Hi gang,
It has been quite awhile since I have posted. I have been trying to work out the kinks with a new OTA that I purchased a couple of months ago. It is a Planewave CDK 12.5 and uses a corrected Dall-Kirkham design. So far, I am pretty impressed with the flatness of the field and the views I am getting. Here is a first light image of M13. I didn't have alot of time before onset of clouds, so only 35 minutes lum and 20 RGB each. Here is the link...

http://www.pbase.com:80/kvwood/image/97919335

Thanks to Rick J for giving me some great tips on working out some problems I was having with my guiding. Tried a guide scope, but Differential Flexure is a problem. Ended up guiding with internal chip of the STL 11000.

Take care,
Kent

Looking fine to me. Is that the full frame?

Rick

Beautiful. I am trying to work out an optimum sub duration for GCs at different magnitudes. How long were yours?

__________________
Kind regards
Matt
LXD75 SN 8" f/4, DSI II pro

The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, "hmm.... that's funny...."
Issac Asimov

I don't see much agreement between magnitude and the exposure time I needed to resolve the core. Density of the cluster and the distribution of star magnitudes in the core have more to say about this than overall magnitude.

I run test shots to determine the exposure of a dense globular to where the brightest star's ADU count is about where the non linearity of my ABG chip starts to be noticable. In my case about 25000. Depending on the globular that can be 1 minute to 5. In the case of a sparse globular like NGC 5053 I went 10 as the stars were so well separated in the core and so greatly different in magnitude that a bit of compression didn't hurt a thing.

So I start at 1 minute and see what the core looks like and what the ADU count is. From that, thanks to the linearity of chips I can calculate a good guess for the maximum exposure. If it isn't dense I will up it some from there even. Takes me only a couple minutes. Gives me a check on focus at the same time so really adds very little time to the process. This was a nightmare in film days as film is very non linear from the start and you don't know what you captured until you developed it. After 40 years of that often wrong guessing I'm in heaven with CCD imaging. With film you had no choice but burn out the core to capture outer faint stars. So you had to take a series of exposures then in the dark room, using masks you'd try to combine them. All night smelling the chemicals and a bad rash on fingers might result in one usable print a fraction the quality of what I can do with CCD.

With a DSLR the counts would be less since they top out at 4096 or 16384 in a few cases from what I've been told. Usually non linearity cuts in somewhere around 50% of saturation. Saturation isn't necessarily at the max count and can be less depending on gain settings.

Rick

Kent,

Great shot and great color! It's good to hear that the Planewave guys have a good product. It is an innovative design. What kind of a mount do you have it on?

--Andy

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Observatorio de la Ballona

A Beautiful image it is!, My first image seen in the field of a Planewave scope and looks impressive. Welldone and Clear Skies.

Rick, this is not a full frame, but close to it. I cropped it to help composition a bit. The CDK design really does offer coma free stars clear out to the edges.

Matt, Rick has some great suggestions above regarding subframe duration. I am afraid I am not as methodical in my approach. My subs for M13 were 5 minutes. I stretched the first sub to make sure that the core was not burned out. It looked pretty good, so I proceeded. I was afraid 10 minutes might have been to much, although 10 is usually what I use for deeper stuff, ie less luminous galaxies and nebulae. For Orions Nebula, I used 1 minute subs for the core, and 10 for the nebulosity and then layered the 2 togeher. As long as you haven't saturated an area, you've got something you can work with. I am not sure if this helps any, but again I think Rick offers a more repeatable and dependable method.

Andy, I am using a Paramount ME. I guess I should be more forthcoming with imaging information when I post. Sorry about that. Here is the info for this image:

Mount: Paramount ME
Scope: Planewave CDK 12.5
Camera: STL 11000
LRGB L: 7x300, R: 4x300, G: 4x300, B: 4x300
FWHM for most of imaging session was 2.4-2.7
Processed with CCDStack and Photoshop

Thanks everyone for your comments and input.
Kent

Thank you both Rick and Kent. My subs are still limited to around 2 min without guiding so I will keep your wonderful advice for future reference.

__________________
Kind regards
Matt
LXD75 SN 8" f/4, DSI II pro

The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, "hmm.... that's funny...."
Issac Asimov

Are you using a motor drive on the CDK focuser, or manually adjusting focus? If using a motor, is an encoder included? Is it one of their options or another manufacturer?

Robert

Hi Robert,
I am using a TCP-3 focuser from Optec. I am very pleased with it. Planewave provides a very sturdy manual focuser, as well as a motorized option that I understand works very well. At the time I purchased the scope, their focuser wasn't compatable with Focusmax, but I believe they have developed the ASCOM drivers for it now.

Kent