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International Year of Astronomy | American Association of Variable Star Observers (AAVSO)
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International Year of Astronomy | American Association of Variable Star Observers (AAVSO)
Hi all
Since we're talking about specific models, I've been meaning to ask here for awhile about the suitabilty of using the Pentax K100D. I have one of these and would like to use it for DSLR photometry, but my impression from using it for simple tripod-mounted wide field astrophotography (on bulb setting) is that it produces a fairly noisy result.
I have heard and read negative comments from others.
If the K100D is unsuitable for this work, I'll keep it for happy snaps and buy a Canon or other model.
Input welcome.
Thanks in advance.
Regards,
David Benn
Hi David,
I have no experience of that one but I have a Nikon D70s that is of the same generation, the last using CCD, 6 years ago. I think there should be only small difference between cameras of the same generation of various brands. The issue is more a question of evolution of the technology during these 5 or 6 years.
It's clear my more recent 450D, CMOS, 14 bits is much better than this D70s, but the D70s remains usable for bright enough stars with defocus and a classical full CCD processing.
The D70s had much more fixed pattern noise or luminescence (dark currents) than the 450D (nothing up to 30 sec. ! ) But it cloud be fixed using the classical dark process. The D70s has also some more read noise.
Another difference is the linearity, new CMOS are perfect up to the saturation level. This could be avoided using only the first half of the response curve of the CCD or making a correction by software.
One interesting point is that older cameras have less and larger pixels that's probably better for us. I am a little bit afraid of the so many pixels of the most recent cameras. I have no conclusion about their noise but this a lot of small pixels to process and sometimes difficult for even not so old computers ! The present 1100D seems identical to the 450D and has "only" 12 Mpix that's manageable. I do not interpolate but use the Bayer's cell data as a single RGB pixel. Then the image is limited to 2145 x 1428 without risk of raw data corruption.
I think you could just test your camera using the procedures from C.Buil ( http://astrosurf.com/buil/ ) and compare its FPN and read noise to what he find from various cameras. It's easy, you just need two short exposure darks for the read moise and a set of regular darks for the FPN. There is all needed to check it in IRIS.
Clear Skies !
Roger
Hi Roger
Thanks for the useful advice. I will follow up on this.
Regards,
David
Hi Roger
Thanks for the useful advice. I will follow up on this.
Regards,
David
Hi Doc,
Generally speaking all of the Canon cameras are good for this purpose - the newer models add video and other features. There are a number of good sites that discuss their merits as astrophotography platforms. I would recommend Michael Covington's.
If you are using image capture the biggest difference will be the processor capabilities. A 12 volt adaptor (aftermarket) and good lenses are important as well. The newer cameras that have a poseable screen are particularly helpful for focusing when the camera is pointed at an awkward angle.
Best,
Denis Grey
Toronto
My current camera is a Hutech modified Canon EOS Rebel XT, which I have been able to use with some success.
It does have the ability to save images in Canon RAW format, and I've also been using a cable release. I also have the original manuals, but since this is an older model that the T3 and T2i, I don't know if that helps.
-Mike Durkin
Mike:
You may be able to answer this question then! I managed to find and download the manual to the Canon EOS Rebel T3. It seemed to indicate that the shutter speed could be set from 30s to 1/4000s, which sort of made me mad. However it also indicated a "bulb" option.
Now the only other experience I have with astrophotography of any sort was about 10 years ago with an Olympus OM-1. The OM-1 had a manual mode where the shutter would stay open as long as you pressed the shutter button. Now obviously that would cause MASSIVE camera shake so a bulb was used. With the OM-1 holding the shutter button down and holding the bulb button down did the same thing. Here if you press the shutter button down it just releases the shutter for a programmed amount of time (apparently a max of 30s). Does the bulb hold it open as long as you press it?
----
Doc Kinne
Hi Doc,
No problem, all DSLR should have a classical "bulb" setting, Canon sure ! You just have to use a cord remote that sells for few $ . Those devices have a button lock that enables you to expose as long as you don't unlock it. No risk of camera shacke at all ! You could also set the retarder when the floor is not stable enough leaving you time to get away (my case on my terrace ! )
Another way (for Canon, I don't know others) is to use the USB connection to a PC running the free processing software IRIS from C.Buil. It includes a Canon camera control that allows you to control series of exposures and other parameters. This function works through the USB link and up to 30 sec. that's enough for most photometry cases. You can get further using the remote connection instead (to see IRIS doc)
All recent Canon CMOS DSLR are superbe for what we are doing, far better than a couple of years ago. CMOS are linear up to the saturation point, the dark currents are very low, negligible for photometry up to 30 sec...
Clear Skies !
Roger
Roger:
Very good. Very much like what I wanted to hear. Also great news on the CMOS linearity. Since this is the newest Canon entry-level DSLR I have to assum what you said applies.
THANKS!
---
Doc
Hey Doc,
Yes the Canon XT does have a bulb setting for the exposure. When I set the mode dial to "M" for manual, and then adust the exposure time, after 30 seconds it shows "bulb".
As I remember, there is a 30 second limit on the exposure time if you try to control it through the USB interface. I'm almost certain that if you use the actual cable release port on the camera, it behaves as expected, the shutter will stay open as long as you hold the button down, whether it's the button the the camera or a cable release. I have used a programmable cable release plugged into the proper port to take light frames as long as 5 minutes.
I double check when I get home tonight.
-Mike Durkin
Mike:
There's a difference between the USB interface and the "programmable cable release?" I am expecting (and am now almost convinced) that the manual bulb will hold the shutter open for as long as you hold it down, as I expect. Between that and now confirming that it outputs in RAW format, I may be convinced that this is my camera.
THANKS!
----
Doc
Doc,
Yes there's a difference, at least with the Rebel XT and the models that were made at that time. The cable release port looks like it would accept a 1/8" audio plug (it's actually 3/32"), and the USB port is a standard mini-USB.
The programmable cable release had to be modified because the plug wasn't compatable with the cable release port on the body. The company that sold me the camera (Hutech) also sells the modified programmable cable release. The manual cable release (RS-60 E3) wouldn't need to be modified for my camera.
I think every DSLR that Canon makes saves as JPG, RAW, or both. According the Canon's website, that model will save as RAW:
http://usa.canon.com/cusa/consumer/products/cameras/slr_cameras/eos_rebe...
-Mike Durkin