Which ZWO ASI OSC Cooled fits best my setup?

[oymd]

Morning everyone
 
Aim is DSO, nebulae, clusters, smaller galaxies etc...
 
My kit is:
 
Mount:
Sky-Watcher AZ-EQ6 Pro
 
Telescopes:
1- Nexstar 8SE (OTA)
2- C11 XLT (I guess this will be better off purely visual)
3- SW ED80 Pro-DS (primary imaging scope)
 
Planned purchases:
1- Sky-Watcher x0.85 FR or Altair Astro x0.8 FR for the ED80
2- Celestron f/6.3 FR
3- Very long term, POSSIBLY a Hyperstar for the C11 (I doubt I will invest in this!)

 

I've decided to go OSC, and for now, avoid the extra layer of complexity of shooting mono with filters etc...
 
 
I have narrowed down my options to:
1. ASI294MC Pro
2- ASI533MC-P
3- ASI183MC Pro
4- ASI1600MC Pro
5- ASI071MC Pro just for comparison sake.
 
I have attached an image comparing the cameras primarily on PIXEL SCALE, as I do not understand the Sensitivity, SNR, and WELL specs.
 
I am leaning towards the ASI294MC Pro, as I like the wider image scale, and am not a fan of the square 533MC Pro scale.
 
I realise that the 533 is the newest, and everyone is raving about its sensitivity and no amp glow etc...
 
Please advise me on what would best fit my setup.
 
my table suggests that the ASI1600MCPro is the best fit for scopes?

Lastly, would the cheapest Mono Mini ZWO guide camera be OK for the guiding camera? the ASI120MM Mini?
 
Many thanks
 

 

[Whirlwind]

None of these should be bad choices.  For the image scale you can always bin using the software you using even with the 183.  One advantage of the smaller pixels is the better scale for very short focal lengths if you had any lens's to hand.

For the terminology the sensitivity is the proportion of photons that hit a pixel will be turned into a signal. The higher the better but isn't too much to get hung up on.  The Full well depth is how much data a pixel can hold before it is full (so any further information is then lost) - called saturation.  In effect it limits your exposure time but for CMOS you are usually exposing for short periods of time anyway.  Bigger well depths when you are imaging objects where the brightness between the brightest parts and the faintest parts are substantial (e.g. the orion nebula).  But there are way round this (you take sequences of shorter, mid and long exposures).  Read Noise is how much data a pixel would capture even in a perfectly darkened room.  It's noice that you can't get rid of and you need to expose long enough to overwhelm this signal.  Hence longer exposures are usually better.  For colour imaging, especially in light polluted areas, this is less of an issue (usually what you are observing overwhelms this quickly, for narrowband imaging it is much more important).

The Celestron focal reducer will be terrible for the larger CCDs, you will get distorted stars.  Also these are standard SCTs so this will apply even at their native focal length.  However, they are fine for planetary imaging, but might want to consider and external focuser.  I don't know much about the reducers for the refractor but would want to check they generate a flat field (so no distortions in the stars).  Also it might be worth considering a light pollution filter.  Not only will it help you control any residual colour aberration in the ED80 but would also help with London light pollution.

 

[vlaiv]

I would probably go for ASI294 myself.

[Xsubmariner]

I have the ASI 071 Pro which is a great APS-C sensor. It worked fine with my edge 11” with 0.7 reducer.

[Adam J]

Well for a start the ASI1600mc pro is no longer available.

Appart from that at your focal length the 294 is the best bet. I would not consider the 533 until the focal length was around 400mm due to the smaller FOV.

Adam

 

 

[oymd]

12 hours ago, Whirlwind said:

None of these should be bad choices.  For the image scale you can always bin using the software you using even with the 183.  One advantage of the smaller pixels is the better scale for very short focal lengths if you had any lens's to hand.

For the terminology the sensitivity is the proportion of photons that hit a pixel will be turned into a signal. The higher the better but isn't too much to get hung up on.  The Full well depth is how much data a pixel can hold before it is full (so any further information is then lost) - called saturation.  In effect it limits your exposure time but for CMOS you are usually exposing for short periods of time anyway.  Bigger well depths when you are imaging objects where the brightness between the brightest parts and the faintest parts are substantial (e.g. the orion nebula).  But there are way round this (you take sequences of shorter, mid and long exposures).  Read Noise is how much data a pixel would capture even in a perfectly darkened room.  It's noice that you can't get rid of and you need to expose long enough to overwhelm this signal.  Hence longer exposures are usually better.  For colour imaging, especially in light polluted areas, this is less of an issue (usually what you are observing overwhelms this quickly, for narrowband imaging it is much more important).

The Celestron focal reducer will be terrible for the larger CCDs, you will get distorted stars.  Also these are standard SCTs so this will apply even at their native focal length.  However, they are fine for planetary imaging, but might want to consider and external focuser. 

Can you please advise what is that? I thought I have only ONE option for the 8SE OTA & the C11 OTA: the Celestron f/6.3 focal reducer?

I don't know much about the reducers for the refractor but would want to check they generate a flat field (so no distortions in the stars). 

My ED80mm seems to have only TWO FR options: the matched Sky-Watcher x.85 and the Altair Astro .80 Lightwave. From other threads I read, I think neither gives a very flat field?

Also it might be worth considering a light pollution filter.  Not only will it help you control any residual colour aberration in the ED80 but would also help with London light pollution.

Excellent that you brought that up. I am at a loss at to which filter to get (UV/IR, or CLS, or CLS-CCD, or Opolong L-Pro etc etc...), and more importantly which size? 1.25" or 2"? Say I bought the 295MC Pro, which has a sensor of 23.2mm diagonal size, giving me a FOV of 132 arc-min at stock FL of 600mm of my ED80mm. I went to Astrofest today, just to ask this question, and I did not get any clear answer regarding 1.25 or 2"? Some said 1.25" is more than enough, others said that the 1.25" will cause vignetting, and its safer to go with 2"

 

Many thanks

 

 

[Whirlwind]

An external focuser is something you attach to the back of the telescope.  The focusing mechanism on the SCT can be less precise and repeatable because of it's design (you are moving the main mirror). Because of mirror movement you can find that moving in/out doesn't bring the focus to the same position and hence isn't repeatable.  An external focuser avoids this problem (something like this https://www.firstlightoptics.com/moonlite-focusers/moonlite-cs-dual-speed-sct-crayford-for-325-thread-fr-compatible.html) but there a lot of second hand market versions and you might want to see if there is one that can be adapted to both the 8" and 11").  You can also get motorised versions which helps get more precise focus and means you don't have to touch the telescope.  Celestron have redesigned the focusers for the RASA versions that is meant to better but have yet to see whether they are going to bring it forward for the main line of Edges and SCTs in the future, that would probably remove the need in the future for external focusers.

The old celestron F6.3 reducer was really designed for smaller CCDs going back to the 2000's and has a quoted imaging diameter of 24mm.  Some of your options above are bigger than this so you will get distorted stars at the edges.  ASA made a version with a wider field as well as Optec and Starizona I believe. 

The Skywatcher reducers are probably fine, but just check reviews for what their imaging circle is.  This should be greater than the sensor diagonal otherwise you will get distorted stars at the edge.  There are lots of reducer combinations out there for refractors you don't need to commit to the 'approved' ones - the only advantage is that they are usually threaded for the correct telescope so no adapters are needed.  Just have a browse around and see what people are using successfully (e.g. on astrobin).

As for the filter, it's not just the sensor size that is important but also how close the filter is to the CCD. Light approaches the CCD as a cone so the further away the filter is the larger it has to be.  If you are placing your filter right next to the CMOS sensor you could probably get away with the 1.25" filter with these camera.  However, I'd probably recommend using the 2" filter, you can then use that with any 2" eyepieces you have then (although not ideal to use a CCD version for visual).

[vlaiv]

How much vignetting, if any, will there be from filters on a particular sensor depends on two things. F/ratio of the converging beam and distance of filter to sensor. It also depends on their sizes - you can't use filter that is smaller than sensor itself as that will cause vignetting in any case.

1.25" filter have about 28-29mm of clear aperture and that is large enough to cover 23mm diagonal of ASI294 if mounted fairly close.

To roughly get max distance of filter without vignetting - you need to do a bit of math. First you take free aperture of filter and subtract diagonal of sensor. let's go with 28.2 - 23.2 to get nice round number of 5mm. Divide this value with 2, resulting in 2.5mm. Next thing to know is F/ratio or speed of the beam. If you are using ED80 that has F/7.5 natively, and you use something like x0.8 reducer you will get F/6 beam. Now it is just a matter of multiplication max distance without vignetting = 2.5 * 6 = 15mm (roughly).

With asi294 you already get T2 ring with 1.25" filter thread that screws in camera nose piece (2" outer and T2 female inner diameter). That way filter sits less than 10mm away from sensor, and you are fine down to about F4.5 - F/5 with this combination.

Quick search online gives this image that shows configuration:

You can still use remaining T2 thread or 2" nose piece  for further connection to telescope.

Out of filter choices - you will need at least IR/UV cut filter because you are using refractive optics (and doublet). Might not be needed much for SCTs, but I would use it anyway just in case corrector plate causes a bit of color bloat.

CLS/CLS-CCD is rather aggressive general purpose filter and will cut deep into LP but will make havoc out of your color balance. UHC filter is even worse / better - depends how you look at it - it is very good for emission type targets but should be avoided for star clusters / galaxies and reflection nebulae. It will also throw off color very much.

I don't know about Optolong L-Pro filter, but looking at its response curve it looks like good general LP suppression filter. I do know that Hutech IDAS LPS filters are very good - I use P2 version for my LP levels and type of LP.

If you live in very light polluted area then it is worth having LPS filter, however, because all filters block light - at some point it will do more harm than good, so if you have mild light pollution where you shoot - I would avoid LPS filters.

As for field flatteners and reducers with ED80 - I think they will provide very flat field over 4/3 size chip, so you don't have to worry about that.

As far as SCTs are concerned - if you don't have Edge versions - these scopes have quite a bit of coma and sometimes mild spherical because of focus position (moving mirror changes distance between primary and secondary and only one exact position is free from spherical if surfaces are figured well). Focal reducer designed for SCT does take this into account and I believe they reduce coma - so I would recommend them if you plan on using SCTs for small targets.

However if you do plan to use them - I would suggest that you get OAG and make sure your mount really guides well and that you understand software binning as part of processing workflow. At these long focal lengths you will oversample and you need to recover back SNR by binning your data and have it at proper sampling rate for detail available - which is usually about 1.3-1.5"/px with these larger scopes, 1"-1.3"/px requires excellent seeing and excellent mount and optics.

 

[oymd]

2 hours ago, Whirlwind said:

An external focuser is something you attach to the back of the telescope.  The focusing mechanism on the SCT can be less precise and repeatable because of it's design (you are moving the main mirror). Because of mirror movement you can find that moving in/out doesn't bring the focus to the same position and hence isn't repeatable.  An external focuser avoids this problem (something like this https://www.firstlightoptics.com/moonlite-focusers/moonlite-cs-dual-speed-sct-crayford-for-325-thread-fr-compatible.html) but there a lot of second hand market versions and you might want to see if there is one that can be adapted to both the 8" and 11").  You can also get motorised versions which helps get more precise focus and means you don't have to touch the telescope.  Celestron have redesigned the focusers for the RASA versions that is meant to better but have yet to see whether they are going to bring it forward for the main line of Edges and SCTs in the future, that would probably remove the need in the future for external focusers.

The old celestron F6.3 reducer was really designed for smaller CCDs going back to the 2000's and has a quoted imaging diameter of 24mm.  Some of your options above are bigger than this so you will get distorted stars at the edges.  ASA made a version with a wider field as well as Optec and Starizona I believe. 

The Skywatcher reducers are probably fine, but just check reviews for what their imaging circle is.  This should be greater than the sensor diagonal otherwise you will get distorted stars at the edge.  There are lots of reducer combinations out there for refractors you don't need to commit to the 'approved' ones - the only advantage is that they are usually threaded for the correct telescope so no adapters are needed.  Just have a browse around and see what people are using successfully (e.g. on astrobin).

As for the filter, it's not just the sensor size that is important but also how close the filter is to the CCD. Light approaches the CCD as a cone so the further away the filter is the larger it has to be.  If you are placing your filter right next to the CMOS sensor you could probably get away with the 1.25" filter with these camera.  However, I'd probably recommend using the 2" filter, you can then use that with any 2" eyepieces you have then (although not ideal to use a CCD version for visual).

 

2 hours ago, vlaiv said:

How much vignetting, if any, will there be from filters on a particular sensor depends on two things. F/ratio of the converging beam and distance of filter to sensor. It also depends on their sizes - you can't use filter that is smaller than sensor itself as that will cause vignetting in any case.

1.25" filter have about 28-29mm of clear aperture and that is large enough to cover 23mm diagonal of ASI294 if mounted fairly close.

To roughly get max distance of filter without vignetting - you need to do a bit of math. First you take free aperture of filter and subtract diagonal of sensor. let's go with 28.2 - 23.2 to get nice round number of 5mm. Divide this value with 2, resulting in 2.5mm. Next thing to know is F/ratio or speed of the beam. If you are using ED80 that has F/7.5 natively, and you use something like x0.8 reducer you will get F/6 beam. Now it is just a matter of multiplication max distance without vignetting = 2.5 * 6 = 15mm (roughly).

With asi294 you already get T2 ring with 1.25" filter thread that screws in camera nose piece (2" outer and T2 female inner diameter). That way filter sits less than 10mm away from sensor, and you are fine down to about F4.5 - F/5 with this combination.

Quick search online gives this image that shows configuration:

You can still use remaining T2 thread or 2" nose piece  for further connection to telescope.

Out of filter choices - you will need at least IR/UV cut filter because you are using refractive optics (and doublet). Might not be needed much for SCTs, but I would use it anyway just in case corrector plate causes a bit of color bloat.

CLS/CLS-CCD is rather aggressive general purpose filter and will cut deep into LP but will make havoc out of your color balance. UHC filter is even worse / better - depends how you look at it - it is very good for emission type targets but should be avoided for star clusters / galaxies and reflection nebulae. It will also throw off color very much.

I don't know about Optolong L-Pro filter, but looking at its response curve it looks like good general LP suppression filter. I do know that Hutech IDAS LPS filters are very good - I use P2 version for my LP levels and type of LP.

If you live in very light polluted area then it is worth having LPS filter, however, because all filters block light - at some point it will do more harm than good, so if you have mild light pollution where you shoot - I would avoid LPS filters.

As for field flatteners and reducers with ED80 - I think they will provide very flat field over 4/3 size chip, so you don't have to worry about that.

As far as SCTs are concerned - if you don't have Edge versions - these scopes have quite a bit of coma and sometimes mild spherical because of focus position (moving mirror changes distance between primary and secondary and only one exact position is free from spherical if surfaces are figured well). Focal reducer designed for SCT does take this into account and I believe they reduce coma - so I would recommend them if you plan on using SCTs for small targets.

However if you do plan to use them - I would suggest that you get OAG and make sure your mount really guides well and that you understand software binning as part of processing workflow. At these long focal lengths you will oversample and you need to recover back SNR by binning your data and have it at proper sampling rate for detail available - which is usually about 1.3-1.5"/px with these larger scopes, 1"-1.3"/px requires excellent seeing and excellent mount and optics.

 

Extremely helpful replies!!

thank you so much...that was very very helpful!!

[2STAR]

I have  used my espirit 80mm f5 for imaging (mostly eaa), I have researched in detail and the 533 is a perfect match for my specific 80mm, the sq sensor fits most objects in the FOV I wish to image, and there are LOTS & LOTS of DSO Objects suitable !

I will be using the Optolong L-enhance duo band filter for my bortle 8 backyard where most of my imaging is done, it restricts a lot of LP and additionally brings out colour particularly in Emission Nebulla ,Just waiting for things  to settle down in China and delivery of my 533 from FLO.

ERIC

Edited February 21, 2020 by 2STAR