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Old 21-06-2013, 03:24 PM
DIYman (Doug)
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Matching pixel size with focal length

Hello all
I feel a bit foolish about asking this but despite reading a lot of info on the net about the importance of matching pixel size with FL I cannot say that I have a 'cast iron' understanding.

My scope has a FL of 2286mm and one CCD supplier suggested I need an imager that has 5.4um x 5.4um pixel size. However, when I ran some formulas it looks like I will need about 10um.

As way of background, I am primarily interested in deep sky imaging. The place where I do my photography has average to good seeing most of the year and light pollution is not that bad as I am in transition zone between suburban and rural. Lastly, I am leaning towards a monochrome CCD as I would like the capacity do narrow band imaging.

Since these things are not cheap I would appreciate some impartial advice as to what size pixels would suit deep sky imaging with my scope. (I have read enough to know that size does matter.)

Thanks
Doug
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Old 21-06-2013, 04:02 PM
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Shiraz (Ray)
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depends on your seeing Doug, which will generally be what determines the best resolution you can get. I find that most of the time, good seeing is about 2 arc sec FWHM. If this is the same where you are, you will not need much better than 1 arcsec per pixel to get the best combination of resolution and sensitivity. At 2286mm fl, as you say, you would be best off with one of the 9 micron chips - anything with smaller pixels and you lose sensitivity for no gain in resolution - a chip with 5.4 micron pixels would have about 1/3 the sensitivity, all else being equal. Binning would restore sensitivity, but introduces some other issues such as reduced field of view and transfer cell saturation. Probably worth going for a 9 micron chip with good quantum efficiency though - available chips vary a lot in this parameter. And of course there is the issue of cost, which a whole other minefield. Regards Ray
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Old 21-06-2013, 04:39 PM
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Merlin66 (Ken)
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With that focal length and 2 to 3 arc sec seeing you'll get good sampling with a 9-10 micron pixel camera (Nyquist sampling >2)
star size 22 to 33 micron FWHM at focus
Or 2 x 2 binning of a small pixel size.
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Old 21-06-2013, 04:49 PM
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Hi Ray,

How do you calculate sensitivity to be 1/3rd?

Doug, sorry don't mean to hijack your thread, I've been thinking of a similar issue of matching pixel size with my 10inch F4. I currently use a QHY8 OSC with 7.8micr pixels, what would be best pixel size if I went with an OSC and which one's best for a mono.
again, seeing is not great.

what other factors do we need to consider with sensors, the high end ones have a well depth of 100k, others 20k, others 25k. how does this fit in with your pixel scale recommendation. does it influence it at all?
other factor would be read noise and i'm guessing sampling rate.

would the recommended 9um for Doug's setup change based on these factors from one chip to another?

Cheers
Alistair
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Old 21-06-2013, 05:09 PM
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Merlin66 (Ken)
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Alistair,
The issue with OSC is the Bayer matrix filter...obviously the pixel size is important for good sampling, but depending on the filters and objects being observed, the matrix does reduce the "resolution" somewhere around 60%.
This is where mono CCD's win....
The well depth (and the readout - 16 v's 32 bit) are also issues....
but the starting point is good sampling (Nyquist >2), low read/dark noise and maximum QE - where you need it....
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Old 21-06-2013, 05:22 PM
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Shiraz (Ray)
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Quote:
Originally Posted by alistairsam View Post
Hi Ray,

How do you calculate sensitivity to be 1/3rd?

Doug, sorry don't mean to hijack your thread, I've been thinking of a similar issue of matching pixel size with my 10inch F4. I currently use a QHY8 OSC with 7.8micr pixels, what would be best pixel size if I went with an OSC and which one's best for a mono.
again, seeing is not great.

what other factors do we need to consider with sensors, the high end ones have a well depth of 100k, others 20k, others 25k. how does this fit in with your pixel scale recommendation. does it influence it at all?
other factor would be read noise and i'm guessing sampling rate.

would the recommended 9um for Doug's setup change based on these factors from one chip to another?

Cheers
Alistair
with your permission Doug, re Alistair's questions, as I see it:

1. the sensitivity scales with the pixel area for a given scope - more area means more photons collected in each pixel - but at the expense of less resolution of course.
2. the only "crime" in choosing pixel size is to use ones that oversample, ie they are smaller than those required to extract all of the detail that the atmosphere gives you. then you just lose sensitivity with no extra resolution. You can of course choose to give up some resolution and have large pixels relative to the seeing spot for high sensitivity - for example Bert (Avondonk) does this very successfully with his f3 system - depends on what you want to do.
3. If your choice is for ultimate resolution with the 800mm scope in 2 arc sec seeing, you would want about 4 micron pixels. OSC requires slightly smaller pixels for the same resolution result as mono - Craig Stark did a good analysis on this - see ref
4. read noise is important by allowing you to use more and shorter subs - if the read noise is low you can do more reads for a given noise level. there are lots of system advantages in having short subs - tracking, flex, field rotation etc.
5. large well depth allows you to image longer before you run into saturation and the ABG takes over. However, limited well depth is not a big deal with low read noise chips - you can readily increase the headroom by combining multiple subs. For example, all else being equal, adding 4 x 1 minute subs taken with a chip with 20000 well depth and 5 electron read noise will give the same result (headroom and noise) as a single 4 minute sub from a chip with 80000 well depth and 10 electron read noise. So don't rely on well depth alone - dynamic range (welldepth/readnoise) tells the whole story and most chips on the market have somewhere around 70dB+, which seems plenty for most purposes.

Advice to Doug is: aim for 9 microns: ABG is useful: highest quantum efficiency you can get: lowest read noise. Another possibility is to image with a focal reducer if one is available for the scope. That would bring the chips with smaller pixels into consideration. Also be aware that some chips may require IR flooding to overcome residual bulk image and some older chip designs also have relatively high thermal noise that requires deep cooling - camera makers will generally take care of these issues though.

regards Ray

ref: http://www.stark-labs.com/craig/reso...yering_API.pdf

Last edited by Shiraz; 23-06-2013 at 09:03 PM.
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Old 23-06-2013, 06:58 PM
DIYman (Doug)
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Thanks Guys
I appreciate the advice.

When I questioned the supplier about the recommended size of (5.4um) I was told that over sampling was OK given the 2286mm focal length.

It makes me wonder.
Doug
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Old 23-06-2013, 08:41 PM
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I'm using a GSO RC8 (1625 mm focal length; 50% secondary obstruction) paired up with a 8300 chip. I find that native resolution at 0.685''/pixel takes a loooong time to get good smooth data - ideal exposure calculators give me 80 mins for narrowband and 30 mins for L subs!

At the moment, 2x2 binning (1.4''/px) for narrowband and L seems to work well - 20 min H-alpha subs turn out quite well. My local seeing at home for narrowband subs seems to be around 4-5'', occasionally 3' - so I'm not sacrificing any resolution by binning.

I probably wouldn't go for 2286 mm and 5.4 um pixels (0.48''/px)...
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Old 23-06-2013, 09:05 PM
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Have a play with CCDCalc and see what you need to get around 1-2 arcsec/pixel? Its freeware, so nothing to lose. Most serious astrophotrographers post their OTA and camera info, so you can look at how other people produce images that mean something to you.
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Old 23-06-2013, 09:50 PM
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The elephant in the room is you can 2x2 bin a 5.4u pixel camera, and effectively get the same gain (sensitivity) as say a 9.0u pixel model.

You can't go the other way.

...plus if you ever decide to use an camera lens, 9-10u pixels will start to look pretty blocky...
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Old 24-06-2013, 05:09 PM
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Unless you're on a mountain top you will probably find 2300mm or so starting to be more seeing affected than anything else. That's a plus for the bigger pixel for longer focal length which is the usual wisdom - larger pixels (9 micron) for longer focal length. 2300 is getting up there. 2900-3000mm for sure. You may get away with it. Typical seeing we get in Oz means 1500mm probably is the cutoff for the small pixel. That's just an educateed guess.

8300 chip on my CDK17 is useless unless seeing is really good and then I don't feel inclined to swap cameras. The 9 micron 16803 is a good match and when the seeing is good it simply gets sharper. Thats at around 3 metres. I get blurrier, less sharp images from the 8300 on that scope compared to the 16803.

As Peter says you can always bin but I also find binning 2x2 on narrowband at 3 metres and 16803 works great.

Greg.
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Old 24-06-2013, 08:28 PM
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One obvious choice of small pixel sensor, the KAF-8300, has problems with binning as the horizontal shift register isn't deep enough. So, binning isn't always a good strategy.

Cheers,
Rick.
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Old 25-06-2013, 04:22 PM
DIYman (Doug)
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Well it looks lke the consensus is 9um thanks to Ray, Ken and Dave.
However, I am troubled by Peter's coment:

The elephant in the room is you can 2x2 bin a 5.4u pixel camera, and effectively get the same gain (sensitivity) as say a 9.0u pixel model.

You can't go the other way.

...plus if you ever decide to use an camera lens, 9-10u pixels will start to look pretty blocky...


Also, Rick's advice also needs to be considered.

One obvious choice of small pixel sensor, the KAF-8300, has problems with binning as the horizontal shift register isn't deep enough. So, binning isn't always a good strategy.

I like the suggestion of checking out other astro sites to see what people are using with scopes of a similar focal length. It looks like the quality of seeing is the big issue as has been pointed out.
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Old 25-06-2013, 06:45 PM
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Quote:
Originally Posted by RickS View Post
... the KAF-8300, has problems with binning as the horizontal shift register isn't deep enough.

Rick.
I have no idea what you mean. To quote Pauline: "please explain".
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Old 25-06-2013, 07:10 PM
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Meru (Michael)
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Lol great quote Peter! Yes that KAF-8300 does have that issue, when binning the well depth is identical to a single pixel i.e. 25,000e. So all you end up doing is reaching the limit quicker when binning, without increasing the depth of the well. Atleast that's my understanding of it

Doug, have a think about whether or not you will eventually use a different telescope. 2283mm is tough I'd say to start Astrophotography in, as at that focal length even the smallest of issues will be magnified. I have a 800mm newt and a 1600mm RC8 (and with a reducer, the RC8 becomes 1053mm). I find my 5.4u pixels to work great across all ranges.

Here are links to what I've achieved with a 5.4um CCD (QHY9 Mono) at 1600mm (With the RC8) to give you an idea. That puts the imaging scale to 0.68arc-sec/pixel:


http://www.iceinspace.com.au/forum/s...d.php?t=103655

http://www.iceinspace.com.au/forum/s...&highlight=RC8

http://www.iceinspace.com.au/forum/s...&highlight=RC8

The images are by no means great and most were early out of focus attempts, but it shows you what that would look like. The 5.4um does offer me flexibility when swapping between the focal lengths. If I decided to stick to the RC8 only (like you with your long focal length scope), then I would seriously consider getting a different camera because 0.68"/pixel is a bit ridiculous. In nights of bad seeing (i.e. summer), its totally useless. So on your 2283mm scope, 5.4um would just be a total waste of money I reckon.
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Old 26-06-2013, 11:30 AM
SpaceNoob (Chris)
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As far as I know, the HSR of the 8300 has a 50% higher well depth than a single pixel. Not ideal, but also manageable. Just adjust exposure times when binned.
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Old 26-06-2013, 01:10 PM
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Quote:
Originally Posted by Peter Ward View Post
I have no idea what you mean. To quote Pauline: "please explain".
The KAF-8300 horizontal shift register only has a well depth of approx 50K e- and is not anti-blooming. This can cause horizontal blooming on bright stars when binning. Examples here: http://www.pbase.com/wjshaheen/horiz...he_kaf8300_ccd

You can ameliorate this by reducing gain (I think some KAF-8300 cameras including QSI do this automatically when you bin) but that's hardly ideal...

Cheers,
Rick.
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Old 26-06-2013, 01:29 PM
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[QUOTE=Meru;991316]Lol great quote Peter! Yes that KAF-8300 does have that issue, when binning the well depth is identical to a single pixel i.e. 25,000e. So all you end up doing is reaching the limit quicker when binning, without increasing the depth of the well. Atleast that's my understanding of it

Not entirely true. 2x2 binning on the 8300 does not give you a 4X gain in signal more like 2.2X. There is a gain, just not as much as you would expect.

As mentioned its an architecture type issue with the 8300.

Greg.
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Old 26-06-2013, 05:29 PM
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Well, I have imaged with a KAF3200ME (6.8um pixels, 85% peak QE) at 3m FL (in pretty awefull urban skies) at an image scale of 0.47 arcsecs. And anecdotal evidence anyway was, that this level of oversampling was well worth it.

In one way in particular. Decon simply works better with more data. At times I upscaled at even this image scale and the result was even better.

Sometimes the capture math doesnt match the real actual picture aesthetics post processing. Anyway, thats my experience .
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