RASA 8 vs Refractor

[AXE]

Hi everyone, my first post after lurking the forums for a while.

I'm currently trying to decide on my new imaging scope and haven't been able to find much helpful info so i'm putting it out there to you good folk. For context I've been using an 8" F5 Newtonian and a ZWO 294.

I'm after something with a wider field of view to image nebulas and some of the larger star clusters and i'm trying to pick between three scopes:

1. RASA 8
2. Skywatcher Esprit 80
3. WO Redcat 51

Out of these the Redcat has the widest FOV but i'm a bit concerned about the 51mm aperture and whether it can pull in enough light within reasonable time frames and also under-sampling on the 294 camera. The RASA is obviously going to be the quickest but it's a dedicated astrograph whereas I can use the other two for visual as well. The 80mm is a tried a true option but I like the extra aperture of the RASA.

I suppose for the money I can buy the Redcat and a camera like the ZWO 183MC Pro with smaller pixels for the same as a RASA. Then again the 80mm is a proven concept and should work with the 294.

Any thoughts?

[gregbradley]

Redcat 51 is a good mini scope but its more of a lens than a telescope.

A good 300mm lens would get similar results.

The others are telescopes and the RASA would be the better astrograph if you have the mount for it.

What mount do you have? The mount is more important than the scope.

Greg.

[AXE]

Should have added that the mount is an EQ6R Pro so all three scopes are well within it's limits.

[gregbradley]

Well the Redcat 51 is a great little widefield scope/lens. But as you say 51mm is pretty small. I use mine as a DSLR type widefield scope to get large objects in a broad vista.

There are some fabulous examples of what can be achieved with 71mm scopes on this forum. Andy's work stands out. I am often amazed at what he achieves with a small refractor from his inner city location.

The refractor will give pleasant stars and sharp widefield images.
If you live in light pollution then you can simply get narrowband filters and do narrowband imaging like Andy does.

The RASA is 200mm in aperture and that is pretty large. It would compete with the Skywatcher 8 inch RC scopes that routinely put up great images.

I am not that familiar with RASA images but F2 does sound seductive if the stars look good and its sharp. I suspect its up against a number of good imaging scopes for the same price though. 8-10 inch F4 Newtonians are also popular with lots of fabulous images from them as well.

Greg.

[xelasnave]

I have a 80mm espirit and zwo 1600 mono and it is great.
I was thinking a RASA 8 but if you want narrow band the filters cost and arm and a leg...plus you can't use a filter wheel. Each filter has to be changed manually..that finally put me off.
I also have a eight inch f5 and they are rather good..
But I bought a 115 mm refractor which should be ideal.
Alex

[The_bluester (Paul)]

I have been really considering a RASA 8" to pair with my ASI294, the focal length is more or less the same as my little ED72 so about the same FOV and the 72mm does do pretty well and with enough subs the undersampling can be fairly well recovered using drizzle processing. I don't imagine the RASA being any different in that regard. but at F2 I could do in under 2 minute subs what I am getting out of the 72mm in 5 minute subs. It would make it possible to get in single good night the equivalent integration wise of around ten hours exposure time that I am doing now!

What has stopped me so far (Aside from finances) is not having found a great number of images yet without significant "Issues" Tilt and Focus being the most obvious. At F2 I wouldn't consider one without motorised focus, and also with the small critical focus zone that F2 implies I imagine they will be very, very sensitive to sensor tilt issues.

I have recently found a couple of images on astrobin that indicate those issues can be conquered and I am still pretty tempted. People are doing NB with them too with the ASI1600 and a now available modified filter drawer to enable quick filter changes, but flat frames are an issue as dust bunnies would change with every filter change. I think if you had an obs and a list of targets you could get around that for automated imaging by shooting a number of targets in a night with a single filter so you can shoot and apply one set of flats to the whole night, then change filters for the next night and go again. It is either that or no flat frames, or sitting up with it changing filters and shooting flats all night!

[Startrek (Martin)]

You could keep your 8” f5 and buy a ZWOASI071MC ( APS-C ) which will increase your FOV by 10 to 15 arc minutes
Just an option ?

[gregbradley]

Originally Posted by The_bluester

I have been really considering a RASA 8" to pair with my ASI294, the focal length is more or less the same as my little ED72 so about the same FOV and the 72mm does do pretty well and with enough subs the undersampling can be fairly well recovered using drizzle processing. I don't imagine the RASA being any different in that regard. but at F2 I could do in under 2 minute subs what I am getting out of the 72mm in 5 minute subs. It would make it possible to get in single good night the equivalent integration wise of around ten hours exposure time that I am doing now!

What has stopped me so far (Aside from finances) is not having found a great number of images yet without significant "Issues" Tilt and Focus being the most obvious. At F2 I wouldn't consider one without motorised focus, and also with the small critical focus zone that F2 implies I imagine they will be very, very sensitive to sensor tilt issues.

I have recently found a couple of images on astrobin that indicate those issues can be conquered and I am still pretty tempted. People are doing NB with them too with the ASI1600 and a now available modified filter drawer to enable quick filter changes, but flat frames are an issue as dust bunnies would change with every filter change. I think if you had an obs and a list of targets you could get around that for automated imaging by shooting a number of targets in a night with a single filter so you can shoot and apply one set of flats to the whole night, then change filters for the next night and go again. It is either that or no flat frames, or sitting up with it changing filters and shooting flats all night!

The faster the scope the more sensitive to sensor tilt seems to be true.
I don't think simply changing the filters would render your flats invalid.
A filter wheel does just that anyway. A lot of the dust bunnies are from dust on the sensor glass not the filters.

200mm aperture is pretty tempting if it can be managed with the right camera. Perhaps one shot colour is the go and a light pollution filter if its being used in a light polluted area and stick to the brighter objects of which there are quite a few.

[The_bluester (Paul)]

Always best IMO to flat frame after every time part of the imaging train has been changed, but at least any vignetting should be the same so you should be able to shoot flats per filter and hope dust bunnies doe not really cause issues.

The faster scope will certainly make tilt more of an issue, the faster ratio meaning a small critical focus zone, even if you don't get elongated stars I have seen RASA images where you could see that the sensor was tilted across a diagonal, stars in two corners diagonally opposite were in focus but the other two corners were not.

the 8" RASA is basically intended to be used with an OSC astro cam like my 294, if the pixels were a smidge smaller it could practically be designed for the job. People doing mono with filters have found a way to do it rather than it being the intended configuration.

[AXE]

I'm wondering how tilt can be introduced into the system, aren't all of the connections threaded on the RASA? Unless there's bad machining I would have though tilt would be minimal.


I'm planning on sticking with OSC, even if I do buy another camera its likely to be a ZWO 071 so filter changes aren't much of a concern to me.



I'm in a Bortle 4 zone and usually don't use a filter. I just remove the minimal gradient in PP. It just occurred to me that the faster, wider aperture scope might make glow more of an issue then an 80mm, is this the case?

[ab1963 (Andrew)]

It's pretty simple if you are wanting to solely image go the RASA 8" but if you want a visual/imaging scope go the 80mm Esprit

[The_bluester (Paul)]

The only non threaded connection I can think of is that I understand the camera connection plate to be a machined flat with a large knurled lock ring over the top, similar to the attachment of a Fastar secondary mirror in a Celestron SCT like my C9.25, if it was not done up nicely that could be a cause of tilt, or if anything at all got under one side. Parallel machining of parts would be critical for spacers etc.

I am not sure how touchy they are about focus, at F2 the critical focus zone will be tiny, but without the primary/secondary mirror combo of the SCT they may be in some ways easier to focus and probably don't shift as much with temperature changes. With the double magnifying mirrors, the SCT moves the focus zone by something like 120 microns for about a 10 micron movement of the mirror to focus. In the RASA, 10 microns should be 10 microns.


There is a specific light pollution filter for the RASA 8", they are sensitive enough about back focus that they come pre fitted with a flat glass window in place of a filter that you remove if you fit either the Celestron LPR filter (Goes in the same place as the glass window) or if you fit a filter drawer of some sort.

[lollywater (Paul)]

I have an Esprit 80 and a Rasa 8 and love them both. They are chalk and cheese. I do VA and know nothing about AP, guiding and post processing. I use the Esprit for nebulae and other coloured things. The Rasa 8 is for the dim fuzzies particularly galaxies where I want structure quickly.There is mirror tilt but not enough to bother me
The maximum exposure I ever do is 1 minute ,but mostly about 15secs and I live stack on sharpcap.
They have been purchased over the last 12 months so I am still getting used to them .The LX90 is for visual and the 8"f4 newt is gathering dust- a good scope but takes too much collimating.
cheers
Paul

[AXE]

Well given the replies so far i'm starting to lean towards the RASA8 but I still keep going back to the versatility and simplicity of the 80mm refractor. Given that i'm after an imager and can keep using the 8" Newtonian for visual I suppose the RASA is probably the best choice for light collection at this stage.

[Stefan Buda]

My suggestion is that you should make your decision based on how important it is for you to have nice stars corner to corner. It will be far more difficult to achieve that with the RASA.

[AXE]

Originally Posted by Stefan Buda

My suggestion is that you should make your decision based on how important it is for you to have nice stars corner to corner. It will be far more difficult to achieve that with the RASA.

Is this based on the tilt issues discussed before or does the corrector plate not actually remove all coma/ curvature? I thought the RASA scopes were supposed to be fully corrected across the entire image circle.

[Stefan Buda]

Originally Posted by AXE

Is this based on the tilt issues discussed before or does the corrector plate not actually remove all coma/ curvature? I thought the RASA scopes were supposed to be fully corrected across the entire image circle.

The RASA is supposed to be fully corrected across the entire image circle, but that is theoretical correction. In real life there are a large number of manufacturing tolerances that have to be met, both optical and mechanical.
I have not tested a RASA myself, so I don't know how well made they are, but one of the comments, in this very thread, mentioned image tilt. Well, good luck sorting that out in an f/2 system. I would also warn people about the Veloce 200. Recently I had to rip one apart to find the cause/s of poor performance. I was quite appalled by the poor engineering that I found and even the optics were sub standard. Now this is a scope that looks like a Ferrari and it is supposed to be optically very very good.

[The_bluester (Paul)]

from what I have seen, the RASA can produce some pretty spectacular results in very short subs (As you would expect) but it is probably very demanding on being set up just so. Use a cheap adapter where the shoulders do not turn out to be quite parallel and the results will be endlessly frustrating.

I also would not consider one without a focus motor and automated focus for deep sky imaging instead of the EAA that lollywater is doing. I don't think the focus shift is likely to be as massive as the traditional SCT due to only having the one mirror instead of two curved mirrors which amplify any focus movement either in making focus adjustments or shifting focus due to temperature changes, but the sweet spot will also be F2 tiny instead of F10 (Or even F6.3) forgiving.

Images like this start to show some promise though the second one shows a little tilt or perhaps the spacing not being quite right. But it is based on 120 second subs!

https://www.astrobin.com/400996/B/

https://www.astrobin.com/hewusf/0/

[gregbradley]

That is impressive.

F2 though is a bit of a fooler. Its the aperture that counts. F ratio really only refers to the width of field.

So it won't get an exposure any quicker than any other 8 inch scope. Rather it will get a wider field of view from the same sized sensor.

Those types of images are a bit of a conflict. They are getting magnification by using a tiny sensor. Much like using an APSc camera gets a more magnified view than the same lens on a full frame.

Greg.

[The_bluester (Paul)]

I have to differ there. The F ratio is the deciding factor time wise. Effective focal length will give you your field of view (I say effective to account for reducers etc)

I can tell you that my SCT (235mm diameter) is sloooooow to image with at it's native F10, demonstrably quicker to expose to the same levels with the reducer in it to bring it to F6.3 and the ED72 is faster again for the same levels at F4.9 though it is only a 72mm diameter objective.