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Poll: Which aperture to suit 90mm triplet and ~70mm?
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Which aperture to suit 90mm triplet and ~70mm?

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  #1  
Old 07-07-2021, 03:48 AM
TareqPhoto (Tareq)
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60s or 70s aperture?

Hi all,


Now a budget will come end of this month [July], and it will be split into two or three things or items if possible, one of items i almost sure to go with it 99%, so only two remaining, one of them is about this topic here.



I want to buy a refractor for DSO imaging, and i want cheap small one, i have the choice between 60s aperture scopes or 70s aperture scopes, if 60s then simply i can add 0.8x reducer later and i will be fine, but with 70s aperture then i have to do one of two things, A: To buy a reducer such as 0.65x or 0.7x a must, B: Buy APS-C mono camera which is very pricey for now, but it can be a plan later anyway, So with two choices i have to be careful what should i buy, your opinion.



I decided to keep my QHY163M and not buy APS-C mono now yet, i think i will leave this camera for next year, unless i get lucky and win money somewhere, so i don't know if i should based my choice/decision now on 4/3" cameras or future APS-C mono, and i don't ask which type of refractor because it will be used for NB mostly, so it can be a doublet or triplet not a problem, but cheap, no more than $500, so i can buy two scopes at once, let me see what will be your opinion or choice for me.



Thanks!
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  #2  
Old 08-07-2021, 02:39 PM
JA
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Originally Posted by TareqPhoto View Post
I want to buy a refractor for DSO imaging, and i want cheap small one, i have the choice between 60s aperture scopes or 70s aperture scopes, if 60s then simply i can add 0.8x reducer later and i will be fine, but with 70s aperture then i have to do one of two things, A: To buy a reducer such as 0.65x or 0.7x a must, B: Buy APS-C mono camera which is very pricey for now, but it can be a plan later anyway, So with two choices i have to be careful what should i buy, your opinion.
I would choose neither. From the apertures you've nominated (60 or 70mm) and your desire to do DeepSky imaging using as you have said reducers, it's apparent that you are looking for something like a 300mm focal length (based on a 70mm aperture at say f/6 with a 0.7x reducer =70x6x0.7=294mm), which you could then use with your QHY163 4/3" sensor which as a guide would give you a roughly equivalent field of view to a 600mm focal length on Full frame and if/when you decide to upgrade to an APSc sensor you would get a wider field of view: roughly equivalent to a 450mm focal length on Full frame. If those fields of view are too tight for you, you could scale down the lens to a focal length of ~200mm. There are many fine lenses you could choose from in the 200-300mm range, then it's simply a matter of adapting them to your camera/s, unless there are filter wheels involved as that brings more complexity, not that it can't be overcome.

So why go to all that trouble (scopes/reducers/flatteners/spacing etc...) if you want two ~300mm optics for <$500 each? I would simply look to some high quality full-frame camera lenses, something like 300mm f/4 or perhaps faster for more $. You also then don't have the problem/cost associated with needing buy and experiment with spacing etc for a reducer or flattener as these are typically part of the optics in camera lenses which are designed to image on to a flat plane.

Best
JA

BTW I seem to recall you already have(had?) a 300mm f/2.8 which you could possibly then use.

Last edited by JA; 08-07-2021 at 02:50 PM.
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  #3  
Old 08-07-2021, 04:16 PM
TareqPhoto (Tareq)
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I would choose neither. From the apertures you've nominated (60 or 70mm) and your desire to do DeepSky imaging using as you have said reducers, it's apparent that you are looking for something like a 300mm focal length (based on a 70mm aperture at say f/6 with a 0.7x reducer =70x6x0.7=294mm), which you could then use with your QHY163 4/3" sensor which as a guide would give you a roughly equivalent field of view to a 600mm focal length on Full frame and if/when you decide to upgrade to an APSc sensor you would get a wider field of view: roughly equivalent to a 450mm focal length on Full frame. If those fields of view are too tight for you, you could scale down the lens to a focal length of ~200mm. There are many fine lenses you could choose from in the 200-300mm range, then it's simply a matter of adapting them to your camera/s, unless there are filter wheels involved as that brings more complexity, not that it can't be overcome.

So why go to all that trouble (scopes/reducers/flatteners/spacing etc...) if you want two ~300mm optics for <$500 each? I would simply look to some high quality full-frame camera lenses, something like 300mm f/4 or perhaps faster for more $. You also then don't have the problem/cost associated with needing buy and experiment with spacing etc for a reducer or flattener as these are typically part of the optics in camera lenses which are designed to image on to a flat plane.

Best
JA

BTW I seem to recall you already have(had?) a 300mm f/2.8 which you could possibly then use.



Hi JA,


Thank you very much for your reply, i was waiting, i so appreciate it.


I will give you points so it might help you understand better my issues and my plan and who knows, we might get somewhere or a choice or a conclusion based on those point, so here we go:


1. I used 300mm, a perfect lens, almost perfect FOV, but it is outdated and it has age problems, it needs service and i can't send it to any service, in my country all lenses services are super expensive for worthless payment, and i try hard to find someone to 3D print for me a lens cap for this 300 2.8 with the hood/dew shield so i can stop down the lens to something like F4 or F5, because i don't have a camera to change aperture to F4/F5 and i hate the spikes from lenses stopped down, never same as Newt/reflector spikes.


2. Focus is so critical with lenses, and the lens 300mm has problem with focusing ring, it is almost dead so i have to use force to focus, that will ruin my setup if i keep using force on the mount and entire setup, so i try to avoid doing this, also lenses have many elements or optics, this might affect sharpness for astrophotography, i feel scopes are straightforward and designed precisely for astro.


3. QHY163M isn't my only camera, it is my first ever cooled, i was thinking about selling it and go larger as APS-C mono, but the financial and situations showing me that this upgrade plan is far long time in future and i don't want to miss things right now, so i decided to keep it, i might buy APS-C mono separately without selling QHY163M.


4. I have 4 cooled cameras, 3 monos and 1 color, the color one is the only one in APS-C size, and i was planning to connect this to my new 90mm Triplet and adding 0.65x reducer or 0.6x, so i can go to that FOV i am looking for, in normal cases i can use the scope natively at 540mm or buying dedicated reducer for it and bring it to 432mm, but from some results i did last year by end of year [Nov-Dec] i can't get those FOV out of my head, and i did them only after i ordered 90mm few months earlier and i didn't cancel the order after that wait.


5. Main setup is dual imaging setup, 2 scopes and 2 cameras, so if i am thinking about pairing that APS-C OSC with 90mm and say 0.65x for example then that will leave one of my 4/3" cameras mainly my new one QHY294M Pro for a scope or lens in 270-290mm, and i found that Askar FRA400 with 0.7x giving 280mm is a perfect match for FOV, very slight difference, that Askar is quintuplet, so i can use without any flattener natively at 400mm or reduce it to 280mm, nice results i saw from this scope around, so if i get this then i am almost done there and it will be nice scope all around too, but someone told me to forge this scope idea which will complicate the plan and make me lost again.


6. From point 5 above, if i go with Askar FRA400 which will cost nearly $1100, then i will have little money/budget left, something around $400-600 maximum, i won't buy reducers by now, i will try to spend that budget i will get to get scopes first, making sure those scopes are the correct choices, then later i can add flattener or reducers with another budget later in future, so that i was asking about if i will choose that Askar FRA400 for example, then the left money i only have to buy smaller cheaper scope, such as 60s or 70s somewhere, i can't buy 90mm/80mm aperture scopes again and definitely not two FRA400, but i can just hold on my plan and go with only one scope to match my 90mm FOV reduced, but i don't want to keep 4 cameras like only 1 or 2 in use and the other 2-3 are collecting dust, and will not sell at this moment.



7. My mount is capable to hold two average weight/size setup or almost 3 small/average setup, 90mm with something like 60/70mm scope will not exceed or reach the payload capacity, those scopes are at 2-5KGs mostly as OTA alone, so i can say the three OTAs will max them out to nearly 10KG, then i am left with about 4-6kgs, cameras with and without filter wheel will match that, and with OSC camera i don't need a filter wheel, and i am planning to buy one more mount in future just in case, so i better get scopes now and worry about weight or another mount later, if my current mount can handle it then perfect, if not then i can wait and practice/use with two scopes and i swap one with another just to have both in use regularly, i done waiting stuff month after month or year after year, so i will buy anything or everything as soon as possible and i take my time using them one by one, my mount is allowing enough possibilities.


Sorry with long points, but i am trying to explain in depth as much as i can, if you have any further questions or points you want to ask me then i am willing to answer with all pleasure, i have to make clear good decisions as much as i can without sacrificing or selling my current equipment, i have very bad luck in selling stuff so i won't wait some miracle sales to buy again, and budget won't wait to be spent.


Best
Tareq
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  #4  
Old 08-07-2021, 08:21 PM
JA
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Hi Tareq,

Well there are many possibilities to consider there. I would personally work towards a choice which sets you up in the medium to longer term, rather than chose something for the here and now because you feel you have to.

To that end it would be good to know your vision of what you would like to do with your setup. To me based on what you've said and what I can piece together from "reading between the lines" you'd like to setup a multi-telescope imaging system perhaps with 2 or 3 scopes using your existing mount (10kg payload) for deepsky imaging using some or all of your cameras. Is that correct?

Why would you like to do this: because imaging time would be shorter or increased image quality or other reasons? Will all scopes be on the exact same target field or will they be pointed so as to create a wider image mosaic from 2 or 3 adjoining fields of view.

One of the things that sticks out to me, since I am very interested in doing exactly the same sort of thing, is that the different camera sizes and different focal length scopes introduce unnecessary complexity in potentially matching their fields of view, if indeed that's what you are trying to do. In summary, if you could expand on what your vision is, it might be easier to offer an easier/better path forward, with possibly different/more choices.

Best
JA
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  #5  
Old 08-07-2021, 10:35 PM
TareqPhoto (Tareq)
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Quote:
Originally Posted by JA View Post
Hi Tareq,

Well there are many possibilities to consider there. I would personally work towards a choice which sets you up in the medium to longer term, rather than chose something for the here and now because you feel you have to.

To that end it would be good to know your vision of what you would like to do with your setup. To me based on what you've said and what I can piece together from "reading between the lines" you'd like to setup a multi-telescope imaging system perhaps with 2 or 3 scopes using your existing mount (10kg payload) for deepsky imaging using some or all of your cameras. Is that correct?

Why would you like to do this: because imaging time would be shorter or increased image quality or other reasons? Will all scopes be on the exact same target field or will they be pointed so as to create a wider image mosaic from 2 or 3 adjoining fields of view.

One of the things that sticks out to me, since I am very interested in doing exactly the same sort of thing, is that the different camera sizes and different focal length scopes introduce unnecessary complexity in potentially matching their fields of view, if indeed that's what you are trying to do. In summary, if you could expand on what your vision is, it might be easier to offer an easier/better path forward, with possibly different/more choices.

Best
JA

Hi again,


I can answer you and clarify things better if possible.


- I had two 2 scopes before, then i added one last year, then another one this year but it was ordered last year, 1 is for planetary and lunar so this is out, the other two are 8" F/5 Newt [1000mm] and 6" F/4 Newt [600mm], so i covered 2 focal lengths or FOV, then this 90mm F/6 which is 540mm, close to that 6" Newt.


- I am planing for multi-scope imaging systems for sure, with 2 or 3 scopes, my mount is rated as 18KG or 20KG roughly.


- Trying to use 3 cameras, if i can use all 4 then that is great, but i think my mount can handle only 2-3 scopes of small ones, with larger scopes as 6" or 8" i will buy another mount in future.


- For now i only want to cover WIDE field multi scopes first, so APS-C with ~350mm, QHY294M with ~270-290mm, then ASI1600MM with ~260-280mm, QHY163M is same as ASI1600, i will use ASI1600 because it is connected to 1.25" high quality narrowbanding filters and not QHY163M with moderate filters.


- Want to get data of different filters at same time, even if FOV is slightly different but not very big difference, software like PI or APP can match that FOV difference perfectly, i saw people did with so much different FOV, then i will try to make it for WIDE FOV that is like 500mm-550 on full frame sensor.


- Someone told me that thinking too much about the camera different size or pixel size and different focal length is just nonsense, many are worried or scared to do it and only blinded by calculations, while others did it nicely successfully, in fact i did it, with a lens and a scope reduced, data was bad but i got data from both at same time of different filters, if the data was good then it will give great result, bad data due to out of focus and errors in guiding which wasted like 1 hour of data from two and also back focus between the two, so i didn't bother to process each/both data, but i got data from both which means it is possible, i just didn't want data from that lens and that scope.
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Old 13-07-2021, 12:43 AM
TareqPhoto (Tareq)
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So no more answer or comments?
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Old 13-07-2021, 02:42 PM
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So no more answer or comments?
I'm sure there are plenty of answers and comments out there ..... just ask a specific follow-up question.

Best
JA
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Old 13-07-2021, 06:40 PM
TareqPhoto (Tareq)
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I'm sure there are plenty of answers and comments out there ..... just ask a specific follow-up question.

Best
JA
What do you mean?
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Old 13-07-2021, 07:31 PM
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What do you mean?
Hi Tareq,
You questioned whether there were any more answers / comments when you said ....
Quote:
Originally Posted by TareqPhoto View Post
So no more answer or comments?
Whilst you didn't make it clear who you were asking: whether it was me or whether you were seeking input from others, I simply said that if you wanted more answers/comments that you should perhaps ask a specific question following up on anything you wanted further opinions on, (given that much had already been discussed.)

Best
JA
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Old 14-07-2021, 05:22 AM
TareqPhoto (Tareq)
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Originally Posted by JA View Post
Hi Tareq,
You questioned whether there were any more answers / comments when you said ....


Whilst you didn't make it clear who you were asking: whether it was me or whether you were seeking input from others, I simply said that if you wanted more answers/comments that you should perhaps ask a specific question following up on anything you wanted further opinions on, (given that much had already been discussed.)

Best
JA
Ok, i really don't know how to ask a specific question really, but i will do my best to make it clear or clarify it as much as i can, sorry for that.
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Old 14-07-2021, 05:32 AM
TareqPhoto (Tareq)
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So, i will try to re-phrase my question or make it in better clear way if possible and let see if that will help.

1. I am expecting a budget near the end of this month [July], and i am planning to use or spend something around $1500-1800 maximum.

2. The budget will be towards scopes for wide field let me say around 260-280mm focal length reduced not native and not Petzval design.

3. I was planning to go with Askar FRA400 because it meets many of my requirements, but i don't know if i should keep this plan or forget it? This is one question.

4. I know the price of this Askar FRA400, if i buy it for now and without a reducer then i am able to buy another scope, but it must be cheap.

5. If i will buy another cheap scope at around $500-700 then what will be that scope??? <--- this is second question.

6. And that cheap scope must also give me like 260-280mm focal length reduced, not Petzval, and not another Askar FRA400.

So sum the points up, my questions are basically about if i should buy Askar FRA400 as first main option, if yes then what will be another scope also giving nearly 260-280mm focal length reduced, if no Askar FRA400 then which two scopes to buy within that budget above and both can give me 260-280mm focal length reduced, doesn't matter which focal length natively both will be.

Those two scopes will run beside or next to my 90mm triplet reduced [side by side], it will be a multiple imaging system with ALL of them giving me very similar FOV, doesn't matter their aperture, if i match the FOV as much as i can then processing can do the rest job effectively, don't bring the topic of why using all at same FOV/FL why not different and please forget about sampling, this also doesn't affect my plan.

Hope now it is more clear, if not then please stop by and ask me what is not clear and what do you need to answer me.
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Old 14-07-2021, 06:15 AM
JA
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Long story short... Buy the Askar 400 and use one of you existing cameras if you want, but to me that is a waste. You are buying a short focal length in order to use your existing cameras, but you are throwing away a lot of light using the smaller sensors. It defeats the purpose of having multiple telescopes imaging the same field since you are not using anywhere near the entire full-frame sized image circle of the Askar 400.

It would be far better in light collection terms for you to forget those smaller scopes and instead focus you attention on a larger sensor for your 90mm f/6 if it is a good quality unit. That way you can get a larger field of view without another scope. This could be done with an APS-c sensor on your 90mm f/6, possibly using a focal reducer or using a
full frame sensor, possibly even try a modern DSLR/mirrorless camera rather than expensive (re your budget) astrocam. This would collect ~4 TIMES THE LIGHT of the 4/3 inch sensors. Having 4 times the light is like having 4 telescopes.

If it were me, within your budget, I would get a modern full frame mirrorless DSLR, probably a Nikon Z5 or Z6 or Sony A7models /Canon to use on your 90mm f/6 to do RGB imaging and consider a L-extreme filter to use with it if you want to try narrowband. Or you could save / extend your budget to allow you to purchase an APS -c sensor with or without a reducer.

Best
JA

Last edited by JA; 14-07-2021 at 06:38 AM.
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Old 14-07-2021, 07:19 AM
TareqPhoto (Tareq)
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Long story short... Buy the Askar 400 and use one of you existing cameras if you want, but to me that is a waste. You are buying a short focal length in order to use your existing cameras, but you are throwing away a lot of light using the smaller sensors. It defeats the purpose of having multiple telescopes imaging the same field since you are not using anywhere near the entire full-frame sized image circle of the Askar 400.

It would be far better in light collection terms for you to forget those smaller scopes and instead focus you attention on a larger sensor for your 90mm f/6 if it is a good quality unit. That way you can get a larger field of view without another scope. This could be done with an APS-c sensor on your 90mm f/6, possibly using a focal reducer or using a
full frame sensor, possibly even try a modern DSLR/mirrorless camera rather than expensive (re your budget) astrocam. This would collect ~4 TIMES THE LIGHT of the 4/3 inch sensors. Having 4 times the light is like having 4 telescopes.

If it were me, within your budget, I would get a modern full frame mirrorless DSLR, probably a Nikon Z5 or Z6 or Sony A7models /Canon to use on your 90mm f/6 to do RGB imaging and consider a L-extreme filter to use with it if you want to try narrowband. Or you could save / extend your budget to allow you to purchase an APS -c sensor with or without a reducer.

Best
JA
How should or could i answer you??!!!

Well, i have many DSLRS, some of them are full frame cameras, NONE of them came close to my cooled cameras, even my non cooled planetary cameras are doing better than any of my DSLRs or one mirrorless camera for DSO, so i will never waste time with a DSLR or mirrorless, and i will never send one for modify it for astro and i have no brave at all to do it by myself, so DSLR and mirrorless are out of questions.

Now about the image circle of the scope, this is nothing to worry about or care about, because i can always use that for another targets, and one day i might buy another APS-C in mono, but for now i have smaller sensors cameras, people are using them with whatever scopes, so it is no point from you to tell me to forget about them, and not all astrophotographers using full frame astro cameras or even APS-C, so i have those small sensor cameras then i must use them, and for FOV i want using astronomy.tools site it showed me what focal length i should buy to use those cameras for certain FOV i want, so cameras are staying, i have to buy scopes to give me those FOV, ASKAR FRA400 isn't only designed for full frame, also not all people who have this FRA400 using full frame or APS-C, so why they bought that, in fact even people who bought Takahashi FSQ-106 also some of them using small sensor cameras where the scope is even larger than full frame image circle.

Long story short is making this more complicated, my 90mm triplet is nice quality and designed for full frame also, if i buy a reducer that will make it down for APS-C, and i think not FRA400 but another scope with a reducer will cover up to APS-C, so using smaller sensor won't be a big deal, are you telling me now to have all my scopes designed for full frame and i should have all my cameras as full frame or APS-C???

I have to use multiple scopes, but i hate hate hate to use only one scope for whole imaging data per target, i remember i used my Canon 135mm lens at F2 wide open for Orion collecting HaLRGB, i spent 3 nights for that and the data was bad due to CA, so even F2 didn't help it to make it fast, i will never spend my life doing each target for 3 nights or more, and i will never spend more only to get RASA when a lens at F2 didn't help it as well, so i try to pick up several/some targets and try to collect data from multiple scopes, but i think using multiple scopes is like a bad idea from so many people, in this case no one will understand or help in this case, i saw few used multiple scopes for amazing successful results, each has own FOV or setup, i want to do mine for that wide field for now.
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Old 14-07-2021, 08:33 AM
JA
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How should or could i answer you??!!!

Well, i have many DSLRS, some of them are full frame cameras, NONE of them came close to my cooled cameras, even my non cooled planetary cameras are doing better than any of my DSLRs or one mirrorless camera for DSO, so i will never waste time with a DSLR or mirrorless, and i will never send one for modify it for astro and i have no brave at all to do it by myself, so DSLR and mirrorless are out of questions.
Which Fullframe DSLRs and Fullframe mirrorless cameras do you have and have you tried in a back to back comparison against which Astrocams?

There are many incredible images taken with DSLRs, just take a look on Astrobin. Also the fact that you have a budget which precludes a Fullframe Astrocam or APS-c Astrocam + Reducer led me to suggest a modern Fullframe DSLR or Fullframe mirrorless to use with your 90mm f/6 since you wanted a large FOV. The same field of view your 90mm f/6 is capable of in fullframe equivalent terms.

Quote:
Originally Posted by TareqPhoto View Post
Now about the image circle of the scope, this is nothing to worry about or care about, because i can always use that for another targets, and one day i might buy another APS-C in mono, but for now i have smaller sensors cameras, people are using them with whatever scopes, so it is no point from you to tell me to forget about them, and not all astrophotographers using full frame astro cameras or even APS-C, so i have those small sensor cameras then i must use them, and for FOV i want using astronomy.tools site it showed me what focal length i should buy to use those cameras for certain FOV i want, so cameras are staying, i have to buy scopes to give me those FOV,

It's your money/choice, but whilst astronomy.tools may tell you the FOV with different focal lengths, does it also tell you how much light your telescope has collected and is now lost (not used) by the smaller sensor.

Quote:
Originally Posted by TareqPhoto View Post
... ASKAR FRA400 isn't only designed for full frame, also not all people who have this FRA400 using full frame or APS-C, so why they bought that, in fact even people who bought Takahashi FSQ-106 also some of them using small sensor cameras where the scope is even larger than full frame image circle.
The Askar 400 has a full-frame image circle and if you image with anything other than a sensor capable of using all the light collected by the telescope, be it a 1) Fullframe sensor or 2) APSc sensor with reducer, then you are wasting the light collected by the scope - LOTS of it, in your case:
, if you use a 4/3" sensor you will loose approx 2 stops of Light i.e: 75% light collected is lost or not used. It defeats the purpose of using multiscopes on the same image to save imaging time or go deeper in the same time.
Quote:
Originally Posted by TareqPhoto View Post
On the other issue of people buying smaller sensors for the TAK106 or whatever, that's a choice they make and they are presumably happy to give up lots light collection potential for a tight FOV. You want a large FOV hence my suggestion not to buy the Askar400, but rather work with your 90mm f/6 if it's a good instrument.

Long story short is making this more complicated, my 90mm triplet is nice quality and designed for full frame also, if i buy a reducer that will make it down for APS-C, and i think not FRA400 but another scope with a reducer will cover up to APS-C, so using smaller sensor won't be a big deal, are you telling me now to have all my scopes designed for full frame and i should have all my cameras as full frame or APS-C???
What I am saying is that unless you use the full image circle of a scope you are loosing valuable light collected. In your case up to ~75% of the light your proposed Askar 400 collects will be lost/un-used.


Quote:
Originally Posted by TareqPhoto View Post
I have to use multiple scopes, but i hate hate hate to use only one scope for whole imaging data per target, i remember i used my Canon 135mm lens at F2 wide open for Orion collecting HaLRGB, i spent 3 nights for that and the data was bad due to CA, so even F2 didn't help it to make it fast, i will never spend my life doing each target for 3 nights or more, and i will never spend more only to get RASA when a lens at F2 didn't help it as well, so i try to pick up several/some targets and try to collect data from multiple scopes, but i think using multiple scopes is like a bad idea from so many people, in this case no one will understand or help in this case, i saw few used multiple scopes for amazing successful results, each has own FOV or setup, i want to do mine for that wide field for now.
Using the 135 f/2 lens wide open may not have been the best path forward for astrophotography as it is primarily designed as a general use and portrait lens and has some softness at f/2, small CA which may appear worse in the corners with altered star shapes. If you like that focal length, stop down 1 or 2 stops or perhaps see if you can try a Samyang 135 f/2 or Sigma 135 f/1.8 for comparison.

One further comment:
The budget you mentioned of $1500 to 1800, you don't say if that's $US or $AUD or other, but I would have thought that could have got you an APS-c astrocam possibly new (depending on the make/model) or certainly used for that budget. Then when you have more funds, or if you get a good deal on the APSc camera, you can purchase an appropriate focal reducer.... and then you are good to go on your 90mm f/6.

Best
JA

Last edited by JA; 14-07-2021 at 09:20 AM.
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Old 14-07-2021, 09:27 AM
TareqPhoto (Tareq)
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Originally Posted by JA View Post
Which Fullframe DSLRs and Fullframe mirrorless cameras do you have and have you tried in a back to back comparison against which Astrocams?

There are many incredible images taken with DSLRs, just take a look on Astrobin. Also the fact that you have a budget which precludes a Fullframe Astrocam or APS-c Astrocam + Reducer led me to suggest a modern Fullframe DSLR or Fullframe mirrorless to use with your 90mm f/6 since you wanted a large FOV. The same field of view your 90mm f/6 is capable of in fullframe equivalent terms.




It's your money/choice, but whilst astronomy.tools may tell you the FOV with different focal lengths, does it also tell you how much light your telescope has collected and is now lost (not used) by the smaller sensor.



The Askar 400 has a full-frame image circle and if you image with anything other than a sensor capable of using all the light collected by the telescope, be it a 1) Fullframe sensor or 2) APSc sensor with reducer, then you are wasting the light collected by the scope - LOTS of it, in your case:
, if you use a 4/3" sensor you will loose approx 2 stops of Light i.e: 75% light collected is lost or not used. It defeats the purpose of using multiscopes on the same image to save imaging time or go deeper in the same time.


What I am saying is that unless you use the full image circle of a scope you are loosing valuable light collected. In your case up to ~75% of the light your proposed Askar 400 collects will be lost/un-used.




Using the 135 f/2 lens wide open may not have been the best path forward for astrophotography as it is primarily designed as a general use and portrait lens and has some softness at f/2. I will add more info on this issue here later, if there is interest and if I can find some further data on this lens.

One further comment:
The budget you mentioned of $1500 to 1800, you don't say if that's $US or $AUD or other, but I would have thought that could have got you an APS-c astrocam possibly new (depending on the make/model) or certainly used for that budget. Then when you have more funds, or if you get a good deal on the APSc camera, you can purchase an appropriate focal reducer.... and then you are good to go on your 90mm f/6.

Best
JA
Ok, if i really really can understand your points better then i will ask those questions based on what i understood from your answers/points.

1. So if using for example a RASA scope at F2 with 1" sensor will not be faster than using a 6" refractor for full frame with a full frame camera, say using Ha filter, so a full frame camera will always make faster data than F2 scope?

2. Why people don't use full frame cameras to have faster data if they bought like RASA or Celestron SCT with Hyperstar and end up using smaller sensors? No sense if a small sensor will eat up image circle which will reduce the light gathering, so either they are wrong or you are wrong or they don't care, and honestly speaking i don't care also because i don't want to like change many things.

You still missed the point of multiple scopes, even if i use a smaller sensor, if i use multiple scopes at once it will collect data as fast if not faster as using one scope with a full frame, i never had any night that i can capture a data for 4 hours or longer, so if one filter i need like 1-2 hours means i will not finish many targets in two nights, but if i can use more scopes at once, each filter can have like 2-4 hours of imaging individually at once, with a full frame mono i still need long time exposure, means i have to keep changing filters, and one full frame camera of astro is more expensive than my mount and my camera and filters almost.

I have Canon 1DX and Sony A7R as my latest full frames, i still have Canon 1Ds3 and 1Ds2 and 5D first one but mirror is off, i tried my A7R and 1DX only for Pleiades and Orion against my QHY163M with filters, i live under Bortle 8/9 with high humidity and heat, so noise is too much, i now have APS-C color camera cooled, so kind of i won't re think about DSLR for now, i still learn with this APS-C, and you know what, i used it with my 8" F5 Newt for a cluster for like 2 hours and it didn't do any good, in fact my QHY163M with RGB for 2 hours did better, but i didn't make more clusters with mono as i hate changing filters and re focusing to make sure they are all parfocal or in focus, and many times clouds is coming or guiding lost in the middle so i have to try make more data, simply put under my conditions a mono is always best, and i still will use a mono paired with data from color as well, all told me that the details are mostly coming from Ha and Lum filters, so doing Ha L RGB with one scope for 3-7 nights per target??? No thanks.

Should i show you results where people used multiple scopes and small sensors or you are not interested? Or they used magic to make it work, because sometimes i feel like some people they don't know things about it so they start to give advises and opinions without they did by themselves really, or they failed while others succeeded and they assume all others will fail as well.
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Old 14-07-2021, 09:59 AM
TareqPhoto (Tareq)
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Ok, let's give examples

I will use one example, i don't want to spend lifetime posting thousands examples anyway between us, but this one is good for me, at least to understand better.

Here is first Andromeda image done with a full frame DSLR and a lens from Bortle 6 and 7 hours by one imager well respected:

https://www.astrobin.com/187996/?active=21194

And here is another image of same target using same lens but with a cooled mono camera small sensor from Bortle 7 under nearly 5 hours by same imager:

https://www.astrobin.com/278800/B/?page=2

To my eyes the second one with smaller sensor is nicer and better, and regardless it was Bortle 7 slightly worse and less time it was better than from a full frame and Bortle 6 from same person, same sky but it went from bad to worse as he is living under same sky anyway.

Funny that he went from full frame to cooled cameras, and i didn't see him went with a full frame camera but he went with a scope that gives 88mm image circle or almost full frame reduced, what is the point then? instead he went with a CCD i believe not a full frame either.
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Old 14-07-2021, 10:12 AM
TareqPhoto (Tareq)
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One more example to make it as last to finish the camera topic here

This image of Rosette, with two Takahashi scopes and two cameras one of them is a full frame camera DSLR, done for 12 hours:

https://www.astrobin.com/310329/?page=1

And here is another one with also another Takahashi scope and smaller sensor for 5 hours 32 minutes:

https://www.astrobin.com/271979/C/

I will choose the second one always.

So, the camera topic isn't really entered my mind as people buying expensive amazing scopes and yet to buy a better camera larger sensor with it, so camera thing is not a factor here, and even if people will lose light percent gathering it didn't stop them to use those small sensor cameras, thus a camera thing is not what they or me worried about, and i won't afford any astro cooled full frame camera an soon in future, and APS-C mono could be my next year plan, but i can't wait until next year to decide which scope only after i buy that camera first.
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Old 14-07-2021, 11:04 AM
JA
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Quote:
Originally Posted by TareqPhoto View Post
Ok, if i really really can understand your points better then i will ask those questions based on what i understood from your answers/points.

1. So if using for example a RASA scope at F2 with 1" sensor will not be faster than using a 6" refractor for full frame with a full frame camera, say using Ha filter, so a full frame camera will always make faster data than F2 scope?
OK to answer a question like that definitively you need to provide more information: Which RASA f/2 scope and which 6" refractor are you referring to as one needs the f-ratio, image circle sizes, central obstruction size and Astrocamera phyiscal size (diameter) data to give a well considered answer?

Quote:
Originally Posted by TareqPhoto View Post
2. Why people don't use full frame cameras to have faster data if they bought like RASA or Celestron SCT with Hyperstar and end up using smaller sensors? No sense if a small sensor will eat up image circle which will reduce the light gathering, so either they are wrong or you are wrong or they don't care, and honestly speaking i don't care also because i don't want to like change many things.
People do and don't choose various things for all sorts of reasons; Budget. cost, complexity, performance, ease of use etc... One thing is certain in optical (science) terms is that you will lose light if you use a sensor smaller than the image circle of the optics. It's a fact. If you don't like it, or don't like to change things that's OK.

Quote:
Originally Posted by TareqPhoto View Post
You still missed the point of multiple scopes, even if i use a smaller sensor, if i use multiple scopes at once it will collect data as fast if not faster as using one scope with a full frame, i never had any night that i can capture a data for 4 hours or longer, so if one filter i need like 1-2 hours means i will not finish many targets in two nights, but if i can use more scopes at once, each filter can have like 2-4 hours of imaging individually at once, with a full frame mono i still need long time exposure, means i have to keep changing filters, and one full frame camera of astro is more expensive than my mount and my camera and filters almost.

I have Canon 1DX and Sony A7R as my latest full frames, i still have Canon 1Ds3 and 1Ds2 and 5D first one but mirror is off, i tried my A7R and 1DX only for Pleiades and Orion against my QHY163M with filters, i live under Bortle 8/9 with high humidity and heat, so noise is too much, i now have APS-C color camera cooled, so kind of i won't re think about DSLR for now, i still learn with this APS-C, and you know what, i used it with my 8" F5 Newt for a cluster for like 2 hours and it didn't do any good, in fact my QHY163M with RGB for 2 hours did better, but i didn't make more clusters with mono as i hate changing filters and re focusing to make sure they are all parfocal or in focus, and many times clouds is coming or guiding lost in the middle so i have to try make more data, simply put under my conditions a mono is always best, and i still will use a mono paired with data from color as well, all told me that the details are mostly coming from Ha and Lum filters, so doing Ha L RGB with one scope for 3-7 nights per target??? No thanks.
So if you have a colour APS-c cooled camera , just use that with your 90mm f/6 with an approx 0.7x focal reducer and you will be using all the light the scope has collected. If you do so you will collect more light than using multiple 1" or 4/3" sensors on 1 or 2 extra ASKAR 400mm scopes. (as already discussed).

Quote:
Originally Posted by TareqPhoto View Post
Should i show you results where people used multiple scopes and small sensors or you are not interested? Or they used magic to make it work, because sometimes i feel like some people they don't know things about it so they start to give advises and opinions without they did by themselves really, or they failed while others succeeded and they assume all others will fail as well.
On that basis, I might leave you to your own thoughts now.

JA

Last edited by JA; 14-07-2021 at 11:26 AM.
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Old 14-07-2021, 05:22 PM
TareqPhoto (Tareq)
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OK to answer a question like that definitively you need to provide more information: Which RASA f/2 scope and which 6" refractor are you referring to as one needs the f-ratio, image circle sizes, central obstruction size and Astrocamera phyiscal size (diameter) data to give a well considered answer?



People do and don't choose various things for all sorts of reasons; Budget. cost, complexity, performance, ease of use etc... One thing is certain in optical (science) terms is that you will lose light if you use a sensor smaller than the image circle of the optics. It's a fact. If you don't like it, or don't like to change things that's OK.



So if you have a colour APS-c cooled camera , just use that with your 90mm f/6 with an approx 0.7x focal reducer and you will be using all the light the scope has collected. If you do so you will collect more light than using multiple 1" or 4/3" sensors on 1 or 2 extra ASKAR 400mm scopes. (as already discussed).



On that basis, I might leave you to your own thoughts now.

JA
Thank you very much for your answers and help and efforts.

Clear skies!
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Old 15-07-2021, 05:23 PM
TareqPhoto (Tareq)
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Something might happen that might change the plan or my choice, if it will happen hopefully then in this case i will cancel 60mm aperture out of the plan and keep 70mm, and might add 80mm as an option also, but until now nothing confirmed yet.
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