Reflectors for solar imaging

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Reflectors for solar imaging

Post by TareqPhoto »

Hi,

Is reflectors or Cassegrain safe enough to be used for solar imaging of any type either Ha or WL or CaK?

Reflectors such as a Newt or RC and Casse such as a Mak.


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Re: Reflectors for solar imaging

Post by EGRAY_OBSERVATORY »

SIMPLY, AS LONG AS THE OBJECTIVE-LENS IS COVERED WITH A SUITABLE FILTER - YES, BUT AS YOU ALREADY HAVE SUITABLE NON-REFLECTOR SCOPES + THEIR RELEVANT FILTERS, SO USE THEM FIRST AND PUBLISH THE REULTS FOR SOLARCHAT TO PERUSE...

TERRY :hamster:


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Re: Reflectors for solar imaging

Post by marktownley »

I'm not a fan of reflectors at all.


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Re: Reflectors for solar imaging

Post by TareqPhoto »

EGRAY_OBSERVATORY wrote: Thu Jun 23, 2022 6:09 pm SIMPLY, AS LONG AS THE OBJECTIVE-LENS IS COVERED WITH A SUITABLE FILTER - YES, BUT AS YOU ALREADY HAVE SUITABLE NON-REFLECTOR SCOPES + THEIR RELEVANT FILTERS, SO USE THEM FIRST AND PUBLISH THE REULTS FOR SOLARCHAT TO PERUSE...

TERRY :hamster:
I want to know if i have any hope with my reflectors before i plan or decide to buy one extra refractor and regret later knowing or finding out that my reflector could do the job.


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Re: Reflectors for solar imaging

Post by TareqPhoto »

marktownley wrote: Thu Jun 23, 2022 7:32 pm I'm not a fan of reflectors at all.
So other members here are also not fan of reflectors? Or let's say, there are no solar results on this site from reflectors?


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Re: Reflectors for solar imaging

Post by galaxie427 »

Link below shows solar results from a 300mm purpose build solar reflector....what I call "creme de la creme"!
Such instruments and observing/imaging methods are discussed in Chapter 9 of Christian Viladrich's book "Solar Astronomy".
The book is a fabulous reference for all things solar!

Link to Laszlo Francsics' solar image:
viewtopic.php?p=342478#p342478


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Re: Reflectors for solar imaging

Post by Dennis »

Reflectors are terrible. Better stick to refractors.


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Re: Reflectors for solar imaging

Post by EGRAY_OBSERVATORY »

THE CONSENSUS ABOVE, AND PARTICULARLY FROM DENNIS, IS RELEVANT.

TERRY


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Re: Reflectors for solar imaging

Post by TareqPhoto »

galaxie427 wrote: Fri Jun 24, 2022 1:54 am Link below shows solar results from a 300mm purpose build solar reflector....what I call "creme de la creme"!
Such instruments and observing/imaging methods are discussed in Chapter 9 of Christian Viladrich's book "Solar Astronomy".
The book is a fabulous reference for all things solar!

Link to Laszlo Francsics' solar image:
viewtopic.php?p=342478#p342478
Thank you


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Re: Reflectors for solar imaging

Post by TareqPhoto »

Dennis wrote: Fri Jun 24, 2022 6:08 am Reflectors are terrible. Better stick to refractors.
Ok, then i don't have any choice but to go with refractors, thank


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Re: Reflectors for solar imaging

Post by TareqPhoto »

EGRAY_OBSERVATORY wrote: Fri Jun 24, 2022 7:31 am THE CONSENSUS ABOVE, AND PARTICULARLY FROM DENNIS, IS RELEVANT.

TERRY
Ok, got it


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Re: Reflectors for solar imaging

Post by MalVeauX »

Hi,

So, technically a long focal ratio reflector is ideal as an instrument for all wavelengths because it will perform well in near UV, visible range and IR as a pure reflection system. The problem is thermal load and focal length. The short focal length of a typical reflector produces a small disc image size yet has all the heat of a large aperture concentrated into a small surface area. It makes for a very hard system to thermally regulate. The other issue is the limited imaging train of a typical reflector which typically would need modding (moving the primary mirror forward) to gain back focus for lots of different purposes. Mechanically a reflector is annoying to use due to positioning and focuser location and the issues with focus with various optics. I wouldn't consider a reflector unless it was for high res, 200mm and larger apertures, and with seeing that actually supports this, ie, sub-arc-second seeing.

SCT and classical cassegrains can be great for visible spectrum and IR but SCT (specifically SCT, not including classical) suffers for near UV due to the corrector plate, so it has lots of SA in near UV. It's fine for typical continuum to Halpha though and you'll find more people with SCT for big aperture Halpha simply due to convenience (most compact form of big aperture). These instruments have long focal ratios and produce larger disc images which have the heat from the aperture spread over more surface area making it easier to handle the heat with rejection systems. So choosing these a classical cassegrain would be a better option than SCT for "all wavelength" approach because it lacks the corrector plate. I would only consider these at larger apertures of 200mm and up. But even then, no point doing this unless you're focusing on high res and have the seeing conditions to even attempt it.

Ultimately, a long refractor is just way easier to work with and will be fine at smaller apertures and longer focal-ratios. Way more points that heat can be rejected from and just easier to deal with imaging trains. Exhaust refractors before you try to move into mirrors because there's not a compelling reason to use a mirror for most people outside of high res work in my opinion since few people are limited by their aperture and virtually always limited by seeing. If you're working in the 80mm~150mm aperture range, it's just way easier and less expensive to work with a long focal ratio refractor.

Very best,


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Re: Reflectors for solar imaging

Post by TareqPhoto »

MalVeauX wrote: Fri Jun 24, 2022 2:00 pm Hi,

So, technically a long focal ratio reflector is ideal as an instrument for all wavelengths because it will perform well in near UV, visible range and IR as a pure reflection system. The problem is thermal load and focal length. The short focal length of a typical reflector produces a small disc image size yet has all the heat of a large aperture concentrated into a small surface area. It makes for a very hard system to thermally regulate. The other issue is the limited imaging train of a typical reflector which typically would need modding (moving the primary mirror forward) to gain back focus for lots of different purposes. Mechanically a reflector is annoying to use due to positioning and focuser location and the issues with focus with various optics. I wouldn't consider a reflector unless it was for high res, 200mm and larger apertures, and with seeing that actually supports this, ie, sub-arc-second seeing.

SCT and classical cassegrains can be great for visible spectrum and IR but SCT (specifically SCT, not including classical) suffers for near UV due to the corrector plate, so it has lots of SA in near UV. It's fine for typical continuum to Halpha though and you'll find more people with SCT for big aperture Halpha simply due to convenience (most compact form of big aperture). These instruments have long focal ratios and produce larger disc images which have the heat from the aperture spread over more surface area making it easier to handle the heat with rejection systems. So choosing these a classical cassegrain would be a better option than SCT for "all wavelength" approach because it lacks the corrector plate. I would only consider these at larger apertures of 200mm and up. But even then, no point doing this unless you're focusing on high res and have the seeing conditions to even attempt it.

Ultimately, a long refractor is just way easier to work with and will be fine at smaller apertures and longer focal-ratios. Way more points that heat can be rejected from and just easier to deal with imaging trains. Exhaust refractors before you try to move into mirrors because there's not a compelling reason to use a mirror for most people outside of high res work in my opinion since few people are limited by their aperture and virtually always limited by seeing. If you're working in the 80mm~150mm aperture range, it's just way easier and less expensive to work with a long focal ratio refractor.

Very best,
Taking that last line, i don't know if it is really less expensive, easier yes i agree, but how come it will be any less expensive?

What i mean is that, i don't have any 120-150mm refractor yet, so how much is one? Then i have to buy an ERF for that larger scope, so i pay twice, one for the scope and another for ERF, with my Mak i only pay once even if ERF is expensive but i pay once, i do have 6" [150mm] Newt and 8" [200mm] Newt also, so they are available and i just need an ERF, if you can find any ERF so cheap but only brand new that i can use for say 150mm refractor then i will forget about the reflectors.


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Re: Reflectors for solar imaging

Post by MalVeauX »

TareqPhoto wrote: Fri Jun 24, 2022 2:42 pm Taking that last line, i don't know if it is really less expensive, easier yes i agree, but how come it will be any less expensive?

What i mean is that, i don't have any 120-150mm refractor yet, so how much is one? Then i have to buy an ERF for that larger scope, so i pay twice, one for the scope and another for ERF, with my Mak i only pay once even if ERF is expensive but i pay once, i do have 6" [150mm] Newt and 8" [200mm] Newt also, so they are available and i just need an ERF, if you can find any ERF so cheap but only brand new that i can use for say 150mm refractor then i will forget about the reflectors.
Simple: D-ERF. You cannot really use a big mirror without a DERF or active cooling system. This will cost a lot in addition to the same need for the instrument and same need for the filtering system for the actual narrowband work. A 200mm+ aperture mirror based optical system for H-alpha for example needs a full aperture DERF. Or the mirror itself coated to become the DERF itself (a big HA wedge). Also very expensive. There's no cheap way to do it.

A refractor can use internal sub-aperture DERFs (50mm, 2" filters) way cheaper, and upwards of 127mm and even 150mm can all use sub-aperture internal DERF for all wavelengths. I still suggest a full aperture DERF for 150mm, but it can be done without it. This is a huge cost savings. And again, if you don't have sub-arc-second seeing all the time, it's probably best to not bother with massive apertures.

Refractors are inexpensive optics, because you don't need ED/APO, achromatic doublets are fine.

Very best,


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Re: Reflectors for solar imaging

Post by TareqPhoto »

MalVeauX wrote: Fri Jun 24, 2022 5:18 pm
TareqPhoto wrote: Fri Jun 24, 2022 2:42 pm Taking that last line, i don't know if it is really less expensive, easier yes i agree, but how come it will be any less expensive?

What i mean is that, i don't have any 120-150mm refractor yet, so how much is one? Then i have to buy an ERF for that larger scope, so i pay twice, one for the scope and another for ERF, with my Mak i only pay once even if ERF is expensive but i pay once, i do have 6" [150mm] Newt and 8" [200mm] Newt also, so they are available and i just need an ERF, if you can find any ERF so cheap but only brand new that i can use for say 150mm refractor then i will forget about the reflectors.
Simple: D-ERF. You cannot really use a big mirror without a DERF or active cooling system. This will cost a lot in addition to the same need for the instrument and same need for the filtering system for the actual narrowband work. A 200mm+ aperture mirror based optical system for H-alpha for example needs a full aperture DERF. Or the mirror itself coated to become the DERF itself (a big HA wedge). Also very expensive. There's no cheap way to do it.

A refractor can use internal sub-aperture DERFs (50mm, 2" filters) way cheaper, and upwards of 127mm and even 150mm can all use sub-aperture internal DERF for all wavelengths. I still suggest a full aperture DERF for 150mm, but it can be done without it. This is a huge cost savings. And again, if you don't have sub-arc-second seeing all the time, it's probably best to not bother with massive apertures.

Refractors are inexpensive optics, because you don't need ED/APO, achromatic doublets are fine.

Very best,
Ok, let's discuss little bit as you gave very valid points here.

1. Cooling a reflector, must it be a refrigerator big size? A fan to sides enough speed can't do it? Also if i wrap the scope tube with reflective papers or material so it can reflect something from the scope body and then it can at least reduce the heat? White or silver color for example, and it is not only a Newt or open OTA, my Mak is smaller size and it is a mirror based, can't this be cooled easier than a Newtonian/RC for example?!!!

2. I am going to buy ERF no matter what as long i decided to go larger aperture, so whether i have the scope or not i was thinking why buy another scope and ERF for it if i have a larger aperture scope so i only need ERF regardless of what type.

3. What do you mean by sub-arc-sec seeing? So it means no one or no place on this planet earth has that nice sub-arc-seeing, in my area we have all kind of seeing, so definitely i have no problem, i won't image the sun like 365 like many are doing, but i am sure i can have minimum 100 days with nice seeing, more or less, so how can you determine that? I won't just pretend or assume that the seeing here is poor, then i will assume it is all over the earth, and hence all who bought 150 and larger aperture for solar are wrong decisions.

4. Most likely if i buy that larger scope say 150mm refractor then i decide to stop it down with whatever cap i can have if it didn't come with one, so it will be like reduced to 120mm [depends which f ratio it is native].

5. Let's say if we have nice seeing, will 120 outperform or surpass 150mm then? Or does 120mm f8 native be any better than 150mm f8 reduced to say 120mm f/15 for example?

6. Depends on what solar imaging i will do, so for white light i bought that cheap ND5.0 solar film filter, isn't this safe enough for visual and can be used for imaging? and for CaK, can't we have also something for Mak/SCT/Newt? I mean my both Newt came with that cap cover and small hole as aperture mask, what is the point of those if the scopes aren't used for solar anyway?

7. For Ha, my Quark is the "combo" version again, and from their sites they mentioned that it is used for F/15 or slower scopes, and they mentioned that for SCT it needs frontal off center ERF, and for Mak just frontal ERF, is that correct or not correct?


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Re: Reflectors for solar imaging

Post by MalVeauX »

Hi,

1) The cooling systems are all over the place, but basically the point is to cool the secondary mirror to avoid it experiencing expansion, etc. It's probably more expensive and difficult to rig up a cooling system than it is to just get a D-ERF and secondary D-ERF. I have seen amateur cooling systems on C8 class instruments that use active cooling with fans and jets to control air flow and active temp control on mirror with sensors. It works. It's elaborate. Not something I'd suggest someone do for their first build. DERF is just simpler.

2) You can absolutely get one big DERF and use it on any instrument. I do exactly this. I have one big 214mm triband Aries DERF that I can use on my 200mm and smaller apertures, so I use it on my 200mm F6 Newt, 200mm F10 C8 Edge SCT and my 150mm F8 Refractor.

3) Most places do not have sub-arc-second seeing during the day at just any time and so as you expect, in day time, most people will not benefit a large aperture often. I suggest anyone interested in high res, especially 200mm or larger systems, should measure their seeing for a while or image int heir seeing for a while to get an idea and data that supports what they're trying to assess. Absolutely most people have more aperture than their seeing supports. If you don't know what your seeing is like already, then I suggest you start there before you buy anything significantly big or expensive. The simplest way is to try and resolve convection cells from photosphere with white light filter (cheap!) in high resolution with a red wavelength filter. If you can do that well and reliably, then your seeing is enough to support the same aperture and scale in HA with appropriate DERF & HA filters. Otherwise, measure with SSM or DIY SSM (scintillation monitor).

4) Yup. Totally fine to mask aperture.

5) Seeing determines the resolution, it's that simple. If the seeing is 1.4 arc-seconds, your resolution is 1.4 arc-seconds which is what a 120mm aperture in 656nm could resolve up to. And if you used a 150mm at the same image scale, the resolution would be whatever the seeing is. You can only get higher resolution by using a larger aperture, critically sampled, and only if seeing supports it. So this is lucky imaging. If you have moments of 0.6~0.8 arc-second seeing (sub-arc-second) then you could lucky imaging high res with 150mm to 200mm aperture resolution. But if it's always 1.2 to 2 arc-seconds, then you likely just can't get more than what an 80mm to 120mm would resolve. Seeing is the limit here. Your gear is rarely the limit. Get the biggest aperture you can that is reasonable for your actual seeing, and mask it to whatever is needed for what the seeing is each session. Or just lucky image however you want. Don't get stuck thinking about the instrument. You should always be thinking of seeing as the actual limit of what resolution will be possible. You rarely, if ever, will be aperture limited or diffraction limited, you will always, always be seeing limited.

6) ND5 is safe for visual and imaging. For near UV I would not use SCT (only Mak, classical cassegrain, or Newt, even then, I would suggest cassegrain or newt, but even then, I wouldn't use any of these for near UV because near UV is much more effected by seeing and is almost 60% higher angular resolution, so a 100mm in near UV is like having a 150mm in HA. You don't need a big aperture for near UV (like CaK).

7) The combo simply lacks a telecentric amp. They don't expect you to buy a 200mm+ large DERF costing thousands of dollars. These were designed back when a 80~100mm ERF was big and costly (and it was their yellow ERF, not even DERF, and really barely blocked anything other than UV and some IR). The combo quark needs a long focal ratio to perform well and really needs a telecentric beam. It will work at F15 on an instrument, but not well. It needs to be F30~F50 in a telecentric beam to do well.

I suggest you just work with refractors and get some solid images and experience and especially get an idea of your seeing before you worry about mirrors, bigger apertures, DERF, etc. None of this is new, there's hordes of information out there from the past 20 years of this. Spend more time imaging and less time seeking bigger aperture mirror optics for solar.

Very best,


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Re: Reflectors for solar imaging

Post by Dennis »

Image


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Re: Reflectors for solar imaging

Post by TareqPhoto »

MalVeauX wrote: Fri Jun 24, 2022 6:20 pm Hi,

1) The cooling systems are all over the place, but basically the point is to cool the secondary mirror to avoid it experiencing expansion, etc. It's probably more expensive and difficult to rig up a cooling system than it is to just get a D-ERF and secondary D-ERF. I have seen amateur cooling systems on C8 class instruments that use active cooling with fans and jets to control air flow and active temp control on mirror with sensors. It works. It's elaborate. Not something I'd suggest someone do for their first build. DERF is just simpler.

2) You can absolutely get one big DERF and use it on any instrument. I do exactly this. I have one big 214mm triband Aries DERF that I can use on my 200mm and smaller apertures, so I use it on my 200mm F6 Newt, 200mm F10 C8 Edge SCT and my 150mm F8 Refractor.

3) Most places do not have sub-arc-second seeing during the day at just any time and so as you expect, in day time, most people will not benefit a large aperture often. I suggest anyone interested in high res, especially 200mm or larger systems, should measure their seeing for a while or image int heir seeing for a while to get an idea and data that supports what they're trying to assess. Absolutely most people have more aperture than their seeing supports. If you don't know what your seeing is like already, then I suggest you start there before you buy anything significantly big or expensive. The simplest way is to try and resolve convection cells from photosphere with white light filter (cheap!) in high resolution with a red wavelength filter. If you can do that well and reliably, then your seeing is enough to support the same aperture and scale in HA with appropriate DERF & HA filters. Otherwise, measure with SSM or DIY SSM (scintillation monitor).

4) Yup. Totally fine to mask aperture.

5) Seeing determines the resolution, it's that simple. If the seeing is 1.4 arc-seconds, your resolution is 1.4 arc-seconds which is what a 120mm aperture in 656nm could resolve up to. And if you used a 150mm at the same image scale, the resolution would be whatever the seeing is. You can only get higher resolution by using a larger aperture, critically sampled, and only if seeing supports it. So this is lucky imaging. If you have moments of 0.6~0.8 arc-second seeing (sub-arc-second) then you could lucky imaging high res with 150mm to 200mm aperture resolution. But if it's always 1.2 to 2 arc-seconds, then you likely just can't get more than what an 80mm to 120mm would resolve. Seeing is the limit here. Your gear is rarely the limit. Get the biggest aperture you can that is reasonable for your actual seeing, and mask it to whatever is needed for what the seeing is each session. Or just lucky image however you want. Don't get stuck thinking about the instrument. You should always be thinking of seeing as the actual limit of what resolution will be possible. You rarely, if ever, will be aperture limited or diffraction limited, you will always, always be seeing limited.

6) ND5 is safe for visual and imaging. For near UV I would not use SCT (only Mak, classical cassegrain, or Newt, even then, I would suggest cassegrain or newt, but even then, I wouldn't use any of these for near UV because near UV is much more effected by seeing and is almost 60% higher angular resolution, so a 100mm in near UV is like having a 150mm in HA. You don't need a big aperture for near UV (like CaK).

7) The combo simply lacks a telecentric amp. They don't expect you to buy a 200mm+ large DERF costing thousands of dollars. These were designed back when a 80~100mm ERF was big and costly (and it was their yellow ERF, not even DERF, and really barely blocked anything other than UV and some IR). The combo quark needs a long focal ratio to perform well and really needs a telecentric beam. It will work at F15 on an instrument, but not well. It needs to be F30~F50 in a telecentric beam to do well.

I suggest you just work with refractors and get some solid images and experience and especially get an idea of your seeing before you worry about mirrors, bigger apertures, DERF, etc. None of this is new, there's hordes of information out there from the past 20 years of this. Spend more time imaging and less time seeking bigger aperture mirror optics for solar.

Very best,
Thank you very much

I will brief or conclude it into points so to end it.

- I won't cool reflectors because i won't use them mainly or mostly, but i bought that ND5.0 solar filter so i can use it with one, and my choice is the Mak, for white light, i do have Continuum filter, so does this setup determines seeing then for just WL and then can judge overall?!!!

- I don't have time to test and wait, an i can't test nowadays even with smallest refractors due to the heat and humidity, so i can wait until the weather is cooling down, by all that time waiting i can simply afford a larger scope.

- I don't want to keep buying and upgrading, so i won't buy 100 then later when seeing is nice i go to 120mm and then again upgrade to 150mm, too much waste and test and spending, so that i was thinking why not jump to 150mm and never look back, what could be worse then, i just mask it and stop it down, this way if seeing is great then somehow i can use 150 full aperture maybe, if not then i just reduce it to 120mm or even 100mm, i already have 90mm triplet, so i don't think i will plan for 120mm achro, and definitely won't stop with 90mm as max aperture for solar.

- I don't know what is the difference can be with the sun rather than the moon and planets, even for those targets people keep talking about seeing and turbulence, so if i can do with 150-300mm aperture for those targets, why can't be possible with the sun which is super bright and same path in the sky and maybe same conditions, could be better or worse time to time??!!

- I don't know what telecentric magnification i should buy or use for my Quark combo version, i do have a Barlow and focal extenders, i bought combo version exactly for that reason, so i can choose which magnification to use, with my Mak for white light i can just use f/15, i also have 0.5x reducer if necessary, but if i have a refractor say with f8-f9 then i can use whatever focal extender to reach that desired recommended focal ratio, with 5x i have i can use it on f8 to be at f40, isn't that too much then?


I hope i can decide carefully and not regret.


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Re: Reflectors for solar imaging

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Dennis wrote: Fri Jun 24, 2022 6:21 pm Image
What do you say??!!! :lol:


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Re: Reflectors for solar imaging

Post by MalVeauX »

Dennis wrote: Fri Jun 24, 2022 6:21 pm Image
You're right.
Disappointed_Smoking_Blackman.jpg
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Very best,
Last edited by MalVeauX on Sun Jun 26, 2022 3:46 am, edited 1 time in total.


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Re: Reflectors for solar imaging

Post by marktownley »

MalVeauX wrote: Fri Jun 24, 2022 6:20 pm Seeing determines the resolution, it's that simple. If the seeing is 1.4 arc-seconds, your resolution is 1.4 arc-seconds which is what a 120mm aperture in 656nm could resolve up to. And if you used a 150mm at the same image scale, the resolution would be whatever the seeing is. You can only get higher resolution by using a larger aperture, critically sampled, and only if seeing supports it. So this is lucky imaging. If you have moments of 0.6~0.8 arc-second seeing (sub-arc-second) then you could lucky imaging high res with 150mm to 200mm aperture resolution. But if it's always 1.2 to 2 arc-seconds, then you likely just can't get more than what an 80mm to 120mm would resolve. Seeing is the limit here. Your gear is rarely the limit. Get the biggest aperture you can that is reasonable for your actual seeing, and mask it to whatever is needed for what the seeing is each session. Or just lucky image however you want. Don't get stuck thinking about the instrument. You should always be thinking of seeing as the actual limit of what resolution will be possible. You rarely, if ever, will be aperture limited or diffraction limited, you will always, always be seeing limited.
Sage advice Marty!


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