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I've been using a William Optics Zenithstar 81 for almost 12 months now and find it great for capturing nebula, however, I'm looking to purchase something cheap and cheerful which I can switch my filter wheel and camera across to when capturing galaxies and smaller star clusters. The William Optics Z81 (with 0.8 reducer) has a focal length of 447mm. My ASI 183MM Pro has a pixel size of 2.4. I've been looking at maybe picking up a 800mm or 1000mm newtonian or an 8"Ritchey-Chrétien for capturing these smaller targets and was wondering if anyone uses something similar. I've only ever used smaller refractors but, having just upgraded my mount to a Celestron CGX, I've got a bit of flexibility on payload capacity now. |
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| I would suggest to start from the Newtonian. The jump in focal length from your current refractor to an RC is too big, and RC is more difficult to collimate than Newtonian. |
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| I would say 8" Newtonian is better than 8'' RC. In most places you don't have enough seeing to satisfied a 8'' RC. |
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Hi Graham, Like the others, I recommend the 800mm Newtonian. I have a Skywatcher 8-inch Newtonian and it is an excellent astrograph. The fast focal ratio (f/4) really helps capture faint details quite nicely. After you have developed some experience with the Newtonian, the RC might be a good next step. I purchased a 10-inch RC last year and am very pleased with the high resolution images that it yields. Collimation was a very involved process that required several attempts before I hit the sweet spot. The experience you get from collimating the Newtonian will prove valuable if you decide to move up to an RC in the future. Clear skies, Kevin |
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Thanks for the response guys. Like I say, I'm just after something relatively cheap to start off with since I intend on keeping the WO Z81 and more than likely adding an Esprit triplet to the setup later in the year. I'm assuming it's relatively simple switching things across from my refractor to the reflector? Nothing too involved in terms of additional spacers etc? I'll probably end up picking something up for around the $1000AUD mark to see how I go for the meantime. Been looking at the Sky-Watcher 200 as well as a Bintel BT200 which is made by GSO. |
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You will likely need to purchase a come corrector for the Newt. The Skywatcher coma corrector hides excellent results. Clear skies, Kevin |
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Thanks for that. I'll have a look around and see what is in stock. Might pick something up to have a play around with. After all, you've got to have more than one telescope, haha Clear skies |
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| Hi Graham. I use a WO GTF81 for widefield and an Astrotech 8" RC for the small stuff and couldn't be happier. I think a newt would fall in between these two scopes and wouldn't give you the desired field of view that an 8" RC would. I also use the 8" RC for visual. Good Luck. |
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You are not alone. I had precisely the same Next Scope Dilemma. My first scope (which I still use most of the time) was a SW Esprit 100. I was choosing between a GSO RC8 f8 (1600mm focal length) and SW 200 Quattro (800mm). In the end I went for the RC8, on the basis that it made a big difference to the image scale with the big (5.9um) pixel detector I use, and also that collimation, once achieved, was likely to be more stable than a Newt. I have used the RC8 for about six months, and I like it but don't love it. Initial collimation is a bit of a b****g, but acheivable. And once set it it is pretty robust. I am lucky enough to have my own roll-off roof observatory, so moving the scope is kept to a minimum (only when swapping out with my Esprit). Pros are the image quality, aperture, robustness. Cons are the contrast and the field-of-view. [Note that I don't mention image-scale or speed. I can always bin-up by a factor 2 to give me an equivalent speed of f/4, I lose (slightly) in read noise, but that's tiny in comparison to sky noise, but I do sacrifice field-of-view at f/8.] On balance I think my next telescope will be a Newtonian, 250mm f/4 carbon fibre - which my EQ6-R Pro mount should just be able to cope with. A corrector with a Newt is a must asa mentioned above, and should be added to the cost. But the cost of a good non-laser collimator (Takahashi) should also be added to an RC. All the best with your decision. The only right answer is what is best for you. |
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Thanks for the insight Brian, much appreciated. I think I'll hold off for the meantime and keep doing some research. There isn't much in stock at the moment anyway. If something becomes available then I might look at getting a slightly larger Newtonian Reflector, maybe something in the range of 1200mm focal length or similar. Thanks again for the responses. Certainly a few things to consider when switching over from a refractor. |
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Dear Graham, I use a 6" RC for 2 years now. With a 0,67 Reducer the scope reaches a focallenghth of 900mm. This is a suitable focal lenghth for many galaxies and smaller targets. The point of collimation is important. I tried many methods of collimation. Today I use the TSRCKolli and it works very well and is easy to use. So check out thispoint while choosing the right scope for your plans. Regards Johannes |
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Hey Graham, I was in the same boat as yourself. I recently just purchased a 6” 1370mm FL RC specifically for galaxy season as I am using an 80mm refractor with a FL of 520mm with reducer/flattener. I had a look around, refractors, reflectors and RCs to see what I thought was most suitable for my current mount. With RCs collimation always came up as a pain/problem (I am still to use my RC as I need to finish a few projects on my refractor) but RCs from my research seem to do well with image quality (maybe suffer a little on contrast). Refractors with large apertures and FL are just way too expensive and too heavy. I found that I could get a lighter scope in an RC than the others, less glass. Reflectors seemed like a good option also as you can get some very very fast ones for a fraction of the cost of an equivalent refractor, still too heavy for me though  I will say, I don’t know how difficult it will be to collimate so can’t really add anything on that but am fairly realistic with my expectations........we’ll see. CS Brad |
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Thanks for the input. I thought I would have a look at the sampling with the two scopes I’m looking at based on the camera I’m using at the moment, the ZWO ASI183MM. The 200 or 250 Newtonian Reflector would result in some slight over sampling, however, the RC8 will result in significant over sampling according to the online calculation I used. I’m starting to think the Newtonian Reflector might be the better option for meantime as I only intend on using it intermittently. The refractor will still be my primary imaging telescope for the majority of the year. |
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I think you can't really beat a Newt if cheap, simple and fast is what you want. With the small pixels of the ASI183 aperture is more important than focal length. RCs tend to have huge focal lengths which you will have to reduce (more complexity), and a corrected field about three times as large as your sensor which you won't need. Many excellent f/5 to f/6 5.5-8" Newts around at very good prices, unlike hyperbolic reflectors where the 6" models are bit of a hit and miss and the 8" models are a serious investment, better suited as primary scope. With a f/5 Newt you might even get away without a coma corrector, especially if you are just playing around and do not care about the edges of the field. With a f/6 Newt it is even more likely. Not that a coma corrector would hurt, of course. Also, coma correctors do have distancing requirements (typically 55mm which is standard DSLR backfocus) but are probably the most forgiving kind of corrective optic. Unlike flatteners which are designed for a specific refractor or a specific range of focal lengths, a f/5 coma corrector works with every f/N telescope where N >= 5 (subject to certain physical limitations in focuser diameter and travel). And if the distancing is too close it will simply undercorrect. No extra distortion, just less correction. I have ordered a 130PDS for similar reasons as you. Cheers, Dimitris |
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Brian Boyle: I went back to a 8" RC from my 10" F4 CF Truss Newt. Newts are also no easier to collimate. I find the RC easy. Wind does not affect it, easier to keep dew off of the secondary. |
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I think you are seeing lots of good suggestions and going either path is fine. Here is my take: I have the RC8 and ASI183MM Pro; and other scopes and ZWO cameras (I run 2 rigs). I'd still recommend the RC8. As you realize it's the combination of pixel scale and focal length that is important. The optics in the RC are so good you can push the lower end of the recommended arcseconds/pixel image scale; so you should target 0.67 arcsec/pixel (a bit less, slightly oversampled, is ok). Natively RC8 (1600mm and F8 ) and ASI183MM Pro (2.4 microns) give you 0.3 arcsec/pixel which is a bit more than twice oversampling. So you need to get back a factor of 2 either using a telecompressor or bigger pixel camera or, as mentioned, 2x2 binning.I personally use a telecompressor working at roughly 0.75x (very common recommendation) and ASI1600MM at 3.8 microns meaning 0.67 arcsec/pixel (in practice 0.64 arcsec/pixel actually). So the effective focal length is actually 1212mm measured. I have absolutely no issue at that focal length. My bigger RC10A runs at 1600mm with no problem because with bigger pixels I'm still running at 0.67 arcsec/pixel. When I contemplated a Newtonian I worried about my heavy camera train hanging off the side, the longer tube (more sensitive to wind), getting a specialized coma corrector. The RC camera setup is rigid and on-axis.I initially worried about the RC collimation, but the worry was unfounded. Don't touch the primary and do very minor adjustments on the secondary with an out of focus star, once. I'm pretty sure the Newtonian will get you within 5 percent of the RC in performance, will save you money and is more compelling for visual work. If these elements are not critical then the RC. Some other advice: From Chuck: don't think for a moment you'll disconnect your camera assembly form one telescope to another; it takes time to get these things optimised you don't want to touch it. Tilt, dust, spacing, convenience, etc. You won't be doing it (often). If you are not going to use it much then don't buy new. I see enough of either the RCs or Newtonians on iceinspace in Oz. If whatever you buy doesn't work for you then put it back on the market at the same price. Cheers Rod |
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Rod, Some great advice there, thank you very much. Certainly a fair bit to consider. I've only ever used refractors so it's going to be a bit of a change. The other option, I suppose, is to just buy a larger refractor but I'd be looking at a significant amount of money (and a significant amount of weight) to get something of a similar quality to the WO Z81 and with a noticeable difference in FL. For the meantime I think I'll keep looking and, if anything turns up in stock, I may decide to pick something up. Thanks again for the information and advice. Clear skies. Graham |
