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Hello all, I would like to collect your opionons regarding the pro´s and con´s of the Esprit 100 versus the RASA 8. I own and use the Esprit 100 for about 2 years now and I am quite happy with it. What I am not so happy with are the focus shift during temperature changes - especially in winter, when it gets rather cold in the night and that the Esprit is not really an absolutely perfect apochromat - the focus differences are small, but they are slightly noticeable when checking carefully. Some around me suggested to sell the Esprit and buy the RASA 8 system - as I am not at all an obsever but solely a photographer, it appeals to me. Focal length is somehow similar, especially when I use the Riccardi reducer on the Esprit. The big advantage of the RASA 8 is the aperture - the Esprit has 5,5 and the RASA has got 2.0. That means, I can expose the same target in less than 1/4 of the time compared to the Esprit. I am not using DSLRs, so that is no problem for me to switch to the RASA.... Are there any major drawbacks of the RASA 8, which I might overlook? Any specific advantages of the Esprit, I am not yet aware of? Your opinions and feedback is highly appreciated! Thanks Georg (from Lower Austria) |
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I own the RASA 11 and love it. However, it is not without it's quirks and issues.
Triplet fracs like the Esprit are -way- easier to use and get good results with than the RASAs. I have one incoming next year (waiting on a buddy to get a Tak off backorder) and am really looking forward to having a rig that's a little less 'work'. In your situation, I'd recommend just grabbing an EAF and setting up autofocus on tempshift/HFR change in NINA. $200 for the EAF and free for NINA solves your focus issue and doesn't saddle you with an awesome but fiddly scope. |
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Hello, The F/2 ratio comes at a cost of being incredibly sensitive to any issues in the imaging train. The slightest bit of tilt or incorrect backfocus can ruin the corners of the image. This is a drawback of any fast focal ratio system, but it is magnified with the RASA 8 because Celestron did not think to build any easy way to adjust tilt into the telescope. Most third party solutions are lacking since you can find a lot of filter holders, but not many options for adjusting tilt or backfocus to the precise level that is needed. The only two options that I know of that allow you to adjust tilt without removing the entire camera are the Baader FCCT which is for QHY cameras only, or the Octopi Astro adjustment system which is 750 USD. Due to having the route the camera's cables over the corrector plate of the RASA, you will also have some kind of diffraction effect. This can be minimized by curving the cables as they cross over the plate. That all being said when it does work, it works great! It is amazing what it can do in a short amount of time. The major drawback for me is how frustrating it can be to sort out back focus or tilt issues. For that reason alone I would be hesitant of trading out the Esprit for the RASA 8. While others have had success, on average I would not recommend an APS-C sized sensor on the RASA 8 unless you are prepared to deal with all the headaches involved. I'd keep it at 4/3rds or below. |
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| SemiPro's message reminded me about filter size issues too! f2 means you need big filters close to the sensor to avoid vignetting. You probably want 2" filters even for APS-C, unless you machine your own adapters to get the filters closer to the sensor than is possible with off the shelf components. 36mm filters will vignette hard enough that flats have a hard time correcting it. (I'm using 36mm NB filters and it's painful) |
11.01
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| OR: "downgrade" to a Skywatcher 150dps Newtonian with coma corrector. I say the "downgrade" thing tongue-in-cheek as it's a fine OTA/coma corrector combo. It'll be an F4.5 675mm focal length scope that you can add a filter wheel and autofocuser to with 1.25" filters. It would only cost you about £434 to have a play with the OTA and the coma corrector; you could spend more than that on one filter for the RASA. It's also about a quarter of the price of your current OTA. The two also weigh a few hundred grams less than your OTA. |
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If the primary concern is focus drift due to temperature change throughout the night, a RASA is not the solution. In fact, it will likely be worse. The best solution is to use an electronic focus motor and software such as NINA which allows you to automatically refocus, for example, every hour. With a RASA your optics are much more sensitive to collimation and tilit. You also cannot use filter wheels which makes mono imaging practically impossible. A good triplet like the Esprit is much less painless to get a good image out of. With reducers and modern highly sensitive CMOS cameras, the F2 speed of the RASA is much less of an advantage. |
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Thank you all for your excellent answers and suggestions - maybe more are coming, but so far, I think, I deal with my smaller problems with the Esprit and keep it and be happy with it.... Thanks a lot, CS, Georg |
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7.90
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Georg N. Nyman: That would be wise, IMO. The aperture advantage isn't as great as the square root of the ratio of the square of the diameters would suggest, either. |
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| Actually, given your mount, you could look at the Skywatcher 200 pds or the Skywatcher 250 pds. They do have a narrower field of view though, but are cheap. The only problems with the Newtonians is their physical size. If you live in a windy area they will be affected by it. |
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The best solution is to use an electronic focus motor and software such as NINA which allows you to automatically refocus, for example, every hour. I would agree with this. I tend to do an automatic refocus on each filter change and then make sure that the number of exposures on any one filter doesn't exceed one hour. It works really well. The ZWO EAF 5V autofocusser is a cheap and good solution as all it needs is a standard USB 2.0 connection to power it. |
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A RASA is like a Ferrari, it's very fun but you have to spend a lot of time fine-tuning it. If you like adjusting tilt, collimating optics, this is your instrument. I have a RASA 8 and spent more time adjusting it than taking photos. But when all goes well, results are amazing. Coming from an Esprit, I think that it's going to be difficult. Don't get me wrong, I love my RASA, but I also have a refractor for headache-free astrophotography. Regards Olivier |
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Andy Wray:The best solution is to use an electronic focus motor and software such as NINA which allows you to automatically refocus, for example, every hour. Better still you can also configure filter offsets and there will be no need to refocus on filter change. There's even a NINA plugin that automatically calculates the offsets for you. |
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Better still you can also configure filter offsets and there will be no need to refocus on filter change. There's even a NINA plugin that automatically calculates the offsets for you. I was going down that route and then decided that doing an autofocus about every hour on filter change would take care of both the differences in filters and any temperature changes. FWIW: since I installed my ZWO EAF 5V I have never once worried about the focus on my images. I do an autofocus once at the start of my imaging and then let it and APT take care of the rest. It cost me £184. |
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I would lean towards buying an autofocuser if I were you, just as suggested above. If you are happy with the quality of the images from your Esprit, then getting a new, unproven telescope is much riskier (and more expensive) than getting an autofocuser. I just picked up a PegasusAstro FocusCube 2 for my Esprit, and if the sky ever clears out (new equipment curse in full effect), I'll finally be able to sleep without thinking about focus. FWIW, I also have an Esprit 100ED, and I am not so happy with mine. I've had endless tilt problems since I got it in July 2020. Nonetheless, my friend had a complete RASA-8 system available for sale last year, and I didn't care for the shape/size/quality of the stars in the images he showed me. I could have bought it from him locally at a good price, but I decided not to. He sold it to upgrade the filters that go with his refractor. |
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| I have an Esprit 100 with an autofocuser, mono cam and filter wheel. I wouldn't trade it for a RASA or anything else except maybe a Takahashi scope. I set mine to focus on every filter change or every hr / 2'c temp change and haven't had any focus issues. This is an easy fix vs. going to a tricky mirrored scope. Refractors are bulletproof (just about) and the optics are very good. |
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Jason dain: I'd flip the assertion and go the other way. Reflectors are bulletproof and I wouldn't trade a reflector properly made with a midget scope with lenses. |
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Ref reflector versus refractor: For me, both can achieve excellent results as can be seen on this site. A few comments though: * Refractors are much smaller and as such are easier to handle on any mount and generally more portable (albeit not necessarily lighter). * Being smaller, most refractors are less influenced by local wind conditions. * Most refractors have a nice wide field of view and are well suited for deep sky nebula stuff. * Refractors don't usually need much attention to collimation. That said: * Newtonian reflectors, in particular, provide a much bigger bang for the buck, particularly when it comes to aperture size/light gathering capabilities. * A carefully setup Newtonian combined with the right coma corrector will provide as good, if not better results than a refractor. * It is a myth that Newtonians need a lot of collimation effort .. carefully set up once, they need only very occasional tweaking that can be easily achieved with a laser in minutes rather than hours and probably only every 3 months or so. * Newtonians are big and as such act as sails in a wind, so a good mount is more essential for them. * They may not be heavier, but Newtonians are not exactly portable. * For really deep sky stuff on smaller galaxies the Newtonian will win out. In summary: I'm looking forward to one day being able to afford to purchase a decent wide field refractor, however I have no problem recommending a cheap 8" newtonian with coma corrector to anyone wishing to get into astrophotography. To put that in context: my total equipment costs so far have been £3,760 for 8" Newtonian, coma corrector, mount, filter wheel, LRGB Ha SII and OIII filters, autofocuser, guide scope, guide camera and main imaging camera (total payload weight 10.95 Kg). That's less than the cost (and weight) of a 6" refractor of roughly equivalent focal length and quality on its own. I bought all my equipment new, so I'm sure you could do much better than me if you shop around on the used sites. It's never going to be a cheap hobby (astrophotography), but it doesn't mean you have to sell the house to afford it. |
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| I'd like to add that a small reflector (i.e., a newtonian) can be efficiently be used as replacement for a wide field apo at a fraction of ownership cost. As an example a 130 mm f/4 short newt can be used with a small mount in the same way as on of those 3"-4" f/6-ish chinese doublets or triplets that are flooding the market. In the same vein I recently tested a low budget 6" f/4 newt (metal tube) to efficiently replace a 120mm APO(i-ish) refractor on a lower end mount (GEMtwentyeight). Just for the heck of it I let it run unattended for 5 hours in sub-freezing temperatures without ever touching the focuser. It didn't flinch a bit. At the end of the run it was covered in frost but the main mirror was remarkably free of condensation. |
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No reflector vs. refractor "wars" please. Both have their right to exist and each has its pros and cons. I appreciate the advice to stick to your workhorse and try to optimize it instead of taking a big leap to another scope. I had similar thoughts since my imaging sessions are usually time boxed and the thought to get a lot of light in short time is quite tempting. As you pointed out there is a price to pay, literally and in terms of complexity. Saved me a lot of money and frustration. Thank you all for this! Clear skies Wolfgang |
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Hi Georg, I went through *exactly* the same dilemma as yours. I started out with an Esprit 100 and bought an RC8 about a year later. I love both. The GSO RC8 is relatively straightforward to use and collimate. With the right tools and procedure. ( I use a Takahashi collimator and remove the entire 1ary mirror baffle ). The RC8 holds collimation really well (for months), but I only move it 1 metre from pier to shelf in my observatory. It holds focus really well - I rarely have to focus during a night - unless temperature drops by more than 10C. Being f/8 helps. I really don't mind the "slow" beam when it comes to integration time, after all I can always bin up to match seeing. The small read noise penalty is small by comparison to the sky noise - even in Bortle 3 skies. The 1600mm focal length and CMOS pixel give a image scale about 2-3x too small for ideal sampling at my place - so no penalty in lost resolution by binning. In many respects the RC8 is a fantastic scope for galaxies and the moon - where the focal length with Barlow and lucky imaging at 0.001sec allows me to beat my lousy seeing. However, in my view the RC8 has two big limitations. 1) The field is curved. I think a field flattener is mandatory for all sensor greater than 1inch in size. 2) Even with a field flattener, the corrected, unvignetted field of view is limited - particularly if you are use a larger sensor. This is where the slow beam hurts. Indeed I don't really think of f/8 as slow - but rather narrow field. Ideally there would be reducing flatteners for RCs. But there aren't. There are some - but there are designed for larger focusses and you are stuck with a 2inch focusser. So, I you love the larger field of the Esprit 100, you will be frustrated by the smaller field of the RC8. Its great for galaxies and solar system, but I found it frustrating for nebular work. After a very happy years observing - and 100 or so images - I bought a TS Hypergraph 8. A 200mm Newtonian at f/3.2 with carbon fibre tube. It was a brute to collimate, but once done it has held its collimation very well. At f/3.2, even the Carbon fibre tube needs regular focussing (every 1.5 hours) during typical nights here [where the temperature typically falls by 5-10C]. But collimation and re-focussing is the price you pay for a wide-field, low f-ration astrograph. I looked at a number of other options for a fast large aperture telescope, but discounted the RASA (due to the filter wheel, cable management issue) and more expensive options like the RH telescopes - a large central obstruction giving rather ugly star halos. Anyway, those are just my thoughts. Good luck with the decision. You can't go wrong with the RC8. Its a great scope for what it does. CS Brian Later edit: oops sorry I misread RASA8 for RC8. Please ignore my rembl8ngs. |
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Fellow Esprit 100 User here. Per some have already outlined earlier, RASA comes with its own challenges, namely a very tight CFZ and the lack of ability to do mono with filter wheel (i.e. automation). I think the Esprit 100 is the absolute go-to and I believe it is by far the easiest scope to use both to carry around and for imaging. At F/5.5 you dont need high speed filter but it is plenty fast, at 550mm it is wide enough for most nebulae targets. All in all, Esprit will def be the solution with less headache attached to it. |
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Caleb Kam: Hello Caleb, Thanks for your encouragement, I shall of course keep my Esprit 100.... I was tempted by the aperture and the speed of the RASA but after thinking about all those opinions and reflection from users, I am more than before convinced to keep my Esprit and automate it... CS, Georg |
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Georg N. Nyman: Hi Georg, as a fellow Esprit 100 Owner, I would just like to add the following points (most of which I suppose have been mentioned): * Focus drift This is likely to be much, much worse on the RASA. If this bothers you with the APO, consider getting a ZWO EAF Motorfocus; solves basically all of your problems. And it's just 250€ (360$ AUD?). Acquisition software like NINA will then continuously monitor focus of the image and adjust if needed (including filter changes). Honestly, I could not imagine doing AP without this. * Ease of use You will find a lot of discussions regarding the handling of the RASA. It is a powerful piece of kit, but much, much harder to master than your F5,5 APO. If your adaptation and sensor are not absolutely, perfectly planar, your stars will not be round. And this can be very difficult to sort out (see next point). Same goes for the image focus. So you will need a motor focus either way. Also, you won't be able to use an automated filter wheel (too big), so it's either OSC or manual filter changing. The horror! ;) And if you do not have a permanent setup, the danger of messing up your system during setup and teardown is a real concern too. * Collimation It's an F2 system - if you have never fiddled with something like that, just be warned that it can be an exercise in frustration. I mean, my F4 Newton has driven my nuts at times...I can only imagine the "fun" trying to set up something like a RASA. Essentially, you have to perfectly align two mirrors in all axes, the schmidt plate, and your camera adaptation. There are some videos on youtube to give you a sense of what that means in practice. So yeah, I'd stick with the APO... Just because I know how soul crushing it can be to spent your clear nights with an allen key dancing around your scope and trying to get it to work the way you want. |
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Marc: lol.. there is that i suppose.. @Georg N. Nyman Are you a tinkerer? Or do you just want it to work from the get go pretty much Clear nights doing any sort of equipment fettling isn't everyones idea of fun... |
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Marc: No idea, what you mean with "tinkerer"....I just wanted to get other opinions.... |
