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Hello you astro people, for a very long time, I am dreaming of a scope with a longer focal length to image galaxies. They were always fascinating to me. At the moment, I have nice gear to take photos of nebulas and larger deep space subjects. But with a focal length of about 850mm (with my 8" f/4 Newt and the ES Coma Corrector), I am very limited if it comes to galaxies. I had a lot of time to research some stuff and with good luck, at the end of the year I am able to buy a dedicated scope. There are some decitions I already made. The scope had to work with my EQ6-R Pro mount. Currently, I am using a ZWO ASI183MM with a quite small sensor. I am not able to buy a camera with larger pixels yet, so oversampling is an issue I have to deal with in the beginning. The solution is to use my OSC Touptek camera that uses the same sensor as the ASI1600MC (but is a color camera). So there are some compomises to be made. It turned out, that the Celestron C8 (or maybe the C9) are options I prefer the most at the moment. I know that the 8" has a focal length of 2032mm but an aperture of f/10. If I buy this scope, the Focal reducer of 0.7 is a must to get an f/7 opening. With this setup, I think my EQ6 has no problems with the weight and I can reach some more galaxies. But there are still some thoughts I am certain about. I am living at an altitude of about 300m. So the seeing is definitely in the 2 arcsec range. I know, that a camera with a smaller pixel size will be problematic due to that seeing. So the solution to get rid of the resulting oversampling is to use a camera with smaller pixels (that means basically less pixels for the same area of the sky). If I now increase the focal length of the scope, I had to increase the pixel size any further, didn't I?. So I try to get closer to the subject, but at the same time, I loose resolution in the image because of the increase of the pixel size. And there lies the problem for me. Basically I then can crop the images taken with a shorter focal length and get, beside the noise problems etc., the same effect (at least in theory). Is there a maximum of a focal length that makes sense for us astro photographers? I guess, there must be some things missing in my thoughts, because there are a lot of people using similar scopes as the C8 with great success. If I look at the images taken with my 8" Newt @850mm, they are much softer than the images taken with my TS Quadruplet 350mm. This is surely related to the oversampling if I use my 183MM. It could also be related to the quality of the scope, because the 350mm was more than twice the price of the Newt and although it is a refractor, I guess the quality is better than the quality of the mirror of the Newt. For sure, it is a combination of both facts. It would be great if you help me with this. Will the C8 (or similar scopes) really improve my images and am I missing something, here? Thank you for your help. CS Christian |
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Hi Christian, Personally, I see no reason for you to switch from your Newton to a SCT with same aperture. Your question is one that every astronomer or astrophotographer will ask. The first thing to realize is: resolution isn't everything! If you have an average seeing of 2" and maybe on very few very good nights close to 1", with an 8" aperture scope, you'll be seeing limited. Hence, if you can achieve an image scale (unbinned) of below 1" per pixel (e.g. 0.5"pp), you can always bin to adjust your image scale to the conditions you had. On the best nights, you go unbinned and on the average nights, you can do 2x2 or more. If your current scope achieves this already (you've got 0.61"pp), just a longer focal length won't help you on anything. In most cases, you will lose FOV if you stick with your current camera. What you would have to do is to increase aperture and keep the image scale constant. Since usually aperture and focal length correlate, your image scale will decrease and you need to increase on binning. The larger aperture will then help you on collecting more photons in the same time which will allow bringing out fainter details more easily. Long story short: don't go the route of the 8" SCT if your image quality on the Newton is good. Personally, the issue with the SCTs is the moving mirror and hence I personally don't like to recommend them. If you want to gear up, you'd have to purchase something with significantly larger aperture 11", 14" etc. etc. Clear skies! Björn |
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Hi Christian, Bjorn said it very well. If you are happy with your 8inch f/4 Newtonian, I wouldn't go to a longer focal length. With 2.4um pixel (ASI183) you have 0.6arcsec/pix (equal to the diffraction limit at 200mm) and a longer focal length won't hep. Even with 3.7um pixels you have 1arcsec/pix, ideally suited for 2arcsec seeing. If I had an 8inch f/4 Newtonian, I would probably to look to invest in my imaging train first. That is a large (full frame) sensor and focusser/correcter to match. But that is just me. Brian |
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Brian Boyle: Hi Björn and Brian, the thing is, that I have no clue weather the scope is good or bad. I have nothing to really compare it to. But the time I got my flatfield APO, I was very impressed about its image quality. Compared to the Newton, the images were way sharper. They should, because the cameras pixel size is better suited for this telescopes focal length. So I took it as it was. But on the other hand, beside some exceptions you usually get what you pay for. The Newton was a really cheap telescope compared to most others in a similar range. So I wonder, if an investment in a better scope with (as I expect) better optical elements and of course also a higher price will improve the image quality. Thats why I thought about switching to the SCT. The time, I invest the money, I wanted to adapt the system to my main interest. Getting the camera that fits this system is the next step in the chain. I totally get what you both try to say. Maybe, for me it is about to improve and not to reach subjects from a greater distance. With the image quality of my Flatfield APO in mind, I also thought about getting a larger refractor like the ED127 or similar. But thats just a side note. Of course, I looked at the images shown here or elsewhere, that were taken with my favorite selections. But you never know how much of the quality is real or done by image processing. But the better the subs, the easier it is to process the data, isn't it? So, again I am at the point where I really don't know what to do. It's strange... CS Christian |
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Hi Christian, I myself had always questioned the quality of the optics I own and also made comparisons to the price. However, quality and price often correlate but high price doesn't necessarily mean high quality. If a manufacturer has a lot of waste production, prices will be high but quality might be moderate. He just has to compensate his quality losses through the higher price. A Newton in itself is a rather "simple" optical design and the manufacturing of a parabolic mirror (primary) and a flat mirror (secondary) are things that manufacturers should have a good control of. The mechanical design should be done well but it is not rocket science. Hence no astronomical prices for a pure Newtonian scope. A very important part however is the corrector lens(es) required to correct for the geometric aberrations of the parabolic mirror. An of course, the alignment of the optics itself. Therefore, before pondering a new purchase, try to make an analysis of your optical setup: - Verify the alignment of the mirrors and focuser. - Take test images and see if you notice aberrations over the whole field of view (especially residual coma). (an optional step could also be to get a Ronchi eyepiece, which you can use to assess the optical quality of the mirrors in case you have doubt). As you have noted yourself: the shorter FL scope will give the impressions of a sharper image simply due to the fact that you've a diffraction limited system and most stars are nailed down to a bit more than a single pixel) CS! Björn |
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Hello Christian, While taking nothing away from the advice Björn and Brian give above, my first thought is that maybe you should consider a solution outside the box you've been thinking of. Going f/10 for galaxies would require a huge investment in time (not to mention other headaches). Have you considered what a modified f/2.2 RASA design might do for you? I have a RASA 11 that is specifically designed (constrained) for galaxy & nebula work (but a RASA 8 would produce a similar result). I use an OSC 183mcPro with it specifically because of the limited chip size and small pixel size, which produces an image platescale of 0.799 arcsecs/pixel (0.399 drizzled). With a full-time UV/IR filter in place of the optical window, this results in a constrained wavelength regime and (intentionally limited) off-axis distance. The resulting optical quality is reasonably well suited for galaxy and nebula work. You can look at my gallery to see whether you're satisfied that the scope can achieve what you're trying to do. I started out taking 30 minute integrations of many objects (30-45 secs each at 1/2 unity on the 183), but I find that a 3-4 hour integration produces a pretty nice image, at least of reasonably sized objects. Cropping of course on the smallest objects produces fatter stars, but for Messier, NGC, and many IC sized objects, it's a pretty fair setup. Since you are doing mono work with your 183mm, you might consider the 294mm which can be used in "unlocked" bin 1 mode for even smaller pixel sizes but with a larger field (at the cost of slightly degraded optical quality at the edge). As you said in your OP, there's always a compromise. For me, time is precious. For the scale I want to image, a field constrained RASA + 183mcPro (OSC) made a lot of sense and I'm pretty happy with the resulting images and lower time commitment. The idea of constraining the RASA's large field is counterintuitive, but I think the system is fun and efficient to use and produces fair images. Something to at least think about outside the box.... CS Doug p.s. See (near the bottom) of this post for the spot quality diagram of the field / wavelength constrained RASA+183 system: https://www.astrobin.com/forum/c/astrophotography/equipment/camera-upgrade-183-to-533/ |
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Ok, I guess at the end of the day, my problems or thoughts always lead back to the pixel size of the ASI183MM camera (which I really love). Maybe the better way to go is to turn everything around. Instead of getting a new scope (which I'd really love to have), I should solve the main issue in the first place. It may be better to get a new camera with larger pixels first. I may go for a ZWO AIS2600MM (not sure yet). This camera may fit the 8" Newton much better. It also offers new opportunities with my existing setups, although they were not what I expected. I can fine tune everything and then I will see, what I can get out of what I have. Later on I can invest in a (kind of) C11 and a mount that can handle it well. Thank you for your replies. I still have some time... to change my mind again  |
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Hi Christian, Focal length does not matter in itself. Pixel scale and aperture do. Aperture gives you the theoretically maximum resolving power which is the same for all 8" instruments (well, about the same and let's not get into that discussion). You can reach that maximum either by increasing focal length or by reducing pixel size, it does not matter. Typically, it is impossible to reach it because you are limited by seeing. You can't do better than approximately half your seeing, no matter what you use. If your pixels are smaller (oversampling), you can simply scale down the images. Either at the source (binning) or later. You do not have that flexibility with every camera: if the camera has pixels 9 micron wide, you really need the larger focal length. But you have it with modern CMOS cameras like yours. Which reminds me: pick a relatively good photo that you took with the Newt and downscale it to ~45%, so that it matches the apo. I bet it is going to look a great deal sharper and more contrasty. Conversely, enlarge the apo image 2.2 times to match the Newt. See if it looks that sharp now. Since you are considering a C8, a 8" SCT will generally not be as optically sharp as an 8" Newton (with a corrector, if it is fast like yours). The objective of the SCT design is not to be superior to the Newt. This is not to say a SCT cannot do a good job, but improved optical performance just isn't what that design is about. The design is about packing all this large aperture/FL combo into something much smaller and easier to handle. Even uncorrected, a 8" f/6-f/7 Newt delivers a better image than a C8 but is already almost thrice as long. And a hypothetical 8" f/10 Newt would absolutely blow away the C8. But of course it would be about 2 meters long so rather tricky to use.  In a nutshell, I will agree with Brian and Bjorn: I think you are okay, you are already seeing limited, there is nothing to gain from going at a larger FL. Instead, you will lose field size (might not matter for the galaxy itself but it does for things like plate solving and focusing) and have to learn a very different instrument with different compromises. Cheers, Dimitris |
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Hi Christian, Hello Dimitris, the change of resolution was a nice suggestion. I tried it and beside some minor things the results were indeed as described by you. Now the differences are much less obvious between both scopes. So it is true what I thought about the limitation of the seeing in the first place and you all prooved it. I think there are worse things than saving some money and stay with the tools I have. CS Christian |
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| While I second all of the above - get seeing-limited ("well-sampled " ) in pixel scale and stop worrying about focal length - there is an advantage to having a camera with large pixels paired with a longer focal length. I'm on the other side of the extreme (an ASI183 - tiny pixels - with a RASA 8 ) and I get images that are just about right for my seeing (FWHM on average around 2.4-2.6" while imaging at 1.2"/px) but what I gain in field of view (which is nice!) I sometimes miss in full-well capacity (and thus, dynamic range). Star cores blow out in seconds, while getting some signal into the pixels for faint extended galaxy arms still takes a bit of time. The larger wells that come with a physically bigger pixel would be nice sometimes. |
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Sounds like you've made your decision.. but just in case you reconsider I can report that the eq6r pro handles my Edge HD 8 very well. My altitude is the same as yours, ~300m. I have used an asi533, asi1600 and asi294mm with my Edge. I think the Edge8 combo with the eq6 and asi294 is a solid setup for galaxies. With a 183.. I would probably just bin 2x2.. and run that until you get a new camera down the road. I've never used the none edge 8" sct but I've heard there isnt much if any difference in optical clarity.. I think the main difference is the flat field. You can see examples of how this setup performs in my gallery. CS! |
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Hi Christian, I can completely understand where you are coming form. But I think staying with your Newt and focusing on the camera is a good way to go. The ASI2600MM sounds like a good choice (I have its larger sidling, the ASI6200MM and it is a brilliant camera), provided your corrector covers the APS-C sized sensor area. It sounds like it does. Ultimately is comes down to SNR per resolution element over as wide-field as practical/desirable. Focal length is a means to an end, not the end in itself. Having seen your images, I would be very pleased and proud to have taken them myself. Your processing skills are also much better than mine. My only "complaint" is that I would want to see more i.e. wider field. The TS Newtonian that you have also produces great images for others on this site. That's how I work out whether a telescope is any good. [I also live around 300m (330m). Unfortunately (for astronomy) I am surrounded by 1500m+ mountains, so my seeing is not quite as good as yours. This may influence my advice. Right now, I find myself gravitating towards shorter focal lengths (200mm-600mm) to increase field-of-view. Telescope entendue (AOmega product) is also something to keep in mind. I made a research career out of it!] CS Brian |
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I chuckle when I skim through threads like this. There's a common misconception that seeing limitation means focal length is diregarded and thats simply not true. an 8" F4 at a resolution of 1" will not collect the same data as a 8" F8 at a resolution at 1". I have proven that to myself several times over. You cannot disregard physics. And the physics is that a larger pixel sure is going to have larger noise but a larger capacity but not linearly. I am currently running a Edge925 at 2350 on a AZEQ6 doing 5-10 min subs regularly. I much perfer my image quality at 2350 than even the reduced images at 1645 or anything a newt of the same diameter can do. I own a 10" Newt, a 10" RC and a 5" Refractor and have owned many 8" so I have a lot of scopes to draw experience from. Long story short, big pixels and long focal length = perferable when it comes to resolving detail regardless of sky quality. My average skies are 2-4" FWHM My recent image of the elephant alone shows more detail than I've seen and done from anything with a shorter focal length. https://www.astrobin.com/full/6xbe63/0/ If this were the case then an 8" rasa with a 183m camera should collect the same quality of data as a 8" RC with a pixel size to match the same resolution, 1.16". lol you will never see this. I challenge anyone to prove me wrong. I have shot very small objects with my 8" RC in bad skies that a rasa simply would turn to mush. |
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Hi Matthew, Your experience is very interesting. I suspect most of those, include myself, that you had a chuckle at were aware that an binned pixel has more read noise than an unbinned pixel. Equally, the full well capacity drives one to shorter subs, exacerbating the read noise a little further. Nevertheless, for most modern CMOS cameras, this read noise is still very small compared to the dominant source of noise (sky) in most circumstances. Therefore, something else other that the mathematical SNR/pixel, must be giving you better images at longer focal lengths. Despite having had a 30-year career as professional astronomer, I am still very much a newbie to the field of astrophotography. I am learning new things every day. CS Brian |
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Brian Boyle: So you would argue that an 8" RASA will collect the same detail as an 8" RC both at 1" resolution and a seeing of 2-3" ? I for one would love to be proven wrong, I will sell everything and own a bunch of 8" RASAS if this were truly the case. |
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Hi Matthew, Ok, In the interest of friendly dialog (can we keep this friendly?), I'd like to take up your challenge. You've got a very nice image there of the elephant! This is a particularly POOR example for me to try to compare as the image I have is only a 30 minute integration taken with a RASA / 183 when I was less than 1 year into the hobby. So, my PP skills were crap; I hadn't started to produce decent images yet, and clearly, it's unfair to compare a 30 minute integration to 16+ hours. Even so, I'll throw out the comparison, and ask in return that you consider comparing a different target of roughly similar size. Do you want to compare a Cocoon or Iris nebula (I have recent images of these with longer integration times). How about M51, M101, or M81? I won't suggest that for small planetary nebula, the RASA is a good fit. But for objects the size of galaxies and nebula, I'll suggest the RASA / 183 can hold it's own and provide a nice fit (with tradeoffs that some will like and others will not). In my (bad) image, my stars are *very* fat, but there are clearly more of them even with 1/30th the integration time. Etendue is physics too! The real question is whether the resolution/detail is much better. The Ha filter definitely shows (the OSC can't compare there). Ignoring lack of signal from much less integration, do you really see that much more detail in the keyhole between the images? From my perspective, I'm not so sure. Our skies have similar seeing. I'm imaging at 0.799, drizzled to 0.399. This should be a good seeing limited test. So, for fun and friendly dialog, do you want to compare a better example? I would be interested in the discussion. You can get a preview of a better comparison by looking at the top of my gallery; I just finished multiple objects a few days ago. All with RASA / 183mcPro. If you're game, I'll look forward to learning from the comparison! CS and best wishes, Doug S  |
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Doug Summers: I see a good difference in the hole but also in the whole nose. My data is sharper and more prominent features, something that I just havent seen with shorter focal length even on my own data. I captured the Luminace at 2350mm and 2 panels, only 2.5 hours per panel so one could say its a 2.5 hr image at f10 so your 30 mins at F2 should sink me. I captured the HaRGB at 1645mm, not only were my stars much larger and grosse the data was not sharp and crispy. I have hundreds of hours of my own data that I can compare to but I am more than willing to compare with someone else. |
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 Here is the Lumanince. 2.5 hours at F10. Open image in new tab to see full res. cant compare downsized images. |
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| Need to keep this apples-to-apples if possible. You're imaging with a mono camera that will have a definite resolution advantage over the OSC. That needs to be fairly factored. Still, I'd like to look at a different object. Do you want to suggest another object to compare, or would you agree for me to select one from the gallery? |
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Doug Summers: You could compare your NGC 891 to mine, yours is with an 11" RASA even. ;) |
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 I star matched and scaled yours to mine then cropped this comparison to 50% to fit into this post. 30 mins at F2 is supposed to be what 1x2x2x2x2 30 mins at F2 = 8 hours at F10 ? Thats what people boast. |
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| Hi Matthew, I think you can be more fair than this. You selected a 25 minute exposure of mine from when I was 5 months into the hobby and compared it to your exposure just released 8 days ago with 6+ hours of integration? How about you allow me the last comparison; I'll select one this time that I think is more representative to make final comments about. Then we'll refrain from hijacking the OP thread any more. I'll do that shortly. Cheers, Doug |
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Doug Summers: I can only compare the same target. Its not my fault you use OSC or don't do long integration(but rasa doesnt have to!) I have not been doing this much longer, less than 2 years. You wanted to compare, I can compare to my own data with my own experience. I have owned 9 telescopes now and have combined many data sets. I have looked at thousands of images and researched a lot on the subject. There are no free lunches. You cannot trade focal length and lose nothing. You'd have to have really bad skies consistently to get crappy data in the first place. If this were the case everyone would be using rasa. Even my own 10" f4 compared to my 10" F8 is not the same. The RC data is certainly higher resolution albeit at a smaller FOV. You cannot have it all. |
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Christian, All good advice above. The longer the focal length, the more galaxies you will be able to target. If you use Stellarium or another app to show framing of targets, you can see your FOV and how it will yield the smaller targets which are more plentiful. SCTs have an advatange over Newtonians in that they are more compact in size, have less of a cross section for windy nights, have less of a moment-arm for controlling with a mount, and are more easily collimated. My C925 Edge HD is serving me well. And Mono cams can be binned to control sampling to compensate for oversampling. Good luck, John |
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Ok, let me wrap my comments up on this. In looking at your gallery, we surprisingly don't image many of the same objects. The best examples from your and my gallery to compare seem to be IC342 and M81/M82. Full resolution to full resolution, the IC 342 images don't exactly line up as my magnification is slightly more than yours. I'll note that while you have processed in more color, I have more stars. While I'd venture your dust structure is sharper, it's not so much sharper that the overall image (when at same magnification) differs very much (especially given OSC vs Mono). These objects seem pretty similar IMO. M81 & M82 seem even more similar to me. You have more IFN (which I intentionally suppressed as I didn't want it to distract from M81 & M82 in my image). If you compare the M82 exploding core details, the images are similar enough that it's likely many folks would be happy with either resolution. So, I'll say that IMO, you took your argument(s) a bit far. If folks match a good optical train up to a good camera that gives an image scale sufficient for Nyman sampling against the seeing, the image quality is likely to be limited by that seeing and post-processing prowess. RASA has a very flat field. The 183 has very small pixels. The result is a ~1 degree field with 0.799 native resolution that can be used well in 2-3 arcseconds seeing with a very acceptable time commitment. 10x expended effort in time doesn't always justify a longer f ratio. Where longer f ratio *is* justified is for *very* small objects (small planetaries come to mind), but only in best seeing conditions. The M81/M82 images are best compared by folks themselves by going to the galleries and looking at full res. People can draw their own conclusions. Ignore color and the IFN (I suppressed it in my photo on the left); it's the resolution that needs to be compared. Again, I'm not making excuses for OSC. It's an intentional choice with a tradeoff I personally like. The question is whether these images are sufficiently different in resolution to justify the assertions made. Some will say yes, others no. Finally, too bad you don't have a cocoon or Iris to compare; those are my latest images and best PP to date. In any case, thanks for the dialog (and why do you have 9 telescopes in only 2 years of the hobby? ;-) Cheers, Doug  |
