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Old 15-06-2017, 10:29 PM
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Question Of pixels and scopes

Folks,

Being cooped up under the clouds gives the grey matter a little too much (but not enough) time to think about imaging

So let's get down to it...

Imagine that on the same night I had two imaging scopes, with the same f-ratio - a small scope and a larger scope, with 2x the objective area. Let's assume equal optical quality to take that out of the equation.

Imagine I also have a different sensor at the back of each. The pixels in the sensor on the larger scope have 2x the area of those of the sensor on the back of the smaller scope, resulting in the same resolution per pixel. The sensor with larger pixels has comparable QE/sensitivity to the smaller sensor, but costs 2x the read noise of the smaller one.

Then I point both rigs at the same object...let's say it's a galaxy...and the object doesn't fill the FOV of either sensor. The image is exposed sufficiently on both rigs to overcome the read noise and maximise the dynamic range.

What's the difference in the image from the two rigs?

Cheers,
Dunk
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Old 16-06-2017, 05:19 AM
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Larger pixels will allow for a greater dynamic range, so longer subs before stars become saturated. Read noise is not the only source of noise- both cameras will be affected by noise from the sky. Depending on image scale, smaller aperture may be more affected by diffraction (lesser resolution). But a smaller rig of equal quality will be significantly cheaper and more suitable for transportation.
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Old 16-06-2017, 06:41 AM
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Although no exact it would be kinda a like:
1) ZWO 150mm f/4 with ASI1600 (16mp MFT)
2) SW 300 F/4 with Canon 1D MkII (16mp FX?)

In short, both are imaging at the same image scale but the 12" sucks in 4x the amount of photons. #2 may have more read noise but that just means longer subs to over come that so #2 will give a more more detailed image with greater contrast. 4x the number of photons for the same resolution.

Because twice the read noise requires 4x the exposure and twice the aperture equals 4x the photons per second, they may very well cancel each other out. Subs should end up the same time but the #2 will have much greater contrast.
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Old 16-06-2017, 08:01 AM
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Quote:
Originally Posted by Atmos View Post
Although no exact it would be kinda a like:
1) ZWO 150mm f/4 with ASI1600 (16mp MFT)
2) SW 300 F/4 with Canon 1D MkII (16mp FX?)

In short, both are imaging at the same image scale but the 12" sucks in 4x the amount of photons. #2 may have more read noise but that just means longer subs to over come that so #2 will give a more more detailed image with greater contrast. 4x the number of photons for the same resolution.

Because twice the read noise requires 4x the exposure and twice the aperture equals 4x the photons per second, they may very well cancel each other out. Subs should end up the same time but the #2 will have much greater contrast.
Well, possibly, however the cameras, in this example, are not equivalent, the Canon, in its natural state, is uncooled and the thermal noise will be significant compared to even an uncooled ASI1600 (which uses the whole body as a heatsink) and has minimal internal processing which generates heat). Shooting longer subs on the Canon will substantially increase the thermal noise effect and as sub numbers increase the sub quality will decrease - without substantial cool down delays between subs ( which are not very effective in dslrs anyway).

For comparative purposes, the cameras would need to have equivalent thermal noise impacts throughtout the duration of the test.

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Old 16-06-2017, 08:20 AM
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Well, possibly, however the cameras, in this example, are not equivalent, the Canon, in its natural state, is uncooled and the thermal noise will be significant compared to even an uncooled ASI1600 (which uses the whole body as a heatsink) and has minimal internal processing which generates heat). Shooting longer subs on the Canon will substantially increase the thermal noise effect and as sub numbers increase the sub quality will decrease - without substantial cool down delays between subs ( which are not very effective in dslrs anyway).

For comparative purposes, the cameras would need to have equivalent thermal noise impacts throughtout the duration of the test.

Unless of course you're using the ASI1600MC which is an uncooled colour camera. Just like the Canon DSLR.
My point wasn't for exactness, just a rough real world example of two systems.
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Old 16-06-2017, 09:04 AM
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Dunk,

If you've kept the same image scale and increased the aperture then you're collecting more photons in every pixel and have improved performance in a fundamental way by reducing the impact of shot noise. If you're taking sky limited subs then that's all that matters (if you assume equivalent QE.)

Cheers,
Rick.
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Old 16-06-2017, 10:30 AM
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My next imaging scope will be an f/3 10" Newtonian. One day.
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Old 16-06-2017, 10:38 AM
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http://www.teleskop-express.de/shop/...ilor-made.html

Not that expensive considering.
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Old 16-06-2017, 10:39 AM
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My next imaging scope will be an f/3 10" Newtonian. One day.
Or this one: f2.8

https://www.teleskop-express.de/shop...strograph.html
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Old 16-06-2017, 02:21 PM
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Colin & Glen - please stop tempting me!
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Old 16-06-2017, 03:25 PM
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Colin & Glen - please stop tempting me!
https://www.optcorp.com/officina-ste...-velrh300.html
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Old 16-06-2017, 04:37 PM
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This strange looking device reminds me of....a beer keg
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Old 16-06-2017, 04:38 PM
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My next imaging scope will be an f/3 10" Newtonian. One day.
I like it. It's cheaper than a hyperstarred sct.
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Old 16-06-2017, 08:54 PM
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Thanks all for the interesting discussion.

Suavi pinned the tail on the backside of the donkey...my angle is what is the least kit I can get away with for a mobile rig and still get decent images?
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Old 16-06-2017, 09:23 PM
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If you want to look at it from that respect then a smaller rig can catch up to a larger one, you just have to put more exposure time in to get there.
Just look at all of the images that get done with small refractors, take the FSQ106 as the perfect example!
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Old 16-06-2017, 09:23 PM
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Did you mean that the larger objective would have twice the area of
the smaller one, or twice the diameter? If twice the area, the improvement
in resolution probably wouldn't be particularly obvious, but if twice the
diameter, fine detail, such as small craters on the moon would be better shown.
raymo
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Old 17-06-2017, 06:31 AM
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Dunk, I think that for a portable setup a small fast refractor combined with a camera with small pixels can yield very pleasing results. That's why I got an f/6 4" triplet and matched it with a Riccardi, so I now have a neat and lightweight f/4.5 imaging tube that should work nicely with ICX814 and 3nm filters. Was quite busy with other commitments, but hoping to put an image together in the coming weeks (school holidays!), so will see if it is a worthwhile path. Could try to go faster with AP's 0.72x Quad Telecompressor Corrector, but it would be a more pricey option than a Riccardi and would need to check if that would work with a smaller refractor. I didn't go the well established FSQ path becuse I like a somehow longer FL of an f/6 4" triplet that gives me a second option of imaging with just a flattener and at a higher resolution.
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Old 17-06-2017, 09:18 AM
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Quote:
Originally Posted by Atmos View Post
If you want to look at it from that respect then a smaller rig can catch up to a larger one, you just have to put more exposure time in to get there.
Just look at all of the images that get done with small refractors, take the FSQ106 as the perfect example!
Yeah I didn't go the good old FSQ route but went with the Esprit 100 and don't regret it - not that I have a frame of reference though! A fast-ish 4" is readily portable, but these apos grow up fast

Of course, that's what got me thinking about imaging newts, despite not being a big fan of diffraction spikes...but I wouldn't be looking to go above, say, 8" otherwise the practicality with an EQ6 goes flying out the window.

Quote:
Originally Posted by raymo View Post
Did you mean that the larger objective would have twice the area of
the smaller one, or twice the diameter? If twice the area, the improvement
in resolution probably wouldn't be particularly obvious, but if twice the
diameter, fine detail, such as small craters on the moon would be better shown.
raymo
raymo, objective twice the area originally, although twice the diameter is another interesting data point!

Quote:
Originally Posted by Slawomir View Post
Dunk, I think that for a portable setup a small fast refractor combined with a camera with small pixels can yield very pleasing results. That's why I got an f/6 4" triplet and matched it with a Riccardi, so I now have a neat and lightweight f/4.5 imaging tube that should work nicely with ICX814 and 3nm filters. Was quite busy with other commitments, but hoping to put an image together in the coming weeks (school holidays!), so will see if it is a worthwhile path. Could try to go faster with AP's 0.72x Quad Telecompressor Corrector, but it would be a more pricey option than a Riccardi and would need to check if that would work with a smaller refractor. I didn't go the well established FSQ path becuse I like a somehow longer FL of an f/6 4" triplet that gives me a second option of imaging with just a flattener and at a higher resolution.
And this is the crux of my inquiry, Suavi. With a nice compact apo and small pixels, how much am I _really_ sacrificing compared with, say, a fast imaging newt? My hunch is that I might lose out to the bigger scopes an excellent night, but how many of those come along on the east coast of Australia

Btw, I'd be keen to hear how you get on, not least with your Riccardi...
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Old 17-06-2017, 09:55 AM
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It is all a level of sacrifice to some extent. Going with smaller pixels on a small refractor increased resolution at the sacrifice of needing more exposure time to get to the same depth.
A larger telescope with larger pixels will allow the same level of resolution but in a significantly shorter time.

http://www.astrobin.com/full/284376/0/?real=&mod=

This is a drizzled tight crop of ETA Carina with a fast 5" and small pixels. Resolution wise, I am basically at my seeing limit and although a bit less portable than a 4", still very portable.
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Old 17-06-2017, 12:00 PM
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I believe it is quite difficult to accurately predict the degree of sacrificing resolution with a smaller telescope-small pixels combo vs large telescope-large pixels combo, because there are many variables, and in the end our pretty astronomical images are subjected to our subjective minds that may or may not like this or that. No doubt a larger scope-larger pixels setup on average allows to go deeper, so for me it is a question of what I can afford without the need of selling my kidneys, and I would rather go for a scaled down but reliable and of good quality setup for astrophotography, rather than a large telescope-large pixels combo but with say inferior filters, focuser flexing or poor guiding that requires baby sitting and frequent tweaking in order to get good data.
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