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tomato

A well sorted RC has the edge over big refractors for imaging small DSOs?

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I’m now seriously thinking about a retirement imaging set up, it will literally be a once in a lifetime opportunity, and it will be a collaborative project so it needs to be the right decision. It looks like an EU purchase so starting to get a bit nervous about what might happen to the GBP post Brexit.

I have been imaging with am AA 102 mm APO on a Mesu 200 and Moravian G2-8300 and enjoying it immensely, but my favourite targets are galaxies so a scope with more aperture and focal length is required. I have thought long and hard about a larger aperture longer FL refractor (eg CFF 140 f7.5), it would probably be the least risky and easiest route to success, but I like the idea of co-owning and mastering a scope that requires a bit more maintenance and well frankly, looks the part. To see what I mean take a look at the attached photos, they are both superbly engineered and crafted instruments, but IMHO one looks like it would not be out of place in an f1 garage or NASA clean room, the other, well, just looks like a telescope. I really enjoyed a thread on SGL by the great Rob Hodgkinson a while back where he retro engineered a GSO RC and ultimately produced some absolutely stunning results, and although I  won’t ever be at that level, that’s what I’m aspiring to.

So with hopefully no offence to refractor aficionados and provided you don’t think I’m mad for  letting a bit of emotion influence the decision, my key questions are:

On a Mesu mount and housed in a dome, and all other things being equal should the 250 mm CFF RC be capable of producing at least as good as (if not slightly better than)images taken with the CFF 140mm f7.5 frac?

The budget could maybe stretch to the 300mm RC, but I think the arc secs per pixel is getting too much for UK skies  and maybe the guiding capability, so would the extra aperture it be worth it?

4992D06D-AFBE-4CC9-A455-D960797AC5F3.jpeg

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My view on the subject is that aperture on given resolution always wins (note on "given resolution" part).

There are other benefits like tighter stars in same conditions.

Problem with larger scopes is finding matching sensor that will provide you with the target resolution.

Given that you plan to put it on Mesu 200 why the worries about guide precision? I would expect that such setup if properly shielded from wind and on good pier with OAG is capable of sub 0.3" RMS guide precision?

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Thanks for the reassurance on the guiding, I currently set up and take down for every session so I have rarely achieved the full guiding performance from the Mesu, but to be fair I can get sub 0.5 arcsecs total RMS, if I sacrifice some imaging time to accurate alignment.

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Hi,

as an option, why not think about a 10" or12" GSO truss F4 newt? 1000mm - 1200mm focal length, fast for DSO's. Cheaper than the RC, and less hassle. You could then simply put a barlow in and image at F8 for small galaxies, planetaries etc. I am seriously thinking about going this way on my Mesu...

I currently have a friend setting this very combination up for a remote imaging site. Being an engineer he is doing a couple of mods o improve it :)

cheers

Gary

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Tomato,

i actually bought Rob's scope and it is an absolutely cracking bit of kit however, and in no way see this as an advert because it is not, I am firmly heading down the refractor route for simplicity and ease. The RC does need some tinkering, though not much, but I now have the big refractor (APM 152/1200 Lomo) as well as a multi-scope widefield rig though this is more about beating the weather. However, you are right, for galaxies (and smaller planetaries) the RC takes a huge amount of beating; it is 2400mm native though Rob often reduced down to 1600mm. My current skillset and general lack of time just can't do it justice so i do have a decision to make.

Properly balanced, the Mesu will have no trouble and be more than capable, and in my SkyPOD (2.2m) the big RC fits, allbeit parked horizontally. Rob had his on a big mount -  AP1200, which i think the Mesu should best, but I am biased :icon_biggrin:.

I would take zero offence with your refractor views. Refractors just suit me at the moment. 

Edited by ampleamp

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I recently changed from an ODK10 to a refractor - The focal length in the frac is slightly less (1200mm as opposed to 1700mm) but when paired with the Sony chip camera the imaging scale is the same so I haven't really lost anything. 

I changed purely because the open tube design was going to start causing me issues in a while when the mirrors needed cleaning and there was no way I was up to the job. I actually don't think that there's much between the frac and the ODK in regards of image quality at the end of the day. The ODK to be fair needed pretty much zero by way of tinkering, it held its collimation perfectly. The frac of course needs nothing doing to it! But value for money as well the refractor does NOT win the contest!

All in all, I do not regret the decision I made, refractors suit me and my technical skill level ..... zero!! Had I been more technically minded and able then I really don't think I'd have sold the ODK at all.

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Hi Tomato

(Still laughing at Sara's comment about having a skill level of zero - not sure how far into the negative skill levels that leaves me!)

Anyway, I'm a serious refractor fan and use a 150 mm apo as my main toy of choice. I second all of the benefits of refractors that previous responses to your post have pointed out. However, you mentioned that your favourite targets are galaxies. That would sway it for me - for most galaxies the plate scale of the refractor would in my view be too small. Sure, you can put an extender in the optical train as I've done, but that makes for a very slow system. So if galaxies are where it's at for you, you're happy with the fact that you're going to have to do some occasional adjustment to keep the system running at the peak of its capability, then I would go for the RC. You could always mount your existing apo on top (personally, if you went for the RC, I'd be tempted to piggyback a smaller apo, something like a 70 or 80 mm, to cover those wide field opportunities.

Also think about plate scales and make sure that your optical and focal plane systems are reasonably matched so that you get the benefit from the plate scale of the RC system.

Nigel

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Tomato,

I am also in the progress of building my "retirement" system.

I already have a WO 110FLT and although not had as much time to use it as I would like, the views are stunning. But for galaxies, especially the more distant ones, you would probably benefit from a longer focal length, as Nigel says. Having said that, I also have a 12" f4 Newt and with careful setting up, has also done well on DSO's...

So, while I absoluteley agree with the feeling the CFF RC would fit well in an F1 garage, I ended up ordering an OO ODK instead.

For around the same price as the 10" CFF is quoted on Telescope Express, you could get the 12" ODK. Extra aperture, with around the same fl. If I remember correctly, Olly had (has) a 14" on a Mesu with no issues.. The OO is a closed tube, so lacks the flash looks of the truss designs, so I am not sure if the appearance of the scope carries any weight with you..

Good luck with your decision.

Gordon.

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Thanks to all for the replies, I’m glad that I don’t appear to have upset any refractor enthusiasts with my visual appeal comparison.

Being retired, I should be time rich but after the scope and dome outlay definitely cash poor, so some of the drawbacks of the RC can be overcome. One of the reasons for going with the CFF is to have a well engineered, gremlin free RC right out of the box. My fellow collaborator and investor in the project is a very able (also retired) mechanical and electronics engineer so between us I think we can manage the technical challenges of this set up.

So far, it looks like the overall view of the refractor being the less hassle option but the RC being able to deliver the optimum results on galaxy targets is still looking good:icon_biggrin:

None of the existing scopes we have are going as part of this job, so it could be we will be testing  the Mesu’s load capacity down the road.....

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Personally I think vlaiv's comment on using a camera that matches the FL of the scope merits a great deal of consideration.
If you are going with a RC of 2 metre FL, or longer, AND a large imaging circle then it doesn't make much sense to "waste" those expensive characteristics with a small sensor camera with small pixels. But going with a camera like a FLI, Moravian or SBIG that can both use the wide image and gives a sensible arc-sec to px match gets expensive - even more so when you are looking at another £2k just for the filters!

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Totally agree about the camera, but the initial spend will have to stop somewhere!

Will seriously be considering another camera, once all the challenges with a new set up have been sorted and this becomes a limiting factor.

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I'm really liking the look of the new range of 16200 based cameras. A good imaging area and pixel size without running into the expense of the 16803 behemoths. Filters are slightly less ruinous too. Mind you, 3nm Astrodons are ruinous in *any* size :eek:.

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On ‎11‎/‎02‎/‎2018 at 00:46, gazza said:

Hi,

as an option, why not think about a 10" or12" GSO truss F4 newt? 1000mm - 1200mm focal length, fast for DSO's. Cheaper than the RC, and less hassle. You could then simply put a barlow in and image at F8 for small galaxies, planetaries etc. I am seriously thinking about going this way on my Mesu...

I currently have a friend setting this very combination up for a remote imaging site. Being an engineer he is doing a couple of mods o improve it :)

cheers

Gary

Hi Gary

Once upon a time I too thought adding a Barlow would increase my focal length and let me image smaller objects. But then I realised that although a Barlow could magnify an object, it wouldn't increase resolution. Worse still, longer exposures needed would decrease the s/n. So all in all adding a Barlow wouldn't be good for imaging. I wish there was such a simple solution  to the problem!

Louise

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Hi!

I used to be a great fan of the RC design but have had much more use of refractors over the years. Setting the spikes/no spikes and halos/no halos issues aside I nowadays think a good quality refractor is the all around winner. Easy to use, handle, clean, transport, store etc. Hubble and all other big ones would be refractors no doubt if it wasn't for weight and engineering drawbacks.

Luckily today we have such a wide range of CCDs/CMOSs to choose from that it's easy to match scopes/cameras vs the seeing you can expect.

What a delightful problem to have on ones hand!

/Jesper

 

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20 hours ago, Thalestris24 said:

Hi Louise,

If you compare an F4 newt + Barlow, with a same size RC imaging at F8, with the same camera,the resolution will be the same as the focal length will be the same as will the required exposures.

Always beats me why people buy RC's that are F8, then desperately try to get a focal reducer to shorten their exposures! 

cheers

Gary

 

Hi Gary

Once upon a time I too thought adding a Barlow would increase my focal length and let me image smaller objects. But then I realised that although a Barlow could magnify an object, it wouldn't increase resolution. Worse still, longer exposures needed would decrease the s/n. So all in all adding a Barlow wouldn't be good for imaging. I wish there was such a simple solution  to the problem!

Louise

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22 minutes ago, gazza said:
21 hours ago, Thalestris24 said:

Hi Louise,

If you compare an F4 newt + Barlow, with a same size RC imaging at F8, with the same camera,the resolution will be the same as the focal length will be the same as will the required exposures.

Always beats me why people buy RC's that are F8, then desperately try to get a focal reducer to shorten their exposures! 

cheers

Gary

 

Hi again

Re: Barlow I think if that were the case then many APers would use a Barlow to increase their focal lengths. But they don't! Resolution is determined by (diffraction limited) aperture. Anything in the optical train either after a primary lens group or primary mirror will do nothing to increase resolution for a given sensor.

https://en.wikipedia.org/wiki/Barlow_lens

I think people use focal reducers to increase speed and widen fov. That's because most DSO's are quite big. For smaller ones that can fit in the f8 fov, reducers aren't necessary.

Louise

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55 minutes ago, gazza said:

Hi Louise,

If you compare an F4 newt + Barlow, with a same size RC imaging at F8, with the same camera,the resolution will be the same as the focal length will be the same as will the required exposures.

Always beats me why people buy RC's that are F8, then desperately try to get a focal reducer to shorten their exposures! 

cheers

Gary

Not quite as easy as that, there are numerous things to consider when choosing resolution to work with.

In long exposure AP, actual resolving power of telescope plays a part, but only a part in achieved resolution. Other two contributing factors are seeing and guide/tracking precision.

"Speed" of telescope is not important in classical sense like with day time photography. Because each telescope can be paired with suitable camera, if one aims at particular resolution, there is no "slow" and "fast" scope. 10" of aperture at 1"/pixel is same regardless if one chooses F/4 newtonian or F/8 RC (with suitable cameras each to achieve 1"/pixel).

What differs is following:

F/8 RC will have flatter field and less aberrations over the same field like F/4 newtonian (with or without barlow). F/8 RC will have less stay light / better baffling than F/4 Newtonian. F/4 Newtonian will be more sensitive to collimation. Mount will carry F/8 RC much easier than F/4 Newtonian for same aperture. Backfocus for RC will not be a problem, as there will be enough room for all accessories.

For example I used to do a bit of imaging with F/6 8" newtonian. Usable field without coma corrector was about 8.5mm. My Heq5 had real problem carrying such a large OTA. Now I image (in 8" range) with TS/GSO 8" RC. I use sensor with diagonal 21.9mm without much aberrations right to the field edge (something like field of 8.5mm in previous setup but without coma, only traces of field curvature in corners). Heq5 manages this OTA quite happily.

If you compare usable fields in these two setups you will see that RC has indeed larger aberration free field. Take a look at this comparison:

http://astronomy.tools/calculators/field_of_view/?fov[]=727||272||1|1|90&fov[]=1||185||1|1|90&messier=51

And this is F/6 Newtonian not F/4.

 

 

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Back on topic, 10" RC paired with KAF 8300 could turn out to work rather well.

With 2x2 binning resolution would be around 1.1"/pixel. It would produce images that are around 1600x1200 - quite nice too look at on today's monitors in full screen (not sure about print resolution, never done much printing).

No need for any sort of field flatteners for that chip size but I suspect 1.25" filters are out of the picture with that sensor size (I use them on ASI1600 with 8" RC and I can get away with slight vignetting).

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29 minutes ago, vlaiv said:

Not quite as easy as that, there are numerous things to consider when choosing resolution to work with.

In long exposure AP, actual resolving power of telescope plays a part, but only a part in achieved resolution. Other two contributing factors are seeing and guide/tracking precision.

"Speed" of telescope is not important in classical sense like with day time photography. Because each telescope can be paired with suitable camera, if one aims at particular resolution, there is no "slow" and "fast" scope. 10" of aperture at 1"/pixel is same regardless if one chooses F/4 newtonian or F/8 RC (with suitable cameras each to achieve 1"/pixel).

What differs is following:

F/8 RC will have flatter field and less aberrations over the same field like F/4 newtonian (with or without barlow). F/8 RC will have less stay light / better baffling than F/4 Newtonian. F/4 Newtonian will be more sensitive to collimation. Mount will carry F/8 RC much easier than F/4 Newtonian for same aperture. Backfocus for RC will not be a problem, as there will be enough room for all accessories.

For example I used to do a bit of imaging with F/6 8" newtonian. Usable field without coma corrector was about 8.5mm. My Heq5 had real problem carrying such a large OTA. Now I image (in 8" range) with TS/GSO 8" RC. I use sensor with diagonal 21.9mm without much aberrations right to the field edge (something like field of 8.5mm in previous setup but without coma, only traces of field curvature in corners). Heq5 manages this OTA quite happily.

If you compare usable fields in these two setups you will see that RC has indeed larger aberration free field. Take a look at this comparison:

http://astronomy.tools/calculators/field_of_view/?fov[]=727||272||1|1|90&fov[]=1||185||1|1|90&messier=51

And this is F/6 Newtonian not F/4.

 

 

The point was that I didn't think it was a good idea to add a Barlow to increase focal length for DSO AP.

Louise

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20 minutes ago, Thalestris24 said:

The point was that I didn't think it was a good idea to add a Barlow to increase focal length for DSO AP.

Louise

My post was primarily addressing Gary's post about F/4 Newtonian, if I somehow quoted you instead, I apologize.

On the matter of using Barlow, I don't see why on its own it would be a bad idea if one knows what they are doing and why they are using a barlow lens. It is true that barlow will not help with limiting resolution of optical system that depends on aperture alone, but most of long exposure AP is done on resolutions quite a bit coarser than limiting resolution (for 8" aperture I believe critical resolution is around 0.25"/pixel), so if system resolution (given camera on a given scope) is defaulting to let's say 2"/pixel - using x2 barlow would bring it down to 1"/pixel - still usable resolution for long exposure AP (if mount and skies support it), or some sort of short exposure / cross with lucky imaging approach that has been used for planetary nebulae for example.

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I think what I'm getting at is being misunderstood.

If someone is thinking of buying an RC, lets say a 10" at F8, they will have an 80" focal length.

They can alternatively get the 80" focal length by using a 10" F4 newt with a 2x barlow, but still have the flexibility if imaging at a 40" focal length if they desire. The resolution will obviously be determined by the aperture and the pixel size of the camera. I am not discussing that at all. There will be no difference between the image scale, the exposure length or the resolution of the 2 systems used at F8, just that a nice truss newt will have the flexibility of short or long focal lengths.

Resolution CAN be increased by using a barlow under the correct circumstances - planetary imagers do it all the time.

Gary

Edited by gazza

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As vlaiv noted, of these two scope combinations (10" F4 Newt with barlow & 10" F8 RC), the RC should give the wider, flatter field, hence why they are usually considered to be more suitable for AP.

Maybe the Newt will be be more flexible, especially if you wish to use it for visual observation (although I understand that RC are ok for visual use), but there is usually a compromise to be made, you just need to determine where your priorities lie. Or buy 2 scopes :evil4:

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4 minutes ago, gazza said:

I think what I'm getting at is being misunderstood.

If someone is thinking of buying an RC, lets say a 10" at F8, they will have an 80" focal length.

They can alternatively get the 80" focal length by using a 10" F4 newt with a 2x barlow, but still have the flexibility if imaging at a 40" focal length if they desire. The resolution will obviously be determined by the aperture and the pixel size of the camera. I am not discussing that at all. There will be no difference between the image scale, the exposure length or the resolution of the 2 systems used at F8, just that a nice truss newt will have the flexibility of short or long focal lengths.

Resolution CAN be increased by using a barlow under the correct circumstances - planetary imagers do it all the time.

Gary

Not at all, I perfectly understand what you are saying, and certainly you are right, using x2 barlow at F/4 Newtonian will offer same light grasp and resolution (or FL) as using F/8 RC.

But it will also introduce additional glass elements in optical path (might or might not be an issue), and again field correction / usable field will not be the same in those two systems. At F/4 newtonian have a loads of coma and usable field without coma corrector is rather small. Using barlow + coma corrector is not going to work good. Remarks on baffling / stray light protection, weight and compactness also stand.

If one plans to give emphasis on galaxies and high resolution work, it makes more sense to choose optical system that is suited for such work out of the box. RC F/8 on the other hand can also be used as wide field instrument. One just needs to change the approach - making mosaics of binned images rather than single frame. Yes it is easier to get wide field by using different instrument, but it just boils down to what is your main preference.

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1 hour ago, vlaiv said:

My post was primarily addressing Gary's post about F/4 Newtonian, if I somehow quoted you instead, I apologize.

 

It's ok - my mistake!

Louise

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On 13/02/2018 at 23:56, vlaiv said:

..........No need for any sort of field flatteners for that chip size but I suspect 1.25" filters are out of the picture with that sensor size (I use them on ASI1600 with 8" RC and I can get away with slight vignetting)......

I use 1.25" filters on the QSI683 and Moravian G2-8300 (Both KAF8300 sensors) - Vignetting? Yes, but flats deal with it :) 

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