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RC vs Edge HD


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I've had my truss tube RC10 for almost 7 years now, and found it very satisfying.  It holds collimation very well, the carbon fibre construction is virtually unaffected by temperature changes, and with my particular setup is easy to clean and collimate (see here).  But I'm getting itchy fingers again.  As I get older and more decrepit I find the relatively heavy scope a bit of a handful to remove and replace on my mount when I want to use a different instrument or clean the mirror.  It's fairly fast at f8, but of course that means a larger central obstruction.  Some people don't like diffraction spikes, but actually I find them quite satisfying - really clean tight spikes are a useful indication of good focus.  The Dawes limit is 0.46 arc-secs - smaller than the 0.56 arcsecs per pixel of my unbinned QSI 683.

Nevertheless I've been considering an alternative, namely a Celestron 9.25 Edge HD.

Pros: protected mirrors in the closed tube - cool down not a problem because it will live permanently in my Observatory. Flat field (but of course my RC is very good), smaller central obstruction so hopefully crisper resolution, and almost 7 Kg lighter so will ride happily on my EQ6 Pro 'star party' mount.  The slightly longer focal length will be useful for smaller targets such as galaxies.  The Celestron 9.25 has always had a good press. and I assume the Edge HD will be the same.

Cons - 'slower' at f10, moving main mirror for focusing, and although it can be locked it relies totally on the original assembly to remain collimated with the rest of the optics.    With the slightly smaller mirror and longer focal length, the Dawes limit is 0.49, close to the 0.47 arc-secs per pixel of the camera.

But of course in both cases the final resolution will be limited by seeing conditions which for me are hardly ever better than 3 arc-secs fwhm unbinned.

So any and all comments/ opinions very welcome before I go spending and perhaps wasting a lot of money.

Cheers,

Peter

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GSO optics are pretty good really. Particularly for the price. ASA (who are extreme quality) tested the bigger GSO RC’s and confirmed as much. The mirrors are quartz which makes for very low expansion so cool down is very minimal unlike the Pyrex mirror of a Celestron. The GSO coating is also very good and very uniform. Its near 100% from 4-700nm and then tails off a bit.

The mirror coatings are a hard dielectric coating. I would assume aluminium but I don’t actually know. 

From whatI’ve been told the GSO mirrors are better then the EdgeHD, you might need to go up market to get a better Astrograph, maybe a planewave, however same time you want lower weight.

The corrector on the SCT still need cleaning , just because it’s sealed does not mean no maintenance. 


Only plus I can see is weight saving.

 

 

 

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16 minutes ago, Deadlake said:

The corrector on the SCT still need cleaning , just because it’s sealed does not mean no maintenance.

Yes indeed, the corrector does need to be cleaned.  But of course the surface is far less fragile than the mirror and easily accessible.  I've had SCT's in the past and with a suitable dew heater and dew shield no problems with the corrector.

But I take on board your comments about the mirrors.  Thanks for responding.   Maybe as you say the only advantage would be weight saving.

Cheers,

Peter

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Hi Peter,

If you have a high resolution rig which works... my advice would be, don't fix it!

I'm not dismissive of statistics in imaging but I'm wary of them. They are often over-ridden by realities which don't appear in all of the numbers.  One mystery concerns SCT star sizes, which tend to be rather large. Why this should be, when they are so good on the planets, I have no idea.  Perhaps the latest processing techniques using Starnet to de-star an image, before a careful re-starring operation, might eliminate this issue, I don't know.

While the corrector plate keeps everything clean, as you say, it is also a big piece of glass bringing issues like dew and scatter into play. Then there's cool-down...

Can you really beat an arcsecond per pixel with the seeing you have? With the QSI you only need just over a meter FL to get to that. I found that there was precious little advantage to be had from a 2.4 metre FL instrument at 0.6"PP over a 1 metre FL instrument at 0.9"PP. Being a 140mm refractor the latter instrument was easier in all practical respects, though it wasn't any cheaper.  I strongly suspect that, after a little over a metre, your FL won't be adding any value.

Nor do I think focal ratio, in itself, matters. What matters is the ratio, area of aperture to area of pixel.

As CMOS cameras with increasingly tiny pixels get better and better it remains to be seen whether amateurs will continue to use long FL scopes. Professionals will still want big scopes but they have access to big pixels.

Olly

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Hi, Olly and thanks for your interest.  Also well done getting that AZ jab when you did.

Anyway, I absolutely take on board what you (and 'Deadlake') say.  As you would see if you followed my link, mirror cleaning on my RC is straightforward and often never needs any attention to collimation after reassembly.  The only thing that hampers collimation is the generally poor seeing here - even using Metaguide to smooth out the oscillations I still find it very difficult to get the main mirror spot on.  But at the moment I think it's very close.  So 'If you have a high resolution rig which works', well, yes I have although with a smaller central obstruction maybe the resolution could be bettered.

The main thing is the weight.  Although at 16 - 17 Kg it's not desperately heavy, but it's very tricky getting the dovetail aligned then holding the scope with one hand while I tighten the clamp screws with the other.  When I had my 12" Meade ACF, it used tube rings.  They were easy to fix first, then I just had to lift the tube onto the rings where it would sit on its own while I clamped the rings.  It was of course 3 m focal length, but often I used a reducer simply because the seeing wasn't good enough to use the full fl.  It also had intermittent oval stars which I never really tracked down, and the focuser would sometimes slip.  None of that now 🙂  But since I sold my business and retired, almost 20 years ago now, and stopped throwing 25 Kg bags of plastic around, my upper body strength has deteriorated considerably.  25 Kg now is HEAVY!!

Never mind, the gym has reopened and after another week to ensure full protection from my second AZ jab, I'll be going back there.   So maybe things will improve a little although at my age reversing muscle loss is not easy😬

Keep well,

Cheers,

Peter

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5 hours ago, petevasey said:

Hi, Olly and thanks for your interest.  Also well done getting that AZ jab when you did.

Anyway, I absolutely take on board what you (and 'Deadlake') say.  As you would see if you followed my link, mirror cleaning on my RC is straightforward and often never needs any attention to collimation after reassembly.  The only thing that hampers collimation is the generally poor seeing here - even using Metaguide to smooth out the oscillations I still find it very difficult to get the main mirror spot on.  But at the moment I think it's very close.  So 'If you have a high resolution rig which works', well, yes I have although with a smaller central obstruction maybe the resolution could be bettered.

The main thing is the weight.  Although at 16 - 17 Kg it's not desperately heavy, but it's very tricky getting the dovetail aligned then holding the scope with one hand while I tighten the clamp screws with the other.  When I had my 12" Meade ACF, it used tube rings.  They were easy to fix first, then I just had to lift the tube onto the rings where it would sit on its own while I clamped the rings.  It was of course 3 m focal length, but often I used a reducer simply because the seeing wasn't good enough to use the full fl.  It also had intermittent oval stars which I never really tracked down, and the focuser would sometimes slip.  None of that now 🙂  But since I sold my business and retired, almost 20 years ago now, and stopped throwing 25 Kg bags of plastic around, my upper body strength has deteriorated considerably.  25 Kg now is HEAVY!!

Never mind, the gym has reopened and after another week to ensure full protection from my second AZ jab, I'll be going back there.   So maybe things will improve a little although at my age reversing muscle loss is not easy😬

Keep well,

Cheers,

Peter

My question would be can you build an observatory? 

I think the GSO 10" is under rated, particularly the new version where the focusers and body are not linked.

If you have a Tak collimation scope then the main complaint about this scope goes away.

 

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1 hour ago, Deadlake said:

My question would be can you build an observatory? 

I think the GSO 10" is under rated, particularly the new version where the focusers and body are not linked.

Thanks again for your interest.  But if you visit my original link (here it is again), particularly the collimation page, you will see that indeed I do have the version with the focuser mounted on the backplate.  And it's the substantial Teleskop Service 3" rack and pinion focuser - very strong and accurate.  By the way, yes I could build an Observatory.  But I don't need to - I've had one for 17 years 😄  If you can be bothered to wade through the page it's all here.  As you will see, being plywood and timber it has suffered from a certain amount of water damage over the years.  But so far still going strong.  Might need a professional carpenter after another couple of years.  We shall see.  If it finally gives up the ghost, perhaps a call to Pulsar will be in order.  But I'd rather keep the current one going if I can.

Cheers,

Peter

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  • 2 months later...
On 19/04/2021 at 08:40, ollypenrice said:

Hi Peter,

.........

Can you really beat an arcsecond per pixel with the seeing you have? With the QSI you only need just over a meter FL to get to that. I found that there was precious little advantage to be had from a 2.4 metre FL instrument at 0.6"PP over a 1 metre FL instrument at 0.9"PP. Being a 140mm refractor the latter instrument was easier in all practical respects, though it wasn't any cheaper.  I strongly suspect that, after a little over a metre, your FL won't be adding any value.

Nor do I think focal ratio, in itself, matters. What matters is the ratio, area of aperture to area of pixel.

As CMOS cameras with increasingly tiny pixels get better and better it remains to be seen whether amateurs will continue to use long FL scopes. Professionals will still want big scopes but they have access to big pixels.

Olly

Hi, Olly,

When the long twilight nights take over here in NE England, the itchy finger syndrome kicks in and I start looking at new equipment, which really I don't need!  But as the years catch up with me, as I said in this thread my nice RC10 has become a bit of a handful.  I'm mindful of your comments above, and have been wondering whether a good refractor of around  6" aperture would do the trick for me - I still like to image galaxies, but as you say smaller pixels at around a metre focal length would probably do just as well on an unobstructed instrument.  I've also been considering changing my monochrome QSI 683 for a one-shot colour CMOS camera, possibly the Altair or ASI offering with the Sony IMX294 sensor, virtually identical in size to the Kodak KAF8300 sensor in my QSI camera, but with slightly smaller pixels.  Even now I usually bin the QSI on my RC10, so the pixel coverage is the same as unbinned on a 1 metre focal length instrument.  But of course that also means shorter exposure times, useful with the UK climate!  Nevertheless the time saved by one-shot colour is tempting, and with the modern tri-band filters narrowband imaging is well covered.

Anyway I've been looking at refractors.  For the sort of focal length we're talking about here I already have a Meade Series 5000 127.  950 mm fl, so f7.5.  Does a good job, particularly since I fitted a better focuser, but the optics are probably 'middle of the road', and I suspect a more modern instrument with a larger aperture and good glass would do better. I would of course sell the Meade and the RC10, the funds would go towards the new refractor.  For wider fields I would still have my TMB105 and TS65 Quad.

I'm considering a handful.  With a slightly shorter focal length to my Meade, there is the quite 'fast' Sharpstar 140, available with a metal tube in the UK, also available with a carbon fibre tube from Teleskop Service.  They also do a specialist range and this 155 mm model is particularly interesting.  Not excessively heavy - 4 Kg lighter than the RC10.  The only worry there is the length of the scope - will it be too obtrusive in my Observatory or catch the wind when away from home on my EQ6 mount?  I need to play with mock up extensions on my Meade.  But although the focal length is a useful size, it's a bit 'slow'at f8.   A third possibility is the 150 mm Skywatcher Esprit.  Seems to be well thought of, nice 1040 mm fl, quite 'fast' at f7,  Weight of the bare tube is quoted as 14.5 Kg, but in another thread here it's stated as 12 Kg.  I would like to see that confirmed.   Anyway for both that instrument and the similarly weighty 155mm TS, I would fit the tube rings to the mount first then simply lift the OTA into the rings, as I used to do with my 12" Meade ACF.  Easy peasy 😉

So I wonder if you and other correspondents have any views you would like to share?

Cheers,

Peter

Edited by petevasey
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I just don't think you can beat refactors and camera lenses for imaging, especially in the UK and our skies.  Having used SCT's and RC's for deep sky, I'd absolutely echo Olly's sentiment that the claimed PP numbers do not tell the whole story.  I use two quality refactors and a 135mm camera lens for all my DSO imaging (TEC140, FSQ85 and Samyang 135m) and simply accept that the more zoomed-in imaging scale that RC's and SCT's with their longer FLs provide are not feasible targets in the UK and choose my targets accordingly.  After all, there are enough of them!  A photographic hunt/study of the UK visible Sharpless objects, for example, would consume every clear night in the UK for years. 

My 2penneth.

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For small targets I use Esprit 150s and ASI 178 cmos cameras  binned 2x2 to image at 0.94 arcsec per pixel, which is usually a bit optimistic for my seeing. I must try the Starnet processing technique to see if I can get tighter stars.

At the ripe old age of 63 I regularly lift the scopes on and off open tube rings in the confines of a Pulsar dome, to date with no drama.🤞

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I used to image with Yves van den Broek's 14 inch ODK at 0.62"PP. A kit shuffle here saw me switch to a TEC 140 at 0.9"PP with an Atik 460. Personally I find little to choose between the results. I did a back to back on here a while ago and some members felt the big scope sometimes captured more useful detail. Personally I found precious little to choose and preferred the rafractor stars to the spiked ones. One or two targets ware very difficult on the big one due to out of shot beams from bright stars. The obvious thing to try would have been binning 2x2 but the SX H36 would not bin satisfactorily. (Dark triangular artifacts next to stellar cores even after electronic adjustment.)

I'll link to some results from the TEC so you can decide what you make of them.

https://www.astrobin.com/omc9sk/

https://www.astrobin.com/full/6f5tfl/0/

https://www.astrobin.com/full/419975/0/

https://www.astrobin.com/full/miqpyu/0/

https://www.astrobin.com/full/393219/0/

Since I have so many instruments on site now I really don't much care for tinkering and like the plug and play way of imaging with a large refractor. However, you're rather an expert at getting the best out of tricky stuff and might enjoy that aspect. We all know that, in the end, a big scope will win but as amateurs in the real world I'm not sure that we are anywhere near 'the end' in question!

Olly

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My ODK 12 runs 0.61"/px with the G3 16200 camera, so may hardware bin 2x2 as I doubt I will lose much, if any, actual resolution. Just means running off another set of calibration frames. Of course, that depends on how well it bins in hardware something to test the next clear night. Next year perhaps.

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Thanks, Guys, a good start.  But if I decide to go all refractor, which of the ones I've mentioned?   I know, Olly, that the TEC140 is reckoned to be one of the best around, but they are very expensive hen's teeth!  And any opinions on one shot colour CMOS cameras?

Cheers,

Peter.

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Here's a carbon fibre 6" triplet.  No problem weight wise.  Pricey!  Shown as out of stock, but available.   I'm not sure about the optics.  Anyone seen one?

Cheers,

Peter

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17 hours ago, ollypenrice said:

I used to image with Yves van den Broek's 14 inch ODK at 0.62"PP. A kit shuffle here saw me switch to a TEC 140 at 0.9"PP with an Atik 460. Personally I find little to choose between the results. I did a back to back on here a while ago and some members felt the big scope sometimes captured more useful detail. Personally I found precious little to choose and preferred the rafractor stars to the spiked ones. One or two targets ware very difficult on the big one due to out of shot beams from bright stars. The obvious thing to try would have been binning 2x2 but the SX H36 would not bin satisfactorily. (Dark triangular artifacts next to stellar cores even after electronic adjustment.)

I'll link to some results from the TEC so you can decide what you make of them.

https://www.astrobin.com/omc9sk/

https://www.astrobin.com/full/6f5tfl/0/

https://www.astrobin.com/full/419975/0/

https://www.astrobin.com/full/miqpyu/0/

https://www.astrobin.com/full/393219/0/

Since I have so many instruments on site now I really don't much care for tinkering and like the plug and play way of imaging with a large refractor. However, you're rather an expert at getting the best out of tricky stuff and might enjoy that aspect. We all know that, in the end, a big scope will win but as amateurs in the real world I'm not sure that we are anywhere near 'the end' in question!

Olly

Many thanks, Olly,

I've now had a look at your images.  And I have to admit they compare very favourably to my versions taken with much larger telescopes.  Having said that, of course your skies are darker and more steady than mine.  But it shows what is possible with a fine refractor.  Hopefully if I go for one of the 6" I'm looking at, the larger aperture will balance out the lesser optics compared to the TEC.  But I need to find some objective reviews before taking any expensive decisions!  Just to add, I've been looking at some of the images from my Meade 127.  And considering that it is much more of a mass produced instrument, the images are quite pleasing.   But it is over 12 years old, although given a new lease of life with a replacement focuser.

Cheers,

Peter.

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When comparing large and small instrument - following applies:

1. if you set your target sampling rate - say 1.1"/px, then something like 5" refractor will be x4 as slow as 10" RC.

Once resolution is set - aperture tells you how fast system is. More aperture - faster the system, simple as that (in fact speed increase is ratio of aperture surfaces - and even large CO in 10" reduces surface by very small amount).

2. In same seeing and with same mount performance - larger instrument will out resolve smaller one.

Just by how much - depends on many factors (or rather relation of these factors - seeing, mount performance and aperture size). Quality of optics as long as both are diffraction limited (Strehl>=0.8) is the least important component for instruments above 2-3" and makes virtually no difference in long exposure imaging.

You don't need particularly good seeing to get measurable difference - 10" will out resolve 5" in 2" FWHM seeing and 0.5 RMS guiding: 1.48"/px vs 1.55"/px, but these measurable differences will be hard to notice by eye in processed image. Even x2 difference in resolution does not cause severe drop in image quality (at least not one we would characterize as x2 drop in resolution).

 

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6 hours ago, vlaiv said:

When comparing large and small instrument - following applies:

1. if you set your target sampling rate - say 1.1"/px, then something like 5" refractor will be x4 as slow as 10" RC.

Once resolution is set - aperture tells you how fast system is. More aperture - faster the system, simple as that (in fact speed increase is ratio of aperture surfaces - and even large CO in 10" reduces surface by very small amount).

2. In same seeing and with same mount performance - larger instrument will out resolve smaller one.

Just by how much - depends on many factors (or rather relation of these factors - seeing, mount performance and aperture size). Quality of optics as long as both are diffraction limited (Strehl>=0.8) is the least important component for instruments above 2-3" and makes virtually no difference in long exposure imaging.

You don't need particularly good seeing to get measurable difference - 10" will out resolve 5" in 2" FWHM seeing and 0.5 RMS guiding: 1.48"/px vs 1.55"/px, but these measurable differences will be hard to notice by eye in processed image. Even x2 difference in resolution does not cause severe drop in image quality (at least not one we would characterize as x2 drop in resolution).

 

Absolutely!  But despite all that, when I compared for example the image of M51 which Olly posted with my own recent image here there is no doubt that Olly's image is superior in the fine detail.  Of course the TEC is an excellent instrument with no central obstruction.  My image was using the camera binned which will give the same resolution as unbinned on a 5" refractor.    I take the point about imaging time.  But bear in mind that I'm also considering a one-shot CMOS colour camera.  It would appear that  CMOS cameras now can outperform CCD in many ways.  And as I said when I started this thread, handling the RC10 truss is a bit of a problem - I dread the day when it gets away from me and crashes to the floor 😬

Cheers

Peter 

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On 20/06/2021 at 16:58, tomato said:

For small targets I use Esprit 150s and ASI 178 cmos cameras  binned 2x2 to image at 0.94 arcsec per pixel, which is usually a bit optimistic for my seeing. I must try the Starnet processing technique to see if I can get tighter stars.

At the ripe old age of 63 I regularly lift the scopes on and off open tube rings in the confines of a Pulsar dome, to date with no drama.🤞

Hi 'tomato',

Would you be so kind as to confirm the weight of the Esprit 150, preferably bare or with tube rings etc.  You may be aware that John Turley reckons the bare tube is 12 Kg rather than the declared 14.5 Kg.  Like yourself I would fit the tube rings first, as I used to with my Meade 12" ACF.  That's part of my problem with the RC10 truss - lots to get hold of but difficult to balance with one hand while tightening the dovetail clamp. At my very ripe old age of 77 I find the 17 Kg quite a handful!  But if I do go for an Esprit 150, it would be good to know the true weight in the hope of being able to use it on my 'star party' EQ6 mount.

Cheers,

Peter

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Hi Peter,

From my notes I have the Esprit 150 weighing in at 12.7 kg, that is with the focuser attached, but no tube rings, camera or filter wheel attached. For lifting the OTA is decidedly front end heavy, no doubt due to the substantial piece of glass at that end.

Good luck with whatever you decide.

Cheers

Steve

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  • 11 months later...

I continued to agonise over which way to jump - eventually looking hard at 6" refractors.  But have decided to stay as I am.  As Olly Penrice stated earlier in this thread 'If you have a high resolution rig which works... my advice would be, don't fix it!' .  And that's what I'm doing for now.  EXCEPT I have done some fixing of my 5" Meade 127 refractor.  Detailed in my thread 'Meade 127 Blue Halos cure' here.  So with the refractor performing better, the itch is much diminished 😉

Cheers,

Peter

 

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3 hours ago, davidc135 said:

Would fixing up some kind of hoist be worthwhile?  David

Nice Idea David.   I had a little hydraulic hoist in my business years ago - very useful for moving electric motors, extruder barrels etc.  Similar to this one.  But no way one would fit in my Observatory. And the Obsy roof certainly wouldn't be happy with a chain hoist hanging from it!  I am able to handle the truss tube RC with care - there are lots of places to hold on to, it's just a matter of getting the balance right.  And I've started going back to the gym 😉

Cheers,

Peter

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On 18/04/2021 at 21:52, petevasey said:

I've had my truss tube RC10 for almost 7 years now, and found it very satisfying.  It holds collimation very well, the carbon fibre construction is virtually unaffected by temperature changes, and with my particular setup is easy to clean and collimate (see here).  But I'm getting itchy fingers again.  As I get older and more decrepit I find the relatively heavy scope a bit of a handful to remove and replace on my mount when I want to use a different instrument or clean the mirror.  It's fairly fast at f8, but of course that means a larger central obstruction.  Some people don't like diffraction spikes, but actually I find them quite satisfying - really clean tight spikes are a useful indication of good focus.  The Dawes limit is 0.46 arc-secs - smaller than the 0.56 arcsecs per pixel of my unbinned QSI 683.

Nevertheless I've been considering an alternative, namely a Celestron 9.25 Edge HD.

Pros: protected mirrors in the closed tube - cool down not a problem because it will live permanently in my Observatory. Flat field (but of course my RC is very good), smaller central obstruction so hopefully crisper resolution, and almost 7 Kg lighter so will ride happily on my EQ6 Pro 'star party' mount.  The slightly longer focal length will be useful for smaller targets such as galaxies.  The Celestron 9.25 has always had a good press. and I assume the Edge HD will be the same.

Cons - 'slower' at f10, moving main mirror for focusing, and although it can be locked it relies totally on the original assembly to remain collimated with the rest of the optics.    With the slightly smaller mirror and longer focal length, the Dawes limit is 0.49, close to the 0.47 arc-secs per pixel of the camera.

But of course in both cases the final resolution will be limited by seeing conditions which for me are hardly ever better than 3 arc-secs fwhm unbinned.

So any and all comments/ opinions very welcome before I go spending and perhaps wasting a lot of money.

Cheers,

Peter

If you plan to image with an SCT, I don't recommend them. I've had nothing but headaches  with it. Mirror moves, corrector plate=dew magnet, cool down is slow, it focuses by moving the mirror ... don't really know why people still buy them. Maybe because on paper they seem to tick many boxes, but the actual experience for me was a waste of money and time.

I have replaced it with a quality refractor, and never looked back. I can actually enjoy the hobby with a frac ... 

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