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Old 17-06-2021, 08:13 AM
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How much faster?

I ordered a RASA 11 inch and a ZWO 2600 mono plus all the filters...probably a very stupid move if I dont get to use them but it was more a "my health is not going to hold me back thing"...anyways... My most recent imaging set up has a 115 mm f5.5 refractor with a ZWO 1600 mono, a very capable unit....now with time on my hands whilst the goods cross the seas I actually started wondering just how fast will this new set up be...what factors will come into play? The larger appeture? the new camera? Certainly the F2.2 has something to do with it...so it started as guesswork whilst drifting off to sleep but really just how do you determine the difference. I expect the only reliable answer will come from in the field experience but I just cant help wondering how long an exposure on the new set up will be needed to match an hour on the old set up. I have found short exposures have some advantages and could do good work with 30 to 60 seconds and with the recent post about "lucky imaging" do wonder the exposure required from the new set up to deliver equivalent to 30 or 60 seconds on the old rig.
Also on the old set up I found I could get away with very high gain can I expect similar with the new set up?

Alex
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Old 17-06-2021, 08:33 AM
JohnF (John)
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Alex 2.2 is a very fast scope. Good luck.

PM me your phone number as may come around the Monday following the 4th Sunday of this month, if that suits you of course.
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Old 17-06-2021, 09:08 AM
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Originally Posted by xelasnave View Post
I ordered a RASA 11 inch and a ZWO 2600 mono plus all the filters...probably a very stupid move if I dont get to use them but it was more a "my health is not going to hold me back thing"...anyways... My most recent imaging set up has a 115 mm f5.5 refractor with a ZWO 1600 mono, a very capable unit....now with time on my hands whilst the goods cross the seas I actually started wondering just how fast will this new set up be...what factors will come into play? The larger appeture? the new camera?
Hi Alex,

Well you've picked all the right variables in terms of what will effect your exposure duration between your old and new setup, but as you're changing the camera as well as the scope there are a few things to consider, in comparing both setups, namely:
  1. differences in f-ratio
  2. differences in camera QE
  3. differences in Camera Sensor size/FOV
  4. differences in Noise
  5. differences in telescope optical aberrations

These will all effect the answer as to what constitutes an equivalent exposure between both systems, in terms of capturing a similarly detailed, similarly deep, similarly bright/exposed image. Fortunately in this comparison the focal length of both the f/2.2 RASA and the f/5.5 Scope are very similar 620mm V 632mm, if we take it that the f/5.5 image circle is not vignetted in any way by a focal reducer then the following comparison is possible....

Just focusing on number 1: differences in f-ratio for now with everything else unchanged and based only on the stated f-ratios of both systems you should expect (5.5/2.2)^2 = 6.25 times more light, so that your previously mentioned 60 second exposure (with your f/5.5 system) would in light gathering terms be equivalent to about a 10 second exposure on your new f/2.2 system.

There's one minor thing in the fine print that often goes unmentioned in these fast optical systems that have a central obstruction : whilst the stated f/ratio is by definition a true ratio of focal length to objective lens diameter it should be used with some correction for the loss of light obscured by the central obstruction when trying to equate exposures on different systems since the difference in exposure is equal to the ratio of Light collecting area of the two systems which can only be approximated by the square of the f-ratio as I did above.

If we look more accurately at the ratio of the light collecting area ratio of both these instruments:
RASA f/2.2 with 279mm objective diameter & 114mm central obstruction diameter then
Light collection Area = 3.1416(279/2)^2 - 3.1416(114/2)^2 = 50,929 mm^2

115mm f/5.5 Scope
Light collection Area = 3.1416 (115/2)2 = 10,387 mm^2
Ratio of light collection Area = 50,929/10,387 = 4.9, so that once the central obstruction of the RASA 11 is accounted for the RASA 11 collects 4.90 times more light than the 115mm f/5.5 scope. So that your previously mentioned 60 second exposure (with your f/5.5 system) would in light gathering terms be equivalent to about a 12 second exposure on the RASA 11 "f/2.2" system.

When accounting for the central obstruction in light gathering terms only one can think of the the RASA 11 as behaving like an f/2.43 system.

Best
JA

Last edited by JA; 17-06-2021 at 10:09 AM.
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Old 17-06-2021, 09:32 AM
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Alex 2.2 is a very fast scope. Good luck.

PM me your phone number as may come around the Monday following the 4th Sunday of this month, if that suits you of course.
I wont be here.
What I am trying to organise is to have the scope dropped off at Casino fire shed. It will be a while for reasons I wint go into.
Alex
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Old 17-06-2021, 09:40 AM
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Thanks JA for taking your time to consider the matter.
It would be wonderful if I find a six to one situation .... Anyways a field test is needed but ultra short exposures seem within grasp.
Because of my health I need to plan on less time imaging...we shall see hopefully.
Alex
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Old 17-06-2021, 09:53 AM
glend (Glen)
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Alex, you probably are well aware of this, but if not, you need to consider the f ratio change and how it impacts your narrowband filters. Standard narrowband filter sets are designed for a particular range of f ratio, and I believe very fast scopes like the RASA perform best with fast narrowband filters. I admit I Don't have all the facts, but I did contemplate converting my Celestron HD8 to Hyperstar at one point, and the advice I received on Cloudy Nights, was that I should go with the 'special' fast narrowband filters designed for RASA. It has to do with a shift in the filter spectrum band pass when you move from your f5,5 to f2. In other words you risk cutting out part of the very spectrum area your seeking to capture. It seems to be less of an issue where you have fairly wide spectrum narrowband filters, but is a big deal when your using narrow spectrum filters.

Info here:

https://www.baader-planetarium.com/e...iii-s-ii).html

Post #8 in this CN thread, has comparative examples,

https://www.cloudynights.com/topic/5...ters-for-rasa/
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Old 17-06-2021, 10:16 AM
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Originally Posted by xelasnave View Post
Thanks JA for taking your time to consider the matter.
It would be wonderful if I find a six to one situation .... Anyways a field test is needed but ultra short exposures seem within grasp.
Because of my health I need to plan on less time imaging...we shall see hopefully.
Alex
Hi Alex,

Yes It will be in that order. In optical terms it is 4.9 times faster as mentioned, but then there is the difference in quantum efficiency of your new camera (ASI2600) to consider. It is stated as 91% by the manufacturer which is huge and I suspect although I can't find it listed is at least 30% higher than the ZWO ASI1600) so there'e many gains to be had there too. If I had to guess and combine the two improvements of optical and camera, I would say it would be in the range of 7* times faster

*TBA as I try to find QE of 1600mm - found it ~= 60%. The 1600mm also has a smaller sensor so FOV & light collection will also be effected- I will try to account for this in a later post.

Based on 4.9 times optically faster (see previous post) and in Light/Electron efficiency (QE) terms 91/60 = 1.52 times more efficient so your new system should be something like 4.9 x 1.52 = 7.43 times faster, so that your previous 60 second exposure could be had in something like 8 seconds in light/electron gathering terms.

Best
JA

Last edited by JA; 17-06-2021 at 10:31 AM.
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Old 17-06-2021, 10:37 AM
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Originally Posted by glend View Post
Alex, you probably are well aware of this, but if not, you need to consider the f ratio change and how it impacts your narrowband filters. Standard narrowband filter sets are designed for a particular range of f ratio, and I believe very fast scopes like the RASA perform best with fast narrowband filters. I admit I Don't have all the facts, but I did contemplate converting my Celestron HD8 to Hyperstar at one point, and the advice I received on Cloudy Nights, was that I should go with the 'special' fast narrowband filters designed for RASA. It has to do with a shift in the filter spectrum band pass when you move from your f5,5 to f2. In other words you risk cutting out part of the very spectrum area your seeking to capture. It seems to be less of an issue where you have fairly wide spectrum narrowband filters, but is a big deal when your using narrow spectrum filters.

Info here:

https://www.baader-planetarium.com/e...iii-s-ii).html

Post #8 in this CN thread, has comparative examples,

https://www.cloudynights.com/topic/5...ters-for-rasa/
I think the filters I ordered take the RASAs needs into account.
Alex
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Old 17-06-2021, 10:46 AM
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Thanks JA as with everything it just keeps getting better.
I had estimated 5.67 times faster but I approximate a great deal and then subtract 7% just for pillow calculations but I usually get rather close.
It will be interesting to see just how far I can push things...2seconds at 350 gain and 2000 saved subs stacked at 3 times drizzle
Alex
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Old 17-06-2021, 11:24 AM
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Originally Posted by xelasnave View Post
Thanks JA as with everything it just keeps getting better.
I had estimated 5.67 times faster but I approximate a great deal and then subtract 7% just for pillow calculations but I usually get rather close.
It will be interesting to see just how far I can push things...2seconds at 350 gain and 2000 saved subs stacked at 3 times drizzle
Alex
Alex,

It was 7.43 times faster based on the increase in light collection area of the RASA 11 compared with the 115mm f/5.5 AND the increase in Quantum efficiency changing cameras from the ZWO ASI1600mm (60%QE) to the ZWO ASI2600mm (91%QE !!!) which is a great result, but I may have mentioned that the sensors are a different size and wanted to account for that, which will drive the improvement even further to over approx 10* times better light collection I'd say.

*I lost my post in the ether somehow/timed out so I will come back with and edit with more exact detail soon.

EDIT: Further to the above, and trying now to correct the estimated improvement created by the sensor increase in size (yes I know the FOV will be different), the sensor area of the ZWO ASI2600mm is 1.56 times that of the ZWO ASI1600mm so that it (the ZWO ASI2600mm) will intercept more of the light collected by the optics (think of it as less of an internal restriction/aperture/field stop in a conventional camera, i.e: akin to a lower f/number in photographic terms).
Taking this in to account you could expect something like 7.43 x 1.56 = ~11.6 times, call it 12 times more light collected by the RASA 11 f/2.2-ZWO ASI2600mm system compared with the 115mm f/5.5 Scope-ZWO ASI1600mm based system.
So in light gathering terms:
what previously was a 60 second exposure is now more like 5 seconds.
Best
JA

Last edited by JA; 17-06-2021 at 05:39 PM. Reason: Added Calc & typo
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Old 17-06-2021, 12:13 PM
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Thanks again JA that is all very exciting.

Glen if you are still there...I raised the matter with John at Bintel and it seems we need to do more...anyways he is looking into the matter. You know I sent him an email about this and he sent a reply immediately which is fantastic service.
Alex
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Old 17-06-2021, 12:31 PM
glend (Glen)
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Originally Posted by xelasnave View Post
Thanks again JA that is all very exciting.

Glen if you are still there...I raised the matter with John at Bintel and it seems we need to do more...anyways he is looking into the matter. You know I sent him an email about this and he sent a reply immediately which is fantastic service.
Alex
Yes Alex, still here. That is good that he is looking into it. As I understand the wider 12nm filters are fine, it's the 3nm filters that can get bumped outside the spectrum range.
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Old 17-06-2021, 01:16 PM
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Thanks Glen...I sent John at Bintel a link to the Cloudy Nights discussion plus some other comments and again an immediate reply...he makes you feel special as if you are the only customer of the shop but of course he is the type if guy that cares and obviously all his clients get great service, .... You know me it takes a lot to impress me ... Again thanks for raising the matter as clearly we were not really up to speed and you have I expect headed off possible disapointement.

Now I am staying calm and not in the least impatient but enjoying the total experience but I would like the "lot" to arrive in one go..as I said to John I dont want to sit looking at the scope and not being able to use it cause I await the camera...I just thought..I probably really need another lap top..my current one is getting slower cause I have too much on it I suspect ... It would be a good idea to set one up just around the new gear...
And all this and your kindness above all really help me cope, I mean I can feel it starting to sink in and fighting feeling sorry for oneself is so much easier with big plans and wonderful friends...
Alex
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Old 17-06-2021, 01:28 PM
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JA I am getting just so excited about what may be achieved with short exposures....I wonder just how many subs you can stack in Deep Sky Stacker...I think it spat the dummy when I tried 750 once but that could be the computer...anyways I am confident at 500...so 500 at 5 seconds seems like a starting point ...and I am going to go all 7 filters RBG so as to get good coloured stars...RBG just for star colour maybe...so much to think about....selecting the good subs seems an area to address...star count probably does not really indicate quality..I wonder...in the past I looked at each sub and decided...I suppose that is the best way...mmmm 500 subs by 7 filters thats 3500 say 10 seconds to judge but then there is the time to go to the next ... A long time but that is what I will need to do.
Alex
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Old 17-06-2021, 01:34 PM
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Hi Alex,

I'm not sure of the limits of Deep Sky Stacker, but certainly can imagine it would be CPU and Memory related. Also one point there's no need to limit to 5 sec exposures, go crazy do 10, 20 , 30 heck 60 seconds whatever, as long as you don't over expose the image. That way you'll have fewer images to stack for the same/similar image depth/noise. Sounds like you're in for a lot of fun with your new gear and dark skies.

Best
JA
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Old 17-06-2021, 01:44 PM
Startrek (Martin)
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Quote:
Originally Posted by xelasnave View Post
JA I am getting just so excited about what may be achieved with short exposures....I wonder just how many subs you can stack in Deep Sky Stacker...I think it spat the dummy when I tried 750 once but that could be the computer...anyways I am confident at 500...so 500 at 5 seconds seems like a starting point ...and I am going to go all 7 filters RBG so as to get good coloured stars...RBG just for star colour maybe...so much to think about....selecting the good subs seems an area to address...star count probably does not really indicate quality..I wonder...in the past I looked at each sub and decided...I suppose that is the best way...mmmm 500 subs by 7 filters thats 3500 say 10 seconds to judge but then there is the time to go to the next ... A long time but that is what I will need to do.
Alex
Alex,
The 2600MC which I have and the 2600MM which your about to get require a lot of storage and RAM memory
My sub files on the 2600MC are 51MB files each
I’d strongly advise you update your old laptop ( if it’s 5 years or older ) to something which has at least an i7 processor , 8GB ram and plenty of storage Hearing your going to stack hundreds and hundreds of subs from these new cameras got my attention !!! not to mention storage and processing
I bought a second laptop early this year , my old laptop, has i7 processor , 1TB and 32GB ram
The new laptop has i7 processor , 1TB storage and 16GB ram
I use the old one for capture and the new one for stacking and processing
They are both interchangeable ie: I can image and process with both , all apps are duplicated
These new generation ZWO and QHY CMOS cooled cameras suck the power and chew the memory and storage
Just a heads up .....
Martin
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Old 17-06-2021, 02:54 PM
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Hi Martin
I appreciate your advice.
Funny this caused me to recall the big day I purchased a flash new computer for my office...a hard drive with one meg storage...640 k ram...
So I just now mentioned my plan to my daughter and she has put her order in for one as well .... So maybe I can get her to set up my new one as a gesture of appreciation...there goes the new mount I guess and the sports car
Alex
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Old 17-06-2021, 05:25 PM
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lazjen (Chris)
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Just anecdotally, I've got both the ASI1600MM and the 2600MM now. The difference is amazing - the 2600MM is just so sensitive in comparison. I've had to really cut back on exposure times otherwise the stars get blown out badly.

Of course this does mean lots more image files of larger size being produced. You'll need that disk space.
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Old 17-06-2021, 07:28 PM
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Alex, I can understand your excitement, the fast f-ratio and new camera will make short work of even your dark sky…the risk of over-exposing is the bigger challenge!
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Old 18-06-2021, 07:12 AM
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Hi Chris
Thank you for your first hand be experience I find it most helpful.
Hi Dunk
Big challenges are the very things I live for...I cant wait.
Alex
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