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Sub exposure time, SNR, and focal ratio...

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#1 CounterWeight

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Posted 10 September 2013 - 03:15 PM

Near and dear to all of us. And very common question for beginning imagers. I am specifically omitting resolution as CCD Calc is worth a thousand words there.

I was playing with the exposure calculator (SkyTools3 pro)last night after plugging in the numbers for the Veloce RH 200. Of interest was the tool has an SNR sort of bent and you can compare exposure times and expected signal to noise ratio, how many subs of a given length or how many subs of a different (shorter or longer) length to get to say an expected SNR of 50 (which going by their numbers usually results in a great image.

Sticking with 20 minute sub exposures, I see I can get to an expected SNR of 50 with about a third as many sub exposure using the f/3 scope as I can using the f/5.3 scope.

In the case of NGC1499 the California nebula to get to ~SNR 50 using my FSQ85 at f/5.3 would take about 18 exposures at 20 minutes.

Using the Veloce 200 at f.3 requires about 6 exposures at 20 minutes.

The tool is extremely versatile in ways (takes airmass into account and other atm), here is a screen clip of the RH 200 stat's...

I'm hoping this thread can help explain to a newcomer to this part of the hobby why the 'need for speed' and the rise of 'astrographs'.

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#2 CounterWeight

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Posted 10 September 2013 - 03:33 PM

And here is a clip of using the FS85. What the upper night bar?
green line is the 2x airmass altitude. red dashed line is the object altitude, and the gold vertical lines are the selected image time window for the night. Light and dark and transition are day and night. Blue dashed line is Luna. The exposure calc uses the scope and the CCD info and displays the CCD info.

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#3 Jeff2011

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Posted 10 September 2013 - 03:42 PM

Thanks for the explanation Jim. That is quite a big difference. The Veloce 200 is a dream scope. The Fastar/HyperStar system I would think is more affordable.

Speed was one of the reasons I went with the f4 AT6IN over the f9 AT6RC.

#4 CounterWeight

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Posted 10 September 2013 - 11:29 PM

Hi Jeff - yes the Hyperstar far more affordable but then there are some tradeoffs there.

What really struck me is the 'more significant than I thought' difference in time needed for image capture. Makes me curious as well about the statistical methods used in the various cal and combine and efficiency with say less than 10 subs. This will be interesting. have to say I'm really impressed and have been for some time at the amount of data, even good dimmer signal the faster Newt's consistently turn out. This is what was egging me for so long, not just that it was faster, but the finished product looked better.

My new imaging mantra is "fast and flat.... fast and flat..."

#5 Jeff2011

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Posted 11 September 2013 - 07:46 AM

The other scope that has my interest is the Tak E-180. An f2.8 newt. I see you have your Veloce 200 on order. Look forward to see your images with it.

#6 CounterWeight

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Posted 11 September 2013 - 08:54 AM

Yes, I was initially looking at that one as well, Tak quality and a good bit less $$, though still 'spendy'. Large image circle, open tube - a lot to like there. One big factor made me opt for the RH was the lack of a spider and the resulting spikes.

#7 Madratter

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Posted 11 September 2013 - 09:19 AM

Wow. That Veloce RH 200 is a monster. If it is lives up to the specs it should be incredible. :)

#8 terry59

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Posted 11 September 2013 - 09:29 AM

I am looking forward to seeing how it works for you. What about the poor little FSQ-85?

#9 CounterWeight

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Posted 11 September 2013 - 09:54 AM

Madratter - I am hoping ! they make a lot of high end gear and the folks I've PM'd and spoke to that are using them (just 2 of them) speak very highly.

Terry - good question. Though the ultimate resolution is in ways different the native fov's are close enough that I want to widen the field on the Tak with their dedicated reducer and Tak it into ~f/4, that way I can justify the FSQ as my 'wide field' scope and call the RH200 my medium field scope.` And call my bank account for this stuff drained.

#10 Footbag

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Posted 11 September 2013 - 11:27 AM

I was looking at the Veloces at the past two NEAF's. They are amazing. Besides the optical quality, they are like pieces of artwork.

I'd sell my soul for a RH, but the devil doesn't think it's worth that much.

I don't say things like this often, but I'd love to see a non-premium vendor start making RH's for the masses. From what I understand, it's not very difficult to build an RH, but there are a lot of pieces of glass to grind.

#11 canukLX90

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Posted 11 September 2013 - 11:58 AM

SkyTools 3 a great piece of software. I love imaging at
F2.8 with the PowerNewt. Next purchase is a high QE CCD for
that "need for speed". Looking forward to your Veloce 200
efforts...wow what a scope!

PJ

#12 CounterWeight

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Posted 11 September 2013 - 02:40 PM

Adam, you're right... all the top shelf stuff is art of engineering, I feel that way about mounts and all the rest as well. But I think maybe a mistake think anything made to a very high standard would be economical to mass produce without sacrificing important consistent qualities somewhere, the faster these things get, the larger the room for error, more to get not just close but exactly right... even then collimation can be a nightmare.

That said it is a great time to be getting into this part of the hobby vs. just 10 years ago - there have been vast improvements and the costs have dropped significantly while innovation continues.

PJ - I was also looking at the BS/PN's. Decided not to buy for altogether different reasons from the Tak which I wont get into. I'm glad you love yours as they and the ASA and Tak's are all amazing scopes. I'm a huge advocate of ST3 pro, in a lot of ways there is nothing like it and I'm always consulting it. Once the UI is understood it's amazing. Some time ago I decided that on the rare occasions I need something to control the mach1 - ST3 would be it, dethroned TheSky6. Funny when I posted on the A-P yahoo group few knew what it was or that it could control their mount (this was a few years ago when I bought the mount)

I have to admit, the Orion or AT f/4 Newt's seem to have the bang/buck in spades - and their support here is fantastic - top shelf. Of all the decisions, not buying one - that was maybe most difficult. And as I mentioned in another thread somewhere, letting go of my TEC 160 and ED80T to get the RH200 was not easy, through all the rationalizations it was more trading one dream for another.

#13 Madratter

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Posted 11 September 2013 - 02:48 PM

One of the things holding me back from buying one of the fast newtonians is I want very flat, not kind of sort of flat.

I have kind of sort of flat with my C8 and the focal reducer/flattener.

#14 Jared

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Posted 11 September 2013 - 11:08 PM

Interesting. Is it accounting for read noise, thermal noise, etc.? If you look just at shot noise, I would have expected the FSQ to require 1.75x as much exposure time rather than 3x. To a first approximation, SNR should improve linearly with focal ratio; an f/2.8 scope should have twice the SNR of an f/5.6 scope for a given integration time. I wonder what additional factors SkyTools is taking into account (assuming it is correct).

#15 CounterWeight

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Posted 11 September 2013 - 11:18 PM

Jared,

Well it takes into account the camera specs (yes read noise and all the rest of the QE stuff), you can enter manually or if it's in the library of CCD's... airmass angle and time, temps and humidity. It's no secret though and the author is very open to any questions.






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