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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

[AXE]

All this talk of the pickiness of the faster F ratio scope is starting to make me look at the refractor again. Yeah, it's slower but its the same ratio as my Newtonian and i'll be able to guide better hopefully given the weight and wider FOV so 5+ minute subs shouldn't be an issue. Plus it's a bit cheaper and just easier as a complete package by the sounds of it.

[The_bluester]

Both have a 400mm focal length, though I would imagine the Esprit needs a reducer/corrector for photography? That will reduce it's focal length and increase the field of view.

At 400mm the both of them (Assuming the refractor does not need a reducer/corrector on top of that) should guide similarly, the advantage to the frac being the ability to use an off axis guider than you can't do with the RASA, that has to have a separate guidescope. The RASA might be a little more wind impacted when guiding due to the bigger surface area, the RASA WILL need a heater strap in cooler weather, the frac MIGHT.

I am still tempted by the RASA myself now that some quality images are appearing, I was not prepared to be an early adopter though.

[AXE]

The Esprit comes with a flattener included, as far as I've read there's no reducer out there that can be used with it.

[gregbradley]

Riccardi reducers should work with it.

Teleskop Services sell them so does APM.

Greg.

[The_bluester]

Well, that makes it a simpler comparison then. The FOV will be similar, the speed will be markedly different, guiding will be probably not easier but potentially more accurate with the refractor due to being able to use an off axis guider.


I was actually looking for a good explanation of factoring the F number to imaging speed and found one on the CN forum.

Without using actual figures. Take a given scope and call it F5, say that particular scope at F5 with your camera is at a scale of 1 arc second per pixel and a particular arc second of sky has 100 photons arrive in a given exposure time, those 100 photons will all fall on 1 pixel. Now double the focal length (F10) of the same diameter objective, the image produced at the focal plane will double in size, each pixel is now seeing half an arc second. 100 photons arriving in that same arc second of sky are now falling on a 2 X 2 pixel grid so it is 25 photons each, you need four times as long to gather 100 photons per pixel compared to the "Faster" setup. The resolution of the image is higher, but if seeing limits the resolution you don't get any benefit from that.

[lollywater]

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

I am with Paul here. I have a 8" f/10 sct and 8" f/4 newt. My latest addition to the 8" sct is a f/4 starizona night owl reducer. Already have a Meade 3.3 and 6.3 reducers. Only have to do a 15 sec capture on any camera to see the difference between the Rasa and the other 8" scopes.
cheers

The Rasa 8 is an ideal scope for VA. I want an image with detail in less than a minute. Beautiful images with pinpoint stars at the corners are not a priority for me.
It appears astrophotographers have different requirements and I can understand why the Esprit 80 is a favourite.
cheers
Paul

I often use a lightwave 0.6 focal reducer flattener on the Esprit. The reducer is meant for refractors f6 or more so I dont think you guys would approve of what I do to that poor little scope.
cheers
Paul

[AXE]

Is this for photography? Do you still use the field flattener or does the reducer also correct curvature?

[lollywater]

Yes Axe ,it is for videoastronomy. My typical image is 10 to 15 secs exposure with a live stack of about 5 on sharpcap.I do spend a bit of time on the histogram. I generally dont save the images unless I want to post something on a forum. So things like darks,flats, curvature are not a big concern. I dont like vignetting which happens a lot with focal reduction on the sct's using the bigger sensor cameras like the zwo 294. That doesnt happen on the Rasa. A few small donut stars out in one corner doesnt bother me. It is all very sloppy from the point of view of an astrophotographer trying to get great images but videoastronomy is all about live viewing
cheers
Paul

[gregbradley]

This is the original article that started this F ratio myth debate:

http://www.stanmooreastro.com/f_ratio_myth.htm

Greg.

[lollywater]

Oh dear!
Not that article again.
One criticism of that article I have read is that "true S/N" is a cropped version of "image S/N".
There is a lot of discussion on other forums as well as this forum done by very informed people and it is obviously not clear cut. Paul's explanation is the way I see it .
So ,what do I see? The Rasa is much faster than my other 8" scopes with/without focal reducers.
cheers
Paul

[gregbradley]

The way I see it is a wider field of view captures more total light.

A faster scope broadens the FOV letting more total light in. A slower F ratio lets in less light and it is more tightly focused on less pixels as you mentioned.

But if you cropped the wider FOV image and compared a similar area of crop to the slower F ratio at the same aperture what would you get?

Greg.

[Slawomir]

I have noticed an increase in SNR when I used a reducer on my refractor as opposed to imaging at the native focal length (used the same camera in both scenarios).

A 15-minute exposure at f/4.5 gave a slightly better SNR for extended objects than a 20-minute exposure at f/6 (same scope, same camera). I tested that for multiple filters and for multiple exposures.

Faster f-ratio does speed up imaging of the extended objects (nebulae, galaxies) if the same camera is being used and same scope, but for the stars it is less of a difference, since these are approximately point sources therefore aperture dictates the signal detected from the stars, not the f-ratio.

[The_bluester]

A quick read of the link, I think this undoes any other argument he puts on the page.

That, very simply put is exactly what is being talked about. Vary the F number, vary the number of photons on a given pixel in a given time, lower F number = more photons on a given pixel in a given unit of time = faster.



This statement is correct but omits half of the equation, it omits how the collected and delivered photons are distributed. Lets say that the entire target fits on the sensor at both focal lengths of a scope, with and without a reducer. The "Faster" ratio with the reducer fitted will take the same number of collected photons from the target and deliver them over a smaller number of pixels (Shorter focal length, smaller image formed) The image will be smaller and brighter as the same number of photons have been distributed over a smaller number of pixels. That is it in a nutshell.

[gregbradley]

Seems right Paul. I think also a reducer focuses more of the light that is missing the sensor back onto the sensor surface so it is focusing more of the light the optics are receiving back onto the sensor. So the total amount of light collected is greater for the same sized sensor.

The cone of light the scope receives is usually quite a bit larger than the sensor so the sensor is missing a lot of the light the scope has collected. Reducers put some more of that onto the sensor.

Greg.

[The_bluester]

You can look at it that way to a degree, a faster ratio with the same objective diameter means a steeper light cone, but that does not add any light to what is already on the sensor, it moves light from objects that was missing the sensor on to the sensor by compressing the view - expanding the field of view. Stuff that was just missing the sensor now falls on to it, everything that was already visible occupies less sensor area and that compression in size delivers the same number of photons per hour from any object (Use a single star as an example) to a smaller area of the sensor, so less pixels divvy up the same amount of photons per unit of time between them.

[gregbradley]

A very few scopes have large corrected fields like the FSQ106 (88mm from memory) and Astrophysics scopes (often 100mm).

So how much of that is being missed by the sensor I wonder? Probably a lot.

Greg.

[The_bluester]

Well, depends on the sensor of course, but putting a bigger sensor behind them wont give you a brighter image, just more sky in one frame. I hate to think what a sensor to cover a 100mm image circle would cost.

[Slawomir]

And a set of filters to cater for a 100mm sensor! Sensor + filters alone would be more than a scope+mount+camera's body+FW+OAG+kidney :lol:

[The_bluester]

I reckon at least one liver is probably on the table too.

[AXE]

Well I pulled the trigger on the 80mm. For anyone who might be interested the things that pushed me towards the refractor, in no particular order, were:

- Easier to set up and use, a plug and play solution
- Can do imaging and visual
- Small and portable
- Collimation isn't really an issue
- Option to move to mono imaging with a filter wheel if I want to
- Skywatcher scope comes as a kit with flattener and diagonal for visual
- Slightly cheaper on it's own, way cheaper when you take into account special filters and other accessories.

In the end the faster F-ratio and aperture didn't outweigh the downsides for me. I'm sure i'd have been happy with the RASA but it seems as though it's one of those things that needs to be 100% dialled in to work. The refractor is just more forgiving and for wide field imaging should do the job just fine.

[The_bluester]

As much as I am keen on a RASA 8 in my future I think you have probably made the right choice. I think the RASA is a scope that is destined to produce some exceptional results for a few, some OK results for a few more but for deep sky imaging rather than EAA I reckon they will break a lot of hearts.

I don't think they will be a scope for the inexperienced or those that are not both experienced and prepared to endlessly run down tilt and spacing issues to get the results that they should be able to give.

I still have one eye on the 6" Sharpstar that was suggested to me on this forum as an alternative to the RASA 8", traditional Newt geometry and enough backfocus to allow for a filter wheel and OAG, just not quite as fast.

[gregbradley]

I think its a good choice.

A good smallish refractor is a lot of fun, easy to use usually and no need for fussy collimation or flexures.

If it comes with a flattener that is even better.

Greg.