Rodrigo Roesch:
Luka Poropat:
A RASA with an OSC camera is significantly slower than an F/3 newtonian with a mono camera and is more similar in speed to a mono camera on an F/5 refractor.
Sorry, I disagree with this statement. I have both RASA 8 and F/5 refractor (using a reducer). Using my ASI 294mC with both RASA and refractor, a comparable ADU value is 120s vs 300s
You can disagree all you want but you can't read. He said that an OSC RASA is more similar in speed to a MONO camera on an F/5 frac. You are not considering the impacts (significant) of the Bayer matrix. It is a square of 2 green, 1 red, and 1 blue pixels. This is especially important because nebulae emit light on specific emission lines. So he is correct, an F/3 Newt absolutely gaps an 8" Rasa. Not to mention actually being able to automate imaging and avoiding buying shifted filters. Not even close. They're on two different tiers. Not even considering optical quality.
Again, that doesn’t work that way, even if you have 50% with Green and 25% of R and B, to create that correlation you will have to expose the same time then you will end with similar results. I see a lot of comparisons when they say 1h of color the same as 1h of each color plus L. Anyways that color vs mono is the never end discussion and it is hacking this tread. But maybe you need to be more polite in the way you write your comments
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F. X. Flinn:
So you guys are saying I'm probably nuts to consider moving from a TAK TSA-120 to a RASA 8?
Because that's exactly what I began daydreaming about a little while ago and decided to see what RASA users were saying and came across this thread!
FXF Yes it is kind of crazy. It is a lot of work to get quality images out of a RASA8. Forget about jumping into the deep end, you would be jumping into the abyss. I would not recommend it. If you really want something faster, check out a quality Newtonian or a quality refractor that is faster than what you have. However if you are new to astrophtography, even a newtonian will be difficult to set up. For everyone else; Also the RASA8 being slower than a newtonian argument makes no sense. It's not the telescopes that matters. It is because you are comparing OSC to Mono. I could say any natively faster scope with an OSC collects SNR at a less efficient pace than a slower scope with a mono setup; up to a point. How you utilize the mono setup matters as well. The bottom line is the RASA8 is not for everyone. It can take really great images if you take care of it, but to be frank, not a lot of people do. This is only compounded by the lack of quality tilt and backfocus accesories suited for the RASA8. Also for those complaining about Chinese products, this is not 2004 anymore lol. You get what you pay for and plenty of western companies are more than happy to drop the ball all the time. One of them even has literal planes dropping out of the sky all the time right now ;)
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I could say any natively faster scope with an OSC collects SNR at a less efficient pace than a slower scope with a mono setup; up to a point. How you utilize the mono setup matters as well.
The bottom line is the RASA8 is not for everyone. It can take really great images if you take care of it Ok, this I agree with, but as you say "up to a point". OSC collects data simultaneously, and mono requires sequential capture. Clock time matters to some folks, and not so much to others. Most telescopes can take great images if you take care of them, and optimizing the scope's strengths while minimizing their weaknesses matters. I'd say look at people's pages, and then decide. If others are doing what you want to do, you probably can too. I don't think it's crazy to consider a RASA OSC setup, and mono could be fine as well. Both can create really nice images if you pay attention to the details. You can screw up either one with sloppy practices. For my time & money, life is short and clock time matters. This lead me to a fast RASA11 (f/2.2) and OSC. I've been happy and wouldn't have changed a thing. That said, I'm in Bortle 1 skies. If I were in the city, I'd have gone mono for sure. CS Doug
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I have the RASA 8” and I use both mono and OSC cameras, in fact, I do narrow band and add color stars or reflection nebula portion to my picture using the OSC.
You are not comparing apples to apples by trying to add slower scope with mono vs faster with OSC since both scopes can use mono.
The RASAs are very good scopes. You can go much deeper with shorter integration time. Your processing skills make also a difference
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Back to your original question… I have ran RASA 8, RASA 11, and Epsilon scopes in a variety of configurations and I have to say that the RASA 11 and the Epsilon would be a good next step. Both with some good and bad.
1. The RASA 11 is a little bit nicer (in my opinion). It's slightly faster f/2.2 and it has 620mm focal length. I like that little extra reach over the 530. The epsilon can do a slight better job in resolution, but you have to be in remote sites with wonderful arcsecond seeing to pull out that extra capacity… The RASA requires filter changes, but my setup has a dome right outside my house and I just run a filter per night.
2. The epsilon on the other hand is a fantastic scope as well, super crisp images if you have skies that can use it's ability. The field isn't quite as flat as the RASA 11, so for me, the combination between collimation and tilt has to be resolved and is a bit more time consuming, whereas the RASA is a bit easier dealing with mostly just tilt. Because it's usually collimated very well from the manufacturer and the epsilon people usually run different focusers/rotators and have to remove and reset the saddle.
I think the epsilon is awesome for filter wheels, nicer remote capabilities, little more tempermental, 530mm focal length. the RASA is a little deeper at 620mm, heavier, filter swaps, but flatter images (in general).
I've compared images as well on same targets, damn near identical, so then there is that…. haha.
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The options you listed aside from the RASA 11 and Starizona Nexus are more or less lateral moves that allow you to use a filter wheel. The RASA 11 gets you more focal length, but then you are still stuck changing the filters. The only way the Nexus allows you to increase your focal length is if you use it on a large newtonian, think 250mm+ of aperture.
I have no experience with the Nexus so I cannot speak towards its quality. I would suggest searching astrobin for newt + nexus combos to see if you like the image quality.
You might just have to accept that with current 'budget' options, a higher focal length comes at the cost of a higher focal ratio. I agree the rasa 8 is 400mm focal length. I use a Starizona Nexus with my 10" f/4.7 Newtonian. With the Starizona it brings it down to f/3.5 and a focal length of 900 mm. I haven't had a chance to try it out with my new 2600mm yet. Another cool thing about that is I used all my other newtonians. I have a 6" and with the Starizona its 458mm focal length and f/3. I have an 8" that's 600mm focal length and f/3 with the Starizona.
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I used a RASA8 for a few years (and will use it again sometime) and although I loved working with it due to it's tremendous light collecting ability, I found that you kind of run out of wide field targets after awhile (unless you want to accumulate mega data over a long period of weeks, months, years etc.). And it's not the friendliest of scopes when it comes to using mono due to not so easy filter swaps (but when I next use it that's exactly what I will do as some users here have done an amazing job with RASA 8's in that regard)
So what was my point? Oh, I'm kind of enjoying long FL shots now of which there are endless possibilities (both RGB and NB).
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Your other option is to just go c11 or c14 with a hyperstar. That way you have the best of both worlds. You can go hyperstar for fast and wide field or native focal length/reducer for longer focal lengths.
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This is a difficult decision.
I have had rasa8 for almost a year, I have had to deal with all the problems related to it.
tilt, backfocus, weight etc.
I have in dual mode a WOGT81 refractor at F4.7 with an OSC (RGB) and the rasa with mono for Lum.
What I can give from my experience is that I would not change my rasa for a slower refractor, the amount of data produced is overwhelming, the frames come out cleaner due to the acquired signal ratio.
They are likes, but I am even about to acquire another rasa to have dual.
Although the focal length is short, I have had a lot of fun with the objects achieved.
clear skies
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Why not use your RASA8 with a different camera pairing like a ZWO 183mcPro? They're pretty inexpensive, and the resolution would be 1.1 arcsecs/pixel (undrizzled), or 0.55 arcsecs/pixel in drizzle 2. The field is 1.1 x 1.7 degrees, but you can always crop for galaxy work (which limits off-axis tilt issues). Personally, I would never go back to slow optical speed, and unless you are in bright city lights, (for which there are other options to consider (e.g. quad filter)), it seems there's no need for harsh adjustments (like switching telescopes). I think folks get screwed up thinking only about focal length, when what's key is targeted resolution. A RASA8 + ASI183 lets you go for wide or narrow field objects @ 0.55 arcsecs/pixel resolution and f/2.2 speed if you're willing to dither and drizzle. Hard to beat that combo (unless you want to switch to a RASA11 and ASI183. ;-)
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Hey Guys!
thank you so much for the wealth of wisdom you shared! That is truely appreciated! I will take that into consideration!
best regards Chris
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Doug Summers:
A RASA8 + ASI183 lets you go for wide or narrow field objects @ 0.55 arcsecs/pixel resolution and f/2.2 speed if you're willing to dither and drizzle. Can the RASA8 produce spot sizes small enough to resolve tiny targets? https://stargazerslounge.com/topic/368514-spot-diagrams-for-rasa-epsilon-and-sharpstar/Looking here it looks like worst-case 700nm (unless you want to try IR imaging) takes about 3-5 boxes of 1.8 microns or about 5.4 - 9 micron spots. The pixels in the 183 are 2.2 microns which sample the spots by 2.45x - 6.8x which is good on the low-end but oversampled on the high-end. Astronomy tools says that at 400mm focal length in the worst-case you'll be able to resolve details 2.78" - 4.64" in size with the RASA8 and with that camera you'll sample those features 3x or better. To me it looks like features below roughly 2.5" will be lost in the blurring of the optics of the RASA8. To get to 0.5" resolution you'd need a 1000mm focal length optic with 2.4 micron spots... Think TOA-130 with the 656 flattener.
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Hi John, I think I read that chart a bit softer than you do, but your right; the spot diagram shows the telescope produces spots larger than the pure resolution math would suggest. Galaxies are going to show broadband light conditions mostly in about 2 or fractionally higher boxes of 1.8 um (only the spots in the upper/middle of the chart apply given expected capture wavelengths and off-axis distance of this smaller sensor. It's true that 650nm light is the worst, but even then, we're looking at a spot sized between the 600nm and 700nm images; something like ~4 boxes RMS of 1.8um, or 7.2 um divided by 2.4 micron pixels is 3 pixels @ 1.1 arcsecs/pixel is 3.3 arcsecs (drizzle 2 to ~1.5 arcsec) resolution. For galaxy work with mostly broadband light, the result should be better, something on the order of 1.7 arcsecs (2 x 1.8um spots divided by 2.4 um pixels @ 1.1 arcsecs/pixel), and drizzled 2 at half that.
Given typical amateur seeing of 2-3 arcsecs, this combo should still be seeing limited, and do a reasonably good job both on galaxies and nebulae (which would have the worst performance owing to being further off axis). A careful search for images using the RASA8 and small pixel cams (183, 294) can reveal reasonably good results. CS Doug
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Doug Summers:
I think folks get screwed up thinking only about focal length, when what's key is targeted resolution. Huh?: well I've been loving long focal length! (F11 @ 3910mm) in Bortle 8 skies (with bad seeing) to boot - I've also done a lot of RASA 8 imaging (F2 @ 400mm) and although it was great, I enjoy the almost limitless amount of smaller targets that longer FL offers... the "fast" and "slow" labels placed upon focal length are somewhat meaningless...
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Well, I still have my RASA8 :-) I enjoy it tremendously. But, from an field of view point of view, i have a Samyang 135 mm to be able to capture really large FOV images, just recently acquired an TS ONTC 8", which has a beautiful build quality, my most favourite scope is a Sharpstar SCA250. So, in my humble opinion, it's all about the FOV you want to have. A RASA11 is undoubtedly a great upgrade, with limitations (which every telescope has). I'm not standing in your shoes, but in your case, i guess I would go for a system with 800 - 1300 mm focal length. The Nexus is a great tool for newtonians, but as previously mentioned: choose your newtonian wisely… Unless you like to tune telescopes, which is an interesting hobby in itself. When using a nexus, the reduction of the f value makes the telescope a lot more critical to tilt, collimation errors, etc. And also, contrary to the RASA8, every reflector needs to be collimated. Which in itself is something you can learn easily.
So still no definite answer… that's up to you!
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Doug Summers:
I think folks get screwed up thinking only about focal length, when what's key is targeted resolution.
...the "fast" and "slow" labels placed upon focal length are somewhat meaningless...
Actually, fast and slow are very meaningful. Try to take a 10 second exposure at f/11 on any target and see how many ADU are produced. Similarly, do the same at f/1.9. Then compare ADU, and see for yourself that optical speed matters. Still unconvinced? For confirmation, try a calculator (there are lots available)...see Astropix.com for an example: Unless you're using Adaptive Optics, lucky imaging, on a mountain top, or otherwise blessed with constantly exceptional seeing, we're all seeing limited. So, as long as you get to seeing limited resolution with proper sampling, that's as good as it gets (long or short focal length). After that, large field or cropped, the target resolution will be the same and then optical speed differential takes over to create higher SNR per unit time. The difference between f/1.9 and f/11 is shown below. Are you sure you wouldn't rather just crop a "faster" field for those smaller targets? ;-) At the lower bound, we need enough pixels to see detail in small targets. I don't use my RASA for ~1 arcminute objects (e.g. small planetary nebulae), but there are thousands of 3-4 arcminute objects that are within reach if you aren't adverse to cropping (e.g. pretty much the whole NGC catalog, plenty of the IC objects, etc.). Like everything else, attention to detail is required to get the best out of any gear (fast or slow).  |
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When I say "meaningless" I mean that in the grand scheme of things, you simply make adjustments in aperture (to some degree), exposure time, gain, binning and processing etc., in order to effectively neutralize the difference and enhance the advantage of each particular FL; between, say, an F2 and an F11. I happen to like, and use, both for different reasons (different targets) and would not shut out either.
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I used a RASA8 for a few years (and will use it again sometime) and although I loved working with it due to it's tremendous light collecting ability, I found that you kind of run out of wide field targets after awhile (unless you want to accumulate mega data over a long period of weeks, months, years etc.). And it's not the friendliest of scopes when it comes to using mono due to not so easy filter swaps (but when I next use it that's exactly what I will do as some users here have done an amazing job with RASA 8's in that regard)
So what was my point? Oh, I'm kind of enjoying long FL shots now of which there are endless possibilities (both RGB and NB). I was just like you - keeping some long FL scopes, RC8 Edge11 10'' Newt etc.. at home and trying to shoot some small stuffs at my B6-7 balcony, because with wide field scope (400mm) there are really very small number of objects especially I have limited view. However, I just setup my remote observatory, and to be honest those images are much better than my home result even under brightest moonlight. I'm losing passions for home AP now... Yingtian
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