Which DSLR for astrophotography?

orly_andico · Post by **orly_andico** » Mon Sep 13, 2010 11:42 am

I already have a Pentax K20D, which is weatherproof yadda-yadda-yadda but has a pretty darn noisy sensor and worst of all, Dark Frame Subtraction cannot be turned off.

What is everybody else using? I don't have the moolah for a Full Frame DSLR so please, no D3/D700/5D Mk II [smilie=angel.gif]

I'm thinking of getting a K-x since they can be gotten cheap ($600 SGD with kit lens) in RED which my wife would love, has better high-ISO than the K20D. But.. it can only do 30 seconds without DFS.

What's the sky extinction limit around here? I expect a 30-second exposure would have lots of green glow. How about with an LPR filter?

My other option is to scrounge for a cheap Nikon, this would have the benefit that I can buy a manual focus Nikon 180/2.8 ED which are dirt-cheap. Of course I have my WO Z70+Flat2 which comes out to 336mm f/4.8 but f/2.8 is still >2 stops faster (and for $200 USD...!)

Of course the old Nikon lenses can actually physically mount on Pentax DSLR's (no adapter needed!) so I could also use it on a K-x.

Anybody else using entry-level DSLR's for astrophotography? (I also don't want to buy a $2K SBIG 8300, and the small-sensor astro cameras have such tiny sensors I don't feel like it).

cloud_cover · Post by **cloud_cover** » Mon Sep 13, 2010 12:57 pm

You sure you don't want a D700/5DMk2? *grin*
The green glow is bad without LPR filters (I don't have any so I can't say). On a reasonably clear night, taking Scorpio which is above 45deg (but facing south so facing the CBD light dome), even 5s shots at f/8, ISO 3200 showed very significant green glow, had to use a negative gradient to remove. If you're going after DSOs the processing gets much more difficult.
I haven't seen many 180/2.8s around for sale but the second hand lens shops price used lenses quite high. Another issue is coma and distortion: The lens was not designed as a astro lens so I'm sure it will have significant coma and distortion at the edges, something which your Z70 has already corrected for

And if you want to reduce the aberrations, you'll have to stop down your lens to er...... 5.6 or 8! So your Z70 may actually turn out to be the better of the two.

orly_andico · Post by **orly_andico** » Mon Sep 13, 2010 1:06 pm

CC,

There's a 180/2.8 ED AI-S on Adorama for $200. And a 180/2.8 ED AF-D on KEH for $349. Both USD. Both cost less than my Z70! Drat!

The 180/2.8 is one of the best lenses for astrophotography.. you should buy one! :-)

http://www.astropix.com/HTML/I_ASTROP/E ... _180MM.HTM

"The lens has little if any coma, astigmatism or distortion when used wide open. It does have a little bit of lateral chromatic aberration that is most prominent in the corners on bright stars. This can be improved by stopping down.

But if you examine this comparison closely, you will see that while the chromatic aberration is improved at f/4, it is not completely removed, especially not on a modified camera. You would have to stop down to f/5.6 or f/8 to see a vast improvement, and the reduced aperture at these f/stops is too high a price to pay for astrophotography, at least in my opinion."

If you say that 5s, f/8, ISO 3200 shows significant green glow, then LPR or HA filter is pretty much mandatory and I don't need to worry about only being at f/4.8 with my Z70...

BTW I realized why DSS was unable to properly stack my M31 shots with the Z70 in Punggai... it's because the "star detection routine" tries to find.. stars. And because of the field curvature, I was getting a lot of egg stars that DSS was getting confused. Which is why I purchased the flattener.. (have not had a chance to use it yet though)

cloud_cover · Post by **cloud_cover** » Mon Sep 13, 2010 1:31 pm

Orly; haha! I stand corrected! Looks like a very nice lens. But you NEED liveview which can zoom in real time or focusing will be extremely painful.
I think at the end of the day its what fl you're looking for: If you can live with 336mm then I think your scope setup is by far the better option (and cheaper, since you've already paid for it!) unless you really don't mind CA and some distortions, even if minimal, at wide open. I'm thinking though that if you're looking for 180 vs 336 then maybe an even wider field lens will be better sicne there are very few single objects that take up the whole FOV at 180mm: Even my shot of M31 at 400mm didn't fill up the whole frame (although I'm sure I missed the fainter arms - we'll get them once I have my mount and scope ;) )

orly_andico · Post by **orly_andico** » Mon Sep 13, 2010 2:00 pm

yeah... I need to grub together a better DSLR for astro though!

one other option I was thinking of... I have an 80mm Bronica (!) medium format lens which I butchered by epoxying a 1.25" barrel to it. Reason I needed a MF lens.. is because 35mm lenses don't have enough back-focus but MF lenses do.

I am supposed to be using that Bronica 80mm f/2.8 as my guider lens (the Toucam mounts quite nicely on the 1.25" barrel) but... I could by a SAC7b or Meade DSI for $100 - $150 and I could do deep sky!

with the teeny tiny sensor of the SAC 7b, I get a 4-degree field. Only downside is the piss-poor resolution (about 0.5 megapixel). But the FL is so short, and even with 5s exposures on the SAC 7b you can hit mag 10, that the mount becomes almost irrelevant....

andeelym · Post by **andeelym** » Mon Sep 13, 2010 4:12 pm

One of the issues of AstroPhoto Insight (which I recently subscribed to), they were comparing 500D (rebel series), 50D and 5D mark II. And they were recommending 5D Mark II due to the very low noise when high ISO is used.

Though I must say that the camera is heavy which will add into the overall weight that the mount must take.

orly_andico · Post by **orly_andico** » Mon Sep 13, 2010 4:16 pm

5D Mk II = too expensive unless you already have a full Canon system and are serious about (non-astro) photography.

Because if you're willing to blow $3K on astrophotography, might as well get the SBIG 8300M which will blow away any 5D Mk II for its intended purpose.

Pentax K-x seems to have the best noise characteristics of APS-C sensors according to DxoMark. But of course FF / FX cameras will always have the best noise characteristics.

I think the Nikon D3 / D700 has even better noise than 5D Mk II because they are only 12 MP.

cloud_cover · Post by **cloud_cover** » Mon Sep 13, 2010 5:23 pm

Hahah! Too late, andy is already a 5DMk2 user (evil!!!!!! - Just kidding - I'm a nikon user)
The noise comparing D700 vs 5DMk2 is comparable, if I remember correctly.
Andy: You have an outstanding camera. Yes, you can get better quality on a dedicated astrocam but unless you have deep pockets, you might find it not worth the additional investment. Your 5DM2 should be able to do ISO 1600 without even breaking a sweat, and that's conservative. If you're stacking (and you should be), then higher ISOs will definitely not be an issue at all. Also your cam will be outstanding at wide field with night landscapes especially if you mate it with a fast lens (say that cheap and good 50mm f1.8)
If you want more light gathering, why not either
1. Get a better mount to allow longer tracking
2. Get an autoguide system to allow longer tracking
3. Get a faster telescope system to allow shorter exposures
Apart from the mount option (which will allow you to mount more equipment or heavier scopes), the other 2 options will be cheaper than a new high end astrocam

orly_andico · Post by **orly_andico** » Mon Sep 13, 2010 5:47 pm

@Stuart, it seems from the 1:1 photos I've seen online that the 180/2.8 ED still has CA around bright stars. Would it (overall) do better than the Z70+Flat2?

I'm not really eager to sink a lot of cash, I just absolutely hated the egg stars of the Z70!! if I can get reasonably round stars across APS-C I will be happy..

My other option is to dig up a DSI/DSI-II or similar and image with my Bronica 80mm f/2.8 lens hahaha! (a used DSI would cost about as much as my Flat2, drat..)