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PostPosted: Fri Feb 04, 2011 12:48 am 
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Hi folks,

A number of companies will modify your DSLR by replacing the filter in front of the sensor with something a bit more friendly to some of the key wavelengths found in astronomical objects. Not a bad idea as DSLR sensors are much cheaper. However check out these two images to be found on George Carey's site here:
    Image
From what I can tell the image to the left shows 18 exposures of 4 minutes with a modified Canon 350D, darks and flats with CLS filter. To the right is 8 exposures of 4 minutes with an SXVF-M25C, no darks, no flats and no CLS filter. For the uninitiated "darks" and "flats" are used to try and remove sensor noise and vignetting and a CLS filter blocks the light of the spectral lines of mercury and sodium-vapour commonly emitted by street lamps. So it seems a lot of effort went into getting the best out of the modified Canon 350D and while I'm sure the image from the Starlight Xpress SXVF-M25C was carefully processed I find it pretty startling how much better its image is.

I suppose the moral of the story is there's no point in spending big bucks on a telescope and then, if you are serious about astrophotography, being content to attach a DSLR to it. :( Mind you one would hope that the SXVF-M25C would do better as it costs about five times as much as a modified 1000D/XS. :shock:

Bob.

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PostPosted: Fri Feb 04, 2011 6:32 am 
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This is quite strange, with such ''long'' exposures and with a modified camera there's much more information in the picture. It needs to be processed a bit better. But maybe he hadnt processed the other shot too...

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PostPosted: Fri Feb 04, 2011 7:22 am 
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One thing springs to mind on that unit...

Quote:
Cooling system: Regulated constant current cooling supply with single stage thermoelectric cooler to give a CCD temperature of approximately -30C below ambient.


I have to guess that helps with the noise a bit! I don't think sticking your DSLR in the freezer before you start will work quite the same...

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PostPosted: Fri Feb 04, 2011 8:25 am 
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That certainly tells an interesting story but I'm not sure its the complete picture (pardon the pun :lol: )

I have seen some great shots done with DSLRs, both modded and unmodded (Check out this site: http://www.astropix.com/HTML/I_ASTROP/NIK_CAN.HTM)

I do agree though, dedicated CCDs will make the job a hell of a lot easier especially with the cooling they possess.

My current goal is to get my guiding sorted and push my DSLR to the limit to see what I can acheive before I start to look at a dedicated CCD (I just know when I start looking Im gonna want to aim high, very high!!!)

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PostPosted: Fri Feb 04, 2011 10:04 am 
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On reflection I think the processing of the 350D sample above might have been improved as the author goes on to offer a better version here (compare with the SXVF version here taken over a year later if the file name for the image is any clue).

One thing I did discover a few years back with a 40D is that if one takes a long sequence of subs without giving the camera a chance to cool off between each shot then the noise level from the sensor increases markedly. From this post it is self-evident that a 30 second rest between 2 minute exposures is way too optimistic: :oops:
    Image Image Image Image
    Image Image Image Image
    Image Image Image Image
    Image Image Image Image
    Image Image Image Image
    Image Image
Bob.

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PostPosted: Fri Feb 04, 2011 6:06 pm 
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I must admit I have read about this but not yet experienced it for myself. I have enough trouble with amp glow from my Nikon let alone having to worry about this as well, lol.

I have debated about getting my DSLR modified but I think if I was going to do that I would possible get a Canon to do it too. But if I was going to do that I would probably end up wanting to get a dedicated CCD.

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PostPosted: Fri Feb 04, 2011 7:30 pm 
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Im very sure dedicated CCD cameras perform much better, but it is sure possible to get good shots with DSLRs, please have a look at this (Dutch) astrosite: http://www.astroforum.nl/forums/314-Astrofoto-s-deepsky
Many professional and amaturephotographers post there their pics, which are sometimes really good!

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PostPosted: Sat Feb 05, 2011 12:38 pm 
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Hi Ruben,

I agree, DSLRs can produce great results, particularly if modified to let more of that valuable Hα through. But there's also no getting away from the fact that if one wants to use, say, a narrowband Hα filter then only one quarter of a DSLR's pixels, or those of a one shot astronomical CCD for that matter, can be active. It's this that's finally pushing me towards a dedicated monochrome CCD because that will allow me to do narrowband imaging on nights when the Moon would otherwise put a stop to things. We seem to get precious few clear nights as it is and when they coincide with a Moon adding to the already ever-present sky glow from the local street lamps it can get pretty frustrating, and that's without considering the demands of a social life. :wink:

But there's still lots of planning and prep work to do before purchase!

Bob.

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PostPosted: Sun Feb 06, 2011 9:09 pm 
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Interesting thread Bob. How long would you recomnmend between DSLR exposures for it to cool down sufficiently?


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PostPosted: Sun Feb 06, 2011 9:26 pm 
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Hi Gordon,

I'm guessing now but when I took the 5D2 out for a test drive a few eeks ago (goodness, has it been that long already! :oops:) I set the EOS Utility intervalometer to give the same cool down time between shots as I spent exposing each shot. I think the numbers would likely change depending on the camera model and the air temperature but maybe a 50% duty cycle is a good starting point and then adjust as needed.

Bob.

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OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
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PostPosted: Wed Feb 09, 2011 6:14 pm 
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I though I read somewhere you were supposed to leave at least as long as the exposure length between shots, but if were looking at 5-10 subs then this can lengthen the session quite a lot!

I think the big advantage of dedicated CCD's as Bob has alluded to is the fact a mono CCD allows you to take advantage of narrow band and emission lights from certain nebulae thus being able to capture further detail not visible to the naked eye, in my opinion, making them better.

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PostPosted: Thu Feb 10, 2011 6:13 pm 
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Funnily enough, I have just read a thread on the subject of letting a sensor cool down between subs on another forum.

The general consensus is that about 15-30s to 60s tops is doable for Canons, whereas Nikons need about half the sub length to reduce the amount of ampglow that they suffer with.

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PostPosted: Thu Feb 10, 2011 9:23 pm 
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I’ve read somewhere that the cooling time must be increased gradually with a DSLR. Not only the sensor produces heat and it accumulates with use.

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PostPosted: Thu Feb 10, 2011 9:36 pm 
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Yeah, that kind of makes sense, although I think there will be a plateau where the heat / noise generated levels out and the cooling time levels out.

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PostPosted: Tue Mar 29, 2011 7:42 pm 
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Here's a nice video showing what can be done with a modified DSLR and a few narrowband filters:
Bob.

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Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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