Thursday, December 02, 2010

Nikon D7000 for Astrophotography

Folks familiar with trying to use Nikon DSLRs are probably aware of the so-called "Star Eater" effect that afflicts the cameras. This is where Nikon applies a form of median filtering against its RAW files, which has a tendency to remove finer details in astro images. Personally, I couldn't detect this in my Nikon D40 but maybe that's just me. The trick that "Nikonians" would use to get around this was the so-called "Mode 3" trick: take the image, then when the camera starts its noise reduction routine, you turn the camera off and it saves a pure RAW file.

Anyway, some analysis has been done on the Nikon D7000, and it looks great. Check this post on the DPREVIEW.COM forums which shows a comparison between the D7000's image processing algorithm and the algorithm used in other Nikon models prior to the D7000. It shows that the 'star eater" phenomenon is no longer an issue! The simulated star in the image remains intact, whereas in the previous algorithm sample image it does not. This bodes very well for those folks interested in using this camera for astrophotography. I am already very impressed with the quality of the astronomical images I have taken using this camera, and now have a little extra confidence that I am not losing any real detail.

To see what I have been able to achieve using the D7000, look here.

16 comments:

starryalley said...

Hello there. I'm also an casual astrophotographer. Thanks for those great sharing about D7000. I purchased it yesterday and mine is showing exactly a problem like this: http://forums.dpreview.com/forums/readflat.asp?forum=1034&message=36838176&changemode=1
Can you confirm to me that yours don't have this problem, or is it normal to D7000? I normally turned of long exposure NR when shooting the stars. This could be a problem? Thanks a lot!

Phil said...

Hi Starryalley.

You will see hot pixels in long exposures. This is fairly typical of all DSLR cameras. To get around this, you need to take "dark" frames (photos of same ISO and time length but with lens cap on) to get a dark image which also points out these hot pixels as well as any other artifacts resulting from long exposures (like amp glow). Then, your astro imaging software (like Deep Sky Stacker) subtracts these artifacts from the final astro image during processing.

Cheers,
Phil

starryalley said...

thanks for your quick response! I'm really grateful! So are you saying that your D7000 without dark frame subtraction or NR, has got photo exactly like this thread?(so many white hot pixels) I see in your post that D7000 seems to have something different from my previous D90 or D70. I am shocked because mine looks like this thread and in D90 it doesn't have so many. Your response is crucial for me to exchange it or not. You are probably the only one I know who shoots mostly astro with a D7000.

Phil said...

No worries.

Here is a sample dark frame fro my D7000 taken for 5 mins at ISO1600.

http://theguvnah.blogspot.com/2010/11/d7000-iso-1600-5-minute-dark-frame.html

You can see a lot of stuff there. Astro image processing programs like Deep Sky Stacker can subtract these artifacts from the final image if you take decent dark frames like this. You basically need to do this with all DSLR cameras.

Admittedly, the D7000 has very good noise reduction at lower ISOs, and I have been shooting at ISO400, 500, 800 and 1000 with very good results.

I reread the forum board you posted. I guess I am in the camp that you will have hot pixels appear in any DSLR camera at long exposures. If you are seeing this in short exposures, I would then be a bit worried.

For astro, it is ALWAYS recommended to take dark frames for subtraction during this process.

Again, if you're seeing hot pixels in short exposures, then I suspect an issue. I would expect to see hot pixels in long exposures.

Good luck!

starryalley said...

Thanks Phil. Your comments are really useful for me! Mine only shows those hot pixels at > 20s exposures. That seems to be normal. I will also check another D7000 again in the shop and see if the amount of hot pixels look the same. Then I should be fine with my D7000 for astrophotography. Thanks again! :)

Anonymous said...

How about Hydrogen-aplha sensitivity? Do you think, that the d7k is better, than the older cameras?
Cheers,
Tom

Phil said...

Hi Anon - apologies for the late delay on this. I only just noticed it. I do not have much knowledge about HA stuff - sorry! The D7000 is still fairly new to market so keep scanning Google (Blogs area) to see if anyone else takes this on...

markogts said...

Hi Phil! Very interesting. I wonder: do you use the D7000 in "mode 3" or in "mode 1"? Is the result consistent? Because anyway there still is some non-linear filter being applied, albeit milder. When I did a dark frame subtraction on my D80 in "mode 1" I got donut-shaped, hollow stars. The possibility to take astro-pictures in "mode 1" would be a great improvement. Thank you!

Phil said...

Hi Markogts! Well, I have been shooting in "Mode 1" or regular mode. Someone on another group (Yahoo! NikonDSLR Astro group) asked if I could do a side-by-side comparison of regular shooting and mode 3 shooting. I have not done that yet. Based on the report featured in the blog post here, it seems that the new Nikon image processing is much better than on prior Nikons and therefore gets rid of the whole "star eater" issue. I'll run a test and see the difference between mode 1 and mode 3 early next month...

Cheers!
Phil

SEONGPOR said...

Sorry, I am totally new to this. By the way what is Mode 1 and 3 means? What about mode 2?

Phil said...

SEONGPOR - Sorry for the delayed response.

Mode 3 = Is a hack, basically. You take a RAW file, then when your exposure is finished, you turn off the camera physically before/while the camera runs through its noise reduction routine. This saves a pure raw file on your SD card that doesn't have Nikon's "star eater" algorithm applied.

Mode 1 = Normal mode with Noise Reduction on and applied

Mode 2 = Umm...no idea :-)

As indicated here, it looks like with the Nikon D7000 they have a much better noise handling capability that doesn't take away fine detail from astro images.

Cheers,
Phil

Mike Farid said...

nice review, i always respected dealing with nikon's RAW files, but i'm a canon guy, and maybe due to my love to the D series i always find that it is the best to get those fainter details,
please check my article about low budget astrophotography with much cheaper devices:
http://astromic.blogspot.com/2010/05/low-budget-astrophotography.html

damian said...

Phil,

I've been a B&W photographer for years (film, not digital) but I also own an old Nikon D70 with a few lenses. I've been an amateur astronomer for years and I'm not getting serious about combining the two. I also want to upgrade from the D70.

Having read Jerry Lodriguss's recommendations (http://www.astropix.com/), I'm torn between getting the D7000 or a Canon. Besides the RAW filtering issue, which as you point out Nikon has improved, it seems that Canons are much easier to control from a computer - setting things like multiple long exposures to go off rather than having to do it manually.

How hard is it, in your experience, to control the D7000 from your laptop?

Thanks,

Damian

Phil said...

Hi Damian,

Admittedly, as interested as I am in astrophotography, this particular hobby is the secondary reason why I bought my D7000. I bought it for other uses, trying to get into the semi-pro arena.

Having said that, it was a huge step up from my D40 for astrophotography. Better noise handling, live view and the 16.7 megapixels makes the camera a decent astrophotography rig.

I don't have a lot of experience with Canons. I don't like getting involved in the Canon vs. Nikon battle that pervades the internet. I just went with Nikon and have had fun with them. maybe one day I will switch but now I have a decent Nikon lens investment.

I have not spent much time trying to control my Nikon from my PC. I am happy to simply use the IR remote and shoot manually. Nikons don't seem to want to let you shoot greater than 30 second exposures when using PC control software. In Bulb, you can't specific actual timed exposures of say, 1 minute or 10 minutes. I DO find that annoying and can't fathom why the D7000 which is basically a PC with a lens mount, simply can't perform times exposures greater than 30 seconds. Annoying.

I know you can buy a special cable that will allow you to do it, can't remember where from. It effectively lets you set a specific number of minutes for an exposure. Again, no experience with this widget.

I am happy enough shooting manually with my IR remote. It gives me something to do while shooting! :-) Still, I am sure that the fact that you can do this with Canons gives them a slight edge in the astrophotography sphere.

Cheers.,
Phil

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Simeon Pilgrim said...

Hi Phil,

Just letting you know the code to turn this off has been found on the D7000, and a public beta patch will be relesed this weekend.

If you are still using Nikon camera's what model's are you using to help target the patching process.

Simeon