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Old 17-09-2017, 06:42 AM
glend (Glen)
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Sensor Size, implications of CMOS Full Frame

With the ZWO release of its first two full-frame OSC cameras in its Pro series, I thought it was a good time to look at the implications and benefits of larger format sensors.
I am happy with an APS-C limitation, and have not considered something larger as a necessity, or even desirable.
What do you think? Would a full-frame sensor really give you something you do not already have? Who really needs them, and why?

Last edited by glend; 17-09-2017 at 07:30 AM.
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Old 17-09-2017, 07:49 AM
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Slawomir (Suavi)
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Hi Glen,

I would like having a 2nd camera with a large sensor, ideally a full frame and with large pixels. My 105mm CFF should handle a full frame with its dedicated field corrector; I would like to have an ability to cover a large portion of the sky with a single frame for narrowband imaging, as I really do not like mosaics because one usually needs to compromise time dedicated to exposure per panel. So large pixels with this scope and for narrowband imaging should be pretty fast.

I also would like to keep my current camera (QSI 690) and ideally put it on an f/3 10" Newtonian for imaging galaxies. Such telescope should ride alright on my mount and 1"pp is fine for chasing fine detail from a costal location. f/3 would mean going deep and fast, given that collimation and tilt were spot on and stable.

That's my long-term plan anyway
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Old 17-09-2017, 10:19 AM
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billdan (Bill)
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Hi Glen,

I have often wondered, do the users of full frame cameras spend 70% of the time cropping their images to highlight the chosen target. Galaxies, planetarys and globs do not need large real estate, as most of the sky will be cropped out.
The only benefit is on large scale structures like the Rosette, Corona Australis, the M42/running man complex etc.

The down side for a newt anyway is maintaining a fully illuminated chip. For a fast newt this would require a larger secondary, 3 inch focuser, bigger OAG and possibly a wider coma corrector. The interface for large frame cameras is a M54 port so all the plumbing must be wider than this as you get closer to the secondary.

Not to mention more expensive filters if a mono camera, even a 2 inch may be too small.

Coma, tilt and field curvature stick out like a sore thumb with a large chip and you need a lot of patience to overcome it.
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Old 17-09-2017, 10:47 AM
glend (Glen)
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Well Bill, that is a good point, and i can not re-engineer my 10" Carbon Strut Newt now. I built it to illuminate a particular spot size, so i gain nothing putting a full-frame on there. As you mentioned, it would mean a larger secondary and the image quality that results. Of my five scopes, only my two APOs could produce the necessary spot size for a full-frame, and that is with their 3" reducer corrector attached. Its not just the cost of the full-frame camera, but the flow on replacement costs in term of your other equipment.
Suavi an f3 newt scares the heck out of me, offset galore, i would not want to wrestle with it, and for imaging galaxies with a 10" newt i believe f5, as mine is, is the logical choice (@1250mm fl). I look at the various fast newt astrographs, with their mid tube corrected focusers and wonder why Hyperstar Edges are not a better choice, at f2.
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Old 17-09-2017, 11:25 AM
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Indeed, I would not like putting a full frame on a fast Newtonian; but my small refractor with its 3.2" FTF and 3.2" field corrector should handle it well at native f/6.

On the other hand, an f/3 10" Newton with its 750mm fl will result in exactly 1" pp with my QSI and a small chip should work well on a fast well built Newtonian. And f/3 means physically smaller instrument. F/3 also scares me, but if I venture this way why not go all the way? It would require a permanent setup for sure and excellent mechanics. I may still go conventional way with a larger refractor; a 130mm f/6.5 refractor would give about 0.9"pp with my QSI and albeit it would be slower than a fast Newt, but also MICH easier to handle
and it should give nice detail on galaxies.

Last edited by Slawomir; 18-09-2017 at 04:55 AM.
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Old 17-09-2017, 11:45 AM
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I'd love one of the new Pro series full frames, as there's heaps of parts of the sky that would suit the FOV with Esprit well, not to mention the even wider FOV from my Z61 and Canon lenses that are clinging to life in Chez Dunk

But as I mentioned in another thread, I can't justify that sort of outlay when I could achieve the results with a few mosaics with the 1600 mono, and I guesstimate in about the same time...
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Old 17-09-2017, 04:50 PM
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I agree with you Dunk, I have been waiting patiently for an APS-C Mono Camera to appear wtih microlens, not a debayered OSC that QHY are experimenting with.

As this is unlikely to happen I have been talking myself into accepting reality and purchase a ZWO-1600MM or QHY-163, this way I can save on filters (1.25"), less stress on the focuser and also the OAG prism can go deeper into the light cone revealing more guide stars.

From what I have read about SGP, doing mosaics is a breeze this days.
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Old 17-09-2017, 05:11 PM
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Making a quality mosaic is actually not that simple. Star correction needs to be very good in the corners, otherwise we will get somehow distorted images when forcing panels to align. One could go deeper with a larger sensor, while imaging time needs to be divided into separate panels with mosaics. On top of that we have many more subs to work with. So a larger sensor does make sense, if one can afford it of course.
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Old 17-09-2017, 06:09 PM
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Granted, a mosaic is only going to look good with a well corrected field.

On th other hand, I don't have a 16MP display let alone one of multiples, or 36MP or otherwise
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Old 17-09-2017, 06:15 PM
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Quote:
Originally Posted by Camelopardalis View Post
Granted, a mosaic is only going to look good with a well corrected field.

On th other hand, I don't have a 16MP display let alone one of multiples, or 36MP or otherwise
When I think about it, perhaps binning may be the answer to making good mosaic more quickly.
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Old 17-09-2017, 06:36 PM
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There are other complications that do need to be considered. For instance, your telescope may have a corrected imaging circle of 44mm for a FX (36x24mm) sensor but what is its correction like off axis?

My 130mm Sky Rover has a 55mm image circle which is considerably more than a FX sensor but the stars at the corner of a Nikon D810 are not perfect. Good (potentially very good in fact) but not perfect. They would be perfect with a 16200 sensor (APS-H) or a 11002 sensor because the pixels are larger. The D810 (QHY367) has 4.88 micron pixels but how many telescopes have a <5 micron spot size 22mm off axis? Not many.
It is easier to have 9 microns at 26mm off axis than it is to be <5 microns 22mm off axis. So in that respect, a 16803 is easier to correct for than a QHY367 even though the sensor has more real estate.

And Suavi, why bother with a 10" F/3 when you can get a 12" F/3! (Officina Stellare RH300
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Old 17-09-2017, 06:48 PM
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RH 300 reminds me of a keg, so no thanks!

But I totally agree that large pixels on a full frame are the way to go
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