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Old 28-04-2015, 08:03 AM
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ZeroID (Brent)
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Of Pixels and Scopes ..

As a result of building the new RN 8F8 scope I've been playing around with CCDCalc (excellent piece of SW) to get an idea of what to expect in capabilities and my imaging options, cameras, barlows etc.

It has raised two questions I am unclear on:

1. With respect to binning. If I select a lower resolution option in my DSLR is this the same as binning ? ie a 4 pixel square as opposed to a single pixel and if that is the case what will be the effect ? Lower noise ? Hot pixel elimination ? Shorter exposure ? Lower resolution images ?

2. The Pixel to Scope ratio issue. I see lots of discussions about matching your camera pixel size to the scopes capability but I am unclear as to why this is so. Smaller pixels = better resolution in my thinking but there are obviously other factors involved.

The first question I intend to test. And compare between my SONY and Canon which have different pixel count sensors.
The second still puzzles me somewhat.

Anyone brave enough to give me a 'Dummy's Guide to ... "

Cheers
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Old 28-04-2015, 08:47 AM
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Amaranthus (Barry)
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A CMOS chip will allow for software binning, but not hardware binning. So you get some gain by binning your DSLR, but not like in a CCD, where the gain in sensitivity of the superpixel can be almost directly proportional. (And that can help with matching pixel scale, see below).

Smaller pixels = lower sensitivity but higher resolution (ability to discriminate fine detail). So it's a trade off. Yes, higher sensitivity means shorter exposures (well, higher per-pixel SNR, provided you have sampled sufficiently).

A good target is to match your pixel size to your average seeing, allowing for Nyquist sampling. Beyond that, it's termed 'wasted resolution' since you are sacrificing sensitivity (photon 'bucket' size per pixel). So let's say you estimate that at your site, on a decent night, you get 2 arcsec seeing. Then aim for a pixel resolution of about 1 arcsec.

As a general rule, the longer the focal length, the larger the pixels you'll want. DSLRs have small pixels, so oversample on many setups. In some ways, undersampling is better, since you can use drizzle stacking to recover some of that lost resolution, and still retain your higher per-pixel sensitivity.
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Old 28-04-2015, 09:08 AM
glend (Glen)
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You could do a mono mod on your DSLR and by de-bayering improved the resolution but at the expense of more capture time required through various filters. As stated before you can Bin in processing. However, seeing and your scopes ability to resolve are limitations.
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Old 28-04-2015, 11:48 AM
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ZeroID (Brent)
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I've done an IR mod to my KM 7D quite successfully but its still turning out colour. Not sure if the sensor construction is suitable for debayering. It's only a 6 meg sensor (3008 x 2000) with 7.8 um pixels.
I'll investigate further when I've finished playing with it in Ha\IR.
I can see me buying another 450D if I can find one at the right price. They seem to be almost the ideal candidate for hacking. Or an 1100D.

I currently have 4 DSLRs, 2 x SONY A77 although one is the wet damaged one with some sensor issues, the old KM 7D with the IR mod and a Canon 450D. Plus of course the ASI 120mc.
And soon to have 5 scopes as well ....

Uh oh ...

And cheers for the 'Dummies Guide' Barry. It's obviously not as clear cut as just being numbers. I'll now have to figure how good my seeing is.
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Old 28-04-2015, 01:10 PM
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Amaranthus (Barry)
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Well Brent, as a general rule of thumb, aim for a pixel scale of 1 to 1.5 arcsec, and you'll be okay in most situations. With my QHY22 I operate at a somewhat undersampled scale for my ED80T (at 2.44), but drizzle stack it, and a little oversampled at about 0.92 with 10" f/4. Quite flexible really.
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Old 28-04-2015, 01:50 PM
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Lower modes on a DSLR are simply smaller files. I don't know that any DSLR does binning. May be they do.

With a CCD 2x2 binning on most CCDs (not all) means a big boost in signal to noise ratio but it comes at a cost of lower resolution.

An 8 inch Newt around F8 should work well with the KAF8300 sensor which is in many cameras and has 5.4 micron pixels. 8 inch F8 is 1600mm focal length which is getting quite long.

The Sony ICX694 sensor is 6mp and 4.54 microns but considerably more sensitive than the KAF8300 and much lower noise. But a bit smaller in physical size. Its also an excellent match for your Newt.

DSLRs - Canon or the Sony would be the go I imagine. Canon has dominated for ages. But as their newer models get more and more pixels and their underlying architecture of their chips does not basically change you are getting noisier chips and less detail in the shadows. Sony excels there -Sony Exmor CMOS sensors are arguably the best on the market.
I could be corrected here but I think the earlier Canon EOS cameras may be a bit better than their later DSLRs for telescope imaging.
Not sure where they peaked but I'd say it was a few years ago now.

Sony A7s or A7 would be hard to beat. Sony though don't do a true 16 bit RAW they do a compressed 14 bit RAW which in 99% of cases makes no difference but nevertheless they do compress their RAWs and its worth knowing. Usually that shows up only in skies that are heavily pushed in processing. But as you push deep sky images I wonder if that will show up.

If you are imaging at F8 200mm = 1600mm focal length you will need bigger pixels. The old Canon 5D would be ideal. I think they are around 9 micron pixels much like the more expensive larger CCDs we use.

I get the idea NZ has reasonable seeing - is that right?

Greg.
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Old 28-04-2015, 06:49 PM
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ZeroID (Brent)
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Some useful info, thanks all.
I have 3 DSLRs to choose from and a ZWO ASI120mc. The most likely DSLR will be the 450D with a pixel size of 5.2 um, oversampling at .66.
The other option would be the SONY A77v which has 3.9 um pixels ( 24 meg ) and comes in at .5

Lower modes = smaller files .. Yes, but still uses the whole sensor so it effectively use larger blocks of pixels as one unit, hence a form of 'binning'.

I know the f8 spec is unusual these days, an older specification, it is an old mirror. But the idea was to make a planetary and galaxy capable scope needing the longer FL. It also reduces the inherent coma of modern short fl scopes.
It will be in some ways a pain to use as it won't fit parked in the Ob, too dang long and high but it's another option and a demanding fun project.

Greg > Yeah, NZ is pretty good mostly, a bit cloudy maybe. Depends a bit on weather direction. Northerlies give warm humid air from the tropics, southerlies can be crystal cold clear. Generally though the air is pretty good, minimal dust and smoke as NZ is not very wide and the prevailing winds push it all out towards Sth America.
I'm in Auckland, about 10 km from the CBD East of me, not the best location but I'm situated part way up the flanks of a small volcano (extinct) so a wee bit of extra altitude to get over the suburban fug. I'm also away from most local street lights and with a good southerly aspect.
Usual seeing is about mag 4 or 5, I can see M42 glowing naked eye on a good night. Don't seem to be too affected by jet streams to my knowledge.
Lucky ...
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