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Georg N. Nyman:Caleb Kam: I am sure this is the right decision Georg. I have the Esprit 150 and it works great with the Pegasus motor focus unit. The NINA autofocus system is excellent! |
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Georg N. Nyman:Marc: Systems like the RASA will require you to fiddle with it, a lot - for some people, like me, this is not actually fun and can lead to tremendous frustration. I wouldn't say I have two left hands, but you get the idea... I am a programmer, not a mechanic. I know some people in the hobby that are engineers, and they will happily build their own adapters, or drill out threads or in other ways optimize their instrument. Me, I just want stuff to work  Anyway, good call on keeping the APO! |
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Georg N. Nyman: Thank you for posting this question Georg! A friend of mine (I’ve listened to him in the past and its caused me to loose a lot of hair 😡) has been telling me to get into a Rasa 8 instead of getting my Stellarvue 80 and I’m well aware of all the issues brought up here! Collimation, Camera in front of the optical path, special filters etc. With all this known I was still considering this option but with me still leaning towards the Stellarvue 80 - 65%/35%. After reading this post I’ve made my decision to stick with my Stellarvue 80 choice! I know its not an Espirit 100 but it does give me the background from others opinions on the OTA option. Dale |
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Hi all, Thank you again for all your comments and remarks - I´d like to summarize, what was said and written (maybe not complete...) RASA: Pro´s: Speed, aperture, maybe also size and weight, much-much lower price compared to similar performing wide-open refractors Con´s: Sensitivity regarding alignment and collimation, shallow depth of field in the image plane (due to large aperture), difficulties when using filters, no automatic filter changer possible, no standard DSLR (except mirrorless), time consuming fine tuning, cables in the light path... APO-Refractor (like Esprit): Pro´s: Stability of alignment and collimation, ease of use, all photographic equipment possible, all filter changer options possible, rigid construction Con´s: Less speed, smaller aperture, refocusing more often necessary (probably), size and weight can become significant - especially for larger refractors, price becomes very high for larger apertures and top quality CS, Georg |
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Georg N. Nyman: Maybe 1-2 comments: Price: A full APO with ~400mm focal length will be cheaper than the 8" RASA. Different story for 11" and 14" RASAs, obviously Focusing: Not sure why you think that the RASA does not need as much re-focusing. It's a much more sensitive instrument, so likely it will require more re-focusing. In either case, this can be automated with a motor focus, so in a head-to-head comparison is probably a non-issue. |
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| My c8/hyperstar) holds focus very well but I can't speak on a Rasa |
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| Depends on what the OP means by holding focus, in the end. The APO holds focus extremely well too - but both systems will need refocusing due to temperature and /or filter changes. And with the very narrow focus of the RASA, this will happen a lot more often. I mean, it is not a pure mirror-system but includes lenses as well. And I am not sure how much of a factor the aluminium tube is in terms of thermal expansion - which in any case would be much more important at this fast focal ratio. |
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Dear Georg! I fully understand your idea to have a faster system with less focus shift. Is there any reason not to think of an fast Newton with corrector. There are the Taks (130 and 160mm) and as well some other manufaturer (I use a sharpstar 200mm f 3,2) with works really fine. I do not have focus drift only at extreme temperature differences. Normally I do have not to refocus all night long. F2 is really demanding and aligning the RASA 8 is a much bigger challenge than any of the mentioned Netwons. The corrected field of all of them is at least APS-C or FF. May be this is something to think about. Cheers Rudi |
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I use both telescope, esprit100 is ultra easy to use ( almost no maintenance is required ) and it works every time out of the box. Rasa8 is ultra fast but extremely difficult to get flat field ( almost impossible ) so you will have always wrong shape of stars at the edge. And you have to maintain everymonth before new moon, and sometime the collimation goes wrong until you drive to the location.. I personally like RASA8 more than esprit100, because it gives me miracle every time  But if you need stable and reliable system esprit is better choise. |
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Riedl Rudolf: I actually have the ES David Levy model which is a Mak-Newt, that I’m planning on trying out. I have attempted it in the past but the very cold temps that I was testing (-10ºf - 13ºf) I believe caused pinching of the optics so with it it now warmed up (in the 30ºf range now) I’m going to give it another try. What I liked about this option was no CC/FF but I do not know how well corrected it will be. Dale |
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Dale Penkala: Sounds like a very nice scope to own, congratulations! Good luck and clear skies Wolfgang |
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Dale Penkala: Thank you! I hope it works out, but we will see. Dale |
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Just to add my opinion (RASA 11, Esprit 100): Of all the optics I have the RASA is the most difficult to get „right“. It‘s sensitive to everything: Tilt, collimation, focus, backfocus. In addition, it is, with only slight alterations, built on the basis of the Celestron SCTs. Those aren‘t bad (and you can get great kmages from them, as many here prove), but they are not without compromises, some by being an SCT and some by budgetary constrains, and you deal with all that entails: Mirror flip and flop, dew, cooling/insulation, tolerances of the connections, etc. - just now you‘re dealing with those at f/2.2 instead of f/10. If I had spent the time I needed to get the RASA to work instead with imaging with the Esprit, I‘ld have more and better images to show. If you have a permanent setup, it‘s certainly worth to spend the time and effort to really dial it in, because you can get very sharp, very deep images, quickly. But if you have to set up each time, and you‘re perfectionist enough not to be content with the Esprit images because of a slight focus drift, I‘ld say this is not the scope for you. Btw, the Esprit has very little color, actually, especially for being as fast as it is. For perfect focus, you‘ll need to refocus with every telescope on a filter change at f/5.5. This is not only due to longitudinal CA (which all refractors exhibit to a certain, sometimes very low, degree) but also due to tolerances in the filters. If you see significant CA, something is off with your sample. |
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I think automating focus really is the most cost effective solution. Plenty of options out there to help you out with temperature related focus drift.Riedl Rudolf: However, if you really are keen on trying something new I echo the Takahashi Epsilon recommendation. I chose the RASA because of the price point and because it compliments my other telescopes. If I was trading a 100mm triplet for the sake of a faster focal ratio and a somewhat bigger aperture I would definitely go with a Takahashi Epsilon. It still presents the challenges of faster focal ratios, but the build quality goes a long way to alleviating some of those headaches. You'd also have to live with the diffraction spikes that come with fast newtonians. Unfortunately in the EU so far as I can the ED is 500 Euros more than the Esprit 100. |
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I think automating focus really is the most cost effective solution. Plenty of options out there to help you out with temperature related focus drift.Riedl Rudolf: Having tried to use one, I can not subscribe to that sentiment. For once, the focuser of the Epsilons is bad: It rotates when changing focus directions and has significant backlash, making repeatable electronic focusing difficult. The inbuilt rotator is unfit for imaging at fast focal ratios as it will tilt the drawtube when used, needing complete recollimation. Also, it is very difficult to motorize because you can only attach the motor with a single bolt, leading to flex and so more (and irregular) backlash. The only currently available replacement is the very expensive Optec Leo and attaching it is not really „drop in“. Here also seems to be a QC problem, because I know a few people who got theirs with bad mirrors (me included). I‘m still waiting for a replacement with good optics, but so far I’m less than impressed. The mechanics do not really warrant the significant premium Takahashi calls up for these. |
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I've read all the above and for me personally the summary is: -- I will probably be buying a Skywatcher 130 PDS newtonian as my next scope/toy. Effectively a downgrade, but one that could give me hours of fun. My reasoning: * It'll cost me about 1/12th the cost of a RASA 8 (i.e. £209) * It'll give me a nice 585mm focal length at F4.5 (much wider field of view than my current 200 PDS) * That'll mean I can capture wider nebula views * All my coma corrector, filter wheels, cameras, focusser etc will work with it * It'll play nicely with my HEQ5 Pro being that much lighter I'm grateful for this post because it could be my best £209 spend. |
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I mean, if we want to go down the road of alteranatives to a RASA (sorry to the OP for derailing this thread now ) - there are obviously also super-fast Newtonians, with F2.8. Prices are in a similar range. The Taks were mentioned; and I heard good things about them. A more budget option could be the Hypergraph Systems by Sharpstar (sold under different labels around the world): https://www.testar.com.au/products/sharpstar-15028hnt It would still be a bit of a pain to collimate, and I cannot see any 360° Rotation on the focuser, but that is solvable with M48-threaded accessories. The advantage would be that the camera does not hang in front of the scope, so all equipment options available. |
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I have both an Esprit 100 and RASA8. I actually like the RASA 8 so much that I now have two of them on a double rig. One thing is clear to me: I can never get as deep with the Esprit as I can with the RASA. I think it does not only have to do with the f/2 but also with the large aperture in itself. I also have a Samyang 135 that is f/2 but it catch much less light than the RASA8 and it definitively does not go as deep. With deep I mean catching things like IFN and faint SNRs. I use color cameras (ASI2600MC) on my RASAs so shooting RGB does not involve any filters. For NB I use a dual-band IDAS NBZ to catch Ha and Oiii simultaneously with the color cameras and it works great. Getting perfect stars in the corners is very difficult but if they are not too bad they can be fixed in processing. So my main argument for getting a RASA8 is that it actually opens up a new world where you can image objects that are virually impossible to image with a smallish refractor (unless you have an eternally cloud-free sky and is prepared to spend weeks exposing, and maybe not even then). Here are two examples of SNRs that I recenty caught by accident - both generally overlooked by others. https://www.astrobin.com/96arhu/B/ https://www.astrobin.com/j6qtu9/C/ And an example of IFN that would be very difficult to catch to this extent with an Esprit 100: https://www.astrobin.com/bftjgc/D/ Cheers, Göran |
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Göran Nilsson: Hi Göran, I am very impressed by your images - all three are really amazingly good! Hearing many voices is so interesting - some rather negative about the RASA8, some very positive and most are rather hesitating - the difficulties of alignment and colimation seem to be quite significant. I shall see, what I do in the future - I definitely keep my Esprit100 and maybe I buy a RASA8 to see what my own experiences are with that instrument - the fast speed is sooo attractive.. Anyway, thanks and CS, Georg |
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I would also encourage you to stay on the Esprit 100ED. this is a very good telescope. Plus. you can get a 0.74x reducer that make it f/4 - 400mm telescope. which is the same FoV of RASA8, and a pretty fast refractor. Here's the reducer: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5120_TS-Optics-REFRAKTOR-0-74x-Reducer-Korrektor-fuer-Apo-und-ED-fuer-Vollformat.html Also you can check this discussion about it: https://www.cloudynights.com/topic/659716-sw-esprit-100ed-focal-reducer/ |
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I think the general recommendation I am aware of is to pair the 100ED with the Riccardi 0.75 reducer; the TS-branded stuff tends to be a bit meh from my experience. https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html But yes, you can turn your 5.5 Apo into 4.1 - with the added challenge of hitting a narrower working distances and such. |
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I turned my Esprit 100ED into an f/3.6 system using the 0.65x Apex-L reducer from Starizona. Do I get perfect stars in the corners? Nope. Adjusting back-focus and tilt is pure torture every time I put the reducer on there. Don't try to pixel peep my images to see the corner stars, because I cropped or processed out the ugly stars  - but it's really difficult to get round stars in the corners.I would like to add to Göran's comment about the RASA going deeper on faint, dusty stuff. I noticed immediately with the Esprit that I can go deeper on dust with the 0.65X reducer and the same telescope/camera from the same location. Take your pick whether it's just the f/ratio or if the fact that the image scale goes from 1.6"/pixel to 2.4"/pixel, putting more photons onto each pixel. Two ways of looking at the same thing? Anyway, I absolutely do believe the claim that the f/2 RASA captures dust or IFN better than the Esprit 100ED. By the way, my 11" SCT at f/7 has an even harder time with dust than the 4" Esprit at f/5.5, so I can't blame the aperture. Looks like the RASA wins for deep, dusty images. Then again, maybe you don't have dark enough skies to chase IFN in every image, especially with an OSC camera? In that case, I guess the RASA will just image the light pollution much more efficiently, and limit your sub length?  |
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I would like to add to Göran's comment about the RASA going deeper on faint, dusty stuff. I noticed immediately with the Esprit that I can go deeper on dust with the 0.65X reducer and the same telescope/camera from the same location. Take your pick whether it's just the f/ratio or if the fact that the image scale goes from 1.6"/pixel to 2.4"/pixel, putting more photons onto each pixel. Two ways of looking at the same thing? Anyway, I absolutely do believe the claim that the f/2 RASA captures dust or IFN better than the Esprit 100ED. By the way, my 11" SCT at f/7 has an even harder time with dust than the 4" Esprit at f/5.5, so I can't blame the aperture. You should always factor in the sky darkness which seems no-one is paying its due in extolling the virtues of their imaging systems. As for the f/ratio what matters is the image scale not the f/ratio both in terms of comparison that in absolute sense. So, you could capture faint stuff regardless of the optical train focal ratio if the image scale is the same, possibly with different detectors having about the same broadband QE. |
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I may be getting the numbers wrong, but the difference between f2 and f5.5 is actually pretty drastic: If I collect 20hours worth of data with the 100ED, I get about "the same" information after just 2.6 hours with the RASA. So even if the APO *can* absolutely capture faint details, the time required would be unrealistic for most users. I mean, I have done 20hour images with the 100ED and I just about see hints of IFN. But 20 hours at F4 (twice as fast, but also more aperture, i.e. 200/800 Newtonian) can reveal a ton of additional details. So imagine going from F5.5 to F2...it's a world of a difference, no doubt. It's just whether you want to put up with the added challenges. |
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andrea tasselli:I would like to add to Göran's comment about the RASA going deeper on faint, dusty stuff. I noticed immediately with the Esprit that I can go deeper on dust with the 0.65X reducer and the same telescope/camera from the same location. Take your pick whether it's just the f/ratio or if the fact that the image scale goes from 1.6"/pixel to 2.4"/pixel, putting more photons onto each pixel. Two ways of looking at the same thing? Anyway, I absolutely do believe the claim that the f/2 RASA captures dust or IFN better than the Esprit 100ED. By the way, my 11" SCT at f/7 has an even harder time with dust than the 4" Esprit at f/5.5, so I can't blame the aperture. I agree with you about the image scale being a deciding factor - but let's not start the never-ending debate about f/ratio vs. aperture vs. image scale vs. "only etendue matters". It will go on for 50 pages with no progress, as usual. Factoring in the darkness of your skies is definitely important. I would love to use a RASA at my dark site to go deep on ]IFN, but I am selling the dark site property soon and imaging only at home from Bortle 4+ due to lack of time for dark site trips. I think I would rather stick with the refractor and have smaller image scale (finer resolution) with my mono cameras and filter wheel. An OSC camera might not be so much fun to use at home, and the IFN isn't going to look so great, either. |
