotation is 6.54 frames per second. Any slower and the image will be blurred by motion. So you need to capture at 15 fps to prevent his. With a 640x480 image size, a star (or prominence or sunspot etc.) will drift from left to right across the entire field of view in 97.9 seconds, which isn't much time especially when your trying to capture images for making mosaic with (after 45 seconds, half the images has shifted out of your field of view), let alone achieving focus.
Finally, I am very interested in
being able to image the entire disc with little (or possibly) no stitching. BTW, the SF70
gives remarkable on-band performance across the entire disc without any detectable
"sweet spot".
To get the the full solar disk on a CCD with say 5% edge space on each side using a 480 mm focal length (assuming a 5.6 micron pixel size), you need a CCD with a smallest dimension of 915 pixels. So a 1280x1024 size would work for single frame and a 1024x768 would work with 2 frame mosaic (a fair bit of vertical overlap in the second case). One nice thing about cameras that supports Format 7 (like Optronics and PGR) is you can specify what portion of the CCD is to be imaged, so if you want to image and capture a 920 x 920 area, then set it to that. That will save space when recording to disk and processing images. But if your on an unguided mount, you can't use such tight tolerances and you'll be hard pressed to fit the image in the CCD and keep it there for any length of time.
I'm thinking larger format chips (1/2-3/4") would suit my needs best and with a
telecompressor would give me a big chunk of the disc through the TV76 (480 mm w/o
reduction). However, I don't know how to gauge whether the frame rates are good
enough. 15 fps seems pretty fast to me for an application where I'm not really light limited
(i.e., solar) but maybe I'm wrong about this? I know capturing even 15 fps full-res with the
largest webcam chips would tax the cpu but don't have a feel for how a current Macbook
(pro?) running ASTRO IIDC would handle the data stream (via firewire).
It should not be a significant factor. Monochrome camera images require very little processing power, so the biggest limitation is how fast your hard drive can write data to disk. All the MacBooks have SATA II Drives which are good to around 40 megabytes per second, and the most you can "in spec" push across the FireWire bus at FireWire 400 speeds is 30.5 megabytes per second.
As for cameras, I've got the following short list - monochrome of course: DMK41 series,
PGR Flea2 (wondering about Optronics but haven't priced yet) or waiting for one of Paolo
Lazzorotti's cameras, which seem promising at the specs currently listed.
We have never tested the Lazzorotti's cameras. And I have not seen any info on what IIDC features they support. The key feature they have to support is the absolute shutter register and if they don't support that, then Astro IIDC won't recognize the camera is even attached.
I take it
Lumenera involves too many headaches for use with a Mac?? Should I not be so wedded to
the idea of larger format at the expense of fps? I'm not so concerned about resolution but
my feeling is that if the cpu can handle the data then why not higher-res?
Lumenera Cameras have USB2 custom interfaces, using completely different protocols and methods of delivering video than IIDC Cameras do. You have to know all the register settings and hardware information to control the Lumenera cameras and then re-write everything to support just their cameras.
I wish I could grab one of the >$15k cooled large chip CCDs on my microscopes outside
for a try but alas, I cannot...
I have found that there is no appreciable difference in image quality when shooting at high frame rates (> 7.5 fps) between cooled and uncooled cameras ( in my case a drop of 22°C due to cooling). For long exposure cooling helps a lot, but not for high frame rate in my own testing.
HTH..
Milton J. Aupperle
President
ASC - Aupperle Services and Contracting
Mac Software (Drivers, Components and Application) Specialist
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