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Doug Summers: um… that m81 and m82 is done at 1000mm focal length and yours clearly is not the same as mine. I have done iris and cocoon. Look again. My ic342 is clearly sharper more detailed and cleaner. I would advise getting an eye doctor perhaps. I've also had 9 telescopes because I can afford to test them and make my own decisions for myself instead of justifying a purchase and dying on that hill. Some of your images are ok but your processing is lacking and the stars are very unpleasing. I've looked at a lot of rasa images and I can't get over that. |
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Hello! I think you can safely stop comparing images. From the signal that your sensors have generated, to the final image, you probably have diverged so much in post-processing that you won't get to the same result. This is absolutely not the way to resolve the question at hand. Matthew Proulx: 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. Yes, let's stick with physics! I love it. Please don't compare apples with oranges (or pears.... seems to depend where one's from). Let's go: We should assume different optics and identical sensor. Let's take two magical lenses (i.e., without any aberrations) both same aperture but focal lengths F and 2*F (e.g., 8" f/4 and 8" f/8 ) and keep image scale constant. Having a sensor which was "ideal" for the f/4, you have to bin 2x2 to have the same image scale on the f/8. While the signal on the super-pixel remains the same, the thermal noise is 4 times larger and the read noise as well, compared to the unbinned pixel on the f/4. Since signal is the same and noise increased, the SNR became worse going from the f/4 to the f/8. (FWD is also determined through the thickness of the photosensitive cell and not just the area.) It may well be that there is a better sensor for the higher FL: For example: the ASI183 sensor has a pixel size, ideal for a RASA. On the other hand, the ASI6200 sensor has pixels with 3.76microns which is 56% larger on the edges and 245% larger on area. The latter has also a higher FWD (corrected for pixel area), lower read noise at unity gain and still keeping dynamic range at maximum, and the dark current is much much lower on the ASI6200. But now we're talking about the sensor and not the original question. Björn |
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Well, no need to get snarky about any of this. On the Iris & Cocoon, these are very good comparisons. Folks should look for themselves. To me, it doesn't seem fair (to you) to compare those images. You don't actually have enough signal to compare favorably given the different f ratios and integration times involved (and it shows). However, that said, at full resolution, a star split test in any portion of those images will show most folks that the resolution is essentially the same. Our processing styles are just different. My eyes are old, but they're not that old! If we can't agree, at least we shared some thoughts that others can consider for themselves. Let's close out disagreeing amicably. I enjoyed the dialog. Cheers, Doug p.s. One thing we can agree on is that my processing skills definitely need to improve. I concur, and I'm working on it. But this wasn't supposed to be about post-processing. Peace.... |
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| You know as camera technology advances, we are seeing lower noise and greater QE. Therefore I think we can now squeeze a bit more out of larger F ratios scopes like F/10 SCTs, and not have the absolute need for super fast optics. Of course having high QE and low F ratio might be the best, collecting the most photons in a given exposure length. I'm sure someone out there knows the math on this. |
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You know as camera technology advances, we are seeing lower noise and greater QE. Therefore I think we can now squeeze a bit more out of larger F ratios scopes like F/10 SCTs, and not have the absolute need for super fast optics. Of course having high QE and low F ratio might be the best, collecting the most photons in a given exposure length. I'm sure someone out there knows the math on this. There’s a trend on which the vast majority of astrophotographers doesn’t have any influence on: the sensor technology and sepcs! While there are many manufacturers of optical systems which make them for the purpose of amateur astronomy, the manufacturers of cameras need to purchase the sensors that are on the market. The trend of the „affordable“ sensor market is clear: smaller overall packages with smaller sensors and smaller pixels. Large sensors (FOV) and large pixels for larger FLs are something that will be in the realm of scientific research where dedicated CCD sensors will be manufactured but which the hobbyist won’t be able to afford in any practical terms. Björn |
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Christian Großmann: Just gonna side step the whole debate going on above for now If you are looking for detail you want a big mirror/lens. You can only resolve so much detail depending on the aperture. Once you hit 8 inches and above the After that you can start thinking about pixel scale. This is where focal length comes into play. In a nutshell, you have to balance focal length with focal ratio. The higher you can crank up the focal length, the smaller your pixel scale will be and the more detail you will get. The trade off is the higher your focal length gets the higher your focal ratio gets. The higher your focal ratio is, the more integration time you will need to attain an acceptable signal to noise ratio. You can mitigate this trade off by going to larger apertures. The bigger the mirror, the more focal length you can cram in while maintaining an acceptable focal ratio. Try not to worry about pixel size so much but if it was me I would try to keep it above 0.6 arc seconds. The optimum sampling rate is 1/3rd the seeing. Something to do with measuring light with square pixels or some such. It's also why the dawes limit is not a ride-and-die rule for us, more like a guideline but I digress. If you really want I can dig up the thread that explains this! So if you say your seeing is in the 2 arc second range then 0.6 is a good time. If anything, once you start going below that you are going to be guiding limited more than anything else unless you truly do have awful skies. For you, I would say the C8 is unwieldy. This is for two reasons. First, its at f/10. If you value your time then unfortunately that is a little too slow for astrophotography at 8". Second, there is no good way to reduce it without sacrificing the quality too much, which was not that great (for DSO imaging, its amazing for visual and planetary!) to begin with. If you are a brave man you can check out RC scopes. You can get an RC8 down to f/6.something with a multitude of reducers. You keep they dynamite pixel scale from the 183mm AND you get a nice focal ratio so you are not collecting photons for the rest of your natural life on one target! If you don't have the stomach for Mr. RC Scope's wild collimation ride, I would check out the Edge HD scopes. Great optics and you can get them down to f/7 with the reducers. F/7 is alright, but eons better than f/10. The 183mm throws a wrench in things though because of the small pixel size. I mean you are already at 0.6 scale. Maybe I'll be the contrarian to say that your dream galaxy scope is already in your garage...  https://www.astrobin.com/99zaj5/F/ There is an example of someone using a scope with a similar aperture and focal ratio as you have now. That's a lot of detail to me! --------------------------------------------------------------------------- As for the RASA debate I will put it like this: If you slapped the same camera, same filters, same mount and same everything except for the telescope, the longer focal length scope is going to win on detail every time. It simply has a smaller pixel scale. Drizzle all you want but you'll never beat the inherent pixel scale advantage of the longer focal length, if both images have the same SNR at the end. Heck, thanks to diminishing returns the longer focal ratio scope doesn't even need to reach the same SNR as the one with the faster focal ratio. Focal length controls the maximum amount of detail attainable via manipulating the pixel scale. Focal ratio affects how fast you can build a good signal to noise ratio but it can only affect detail in so much as reducing the noise to reveal it. Now you know why all the calculators out there for pixels scale and suitability don't care about focal ratio, just focal length. I have a RASA. I used it for all of two hours before it became the worlds most expensive paperweight once I went to Montreal for university. I loved it. But I will never claim it can do what an RC can do with galaxies.. If I paired my reduced RC8 with a 2x2 binned 294mm, it gets to 0.74 arc seconds/pixel. If I binned 1x1 with a 294mm on a RASA 8, it only reaches down to 1.19 arc seconds/pixel. Detail contest over, RC8 wins. Assuming we are not working with telescopes with dollar store quality optics, which ever one can produce the smaller pixel scale is gonna win the detail contest. |
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| Hi semi-pro. We're almost together in your analysis, but not quite. The RASA-11 and 183 have a platescale of 0.799. That's not very different from your RC8 with a 294 camera (0.74). It becomes a seeing game in the margins. The 294 has twice the pixel size as the 183. Better dynamic range, but larger pixel (unless we're talking about "unlocked" mono mode where the pixels are even smaller than the 183). So, what I was saying is that it's not universally true that a RASA will under perform. In your case, the combo (RASA+cam) didn't work as you wanted. In other cases, a careful pairing could be just fine (like with a mono 294 unlocked or 183), The larger aperture and flat field will yield benefits too. The trade between an unlocked 294 and 183 is a field size / image quality trade (RASA is flat, but not perfect, and degrades as a function of off-axis distance). So, it's just not a one-size-fits-all answer about RASA performance. If carefully considered, a good system can be built that will perform well with reduced time required. |
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Thanks for your answer. I think that the F/10 from the C8 (EdgeHD) is really´ slow. At least if you are used to image with an f/4 ratio, this may be a pain to use. So the reducer was a must for me. Instead I could've looked at the RC scopes and I did. But honestly, the discussions above are really helpful and I am glad I asked the question here. It turned out much more interesting as I had hoped. With a night between my initial message and your answers, I really changed my mind. We are talking about a lot of money here. With all the adapters, flatteners, etc. this might be an investment of about 3K (in Europe). This is a serious number for most of us. A new mount to handle a bigger scope (one day I will upgrade to an EQ8 or similar) is another 4k (a step up from the EQ6 is really pricey). The 8" f/4 really is a sweet spot for me, but is not the "One-for-all"-solution (nor is the C8 ). There are a lot of interesting fields to cover and I will go the safe way for now. There's more to come...  My idea is, to maybe visit a star party and get into contact with other astro photographers. Then I may get the opportunity to try some stuff out or to compare different options in a real life scenario myself. CS Christian |
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I'd like to (hopefully) clarify this discussion about RASA performance. There *are* some fair knocks on a RASA imaging system, but thus far, I haven't seen them fairly represented here. Let's all remember that we're in a seeing limited regime. The knocks on a RASA imaging system at lowest seeing capability (for amateur sites, approximately 2+ arcsecs routinely, with rare instances of 1+ arcsecs) are: a. The wind profile of the OTA is higher than a smaller alternative seeing limited system. This puts stress on mount performance. b. The weight of the OTA is higher than alternative seeing limited system(s). This also puts stress on mount performance (and possibly mobility+your back). c. (maybe the biggest): There are only 2 imaging chip choices for the smallest imaging plate scale, forcing other choices: - a 183 OSC (small pixel size, allowing lower imaging time required, but with a resolution loss, and dynamic range hit (12 bit ADC)) - a 183 Mono (same small pixel size, but forced filter work to get best possible resolution, giving up the time advantage. Still that 12 bit ADC disadvantage) - an "unlocked" mono 294 (smallest pixel available, with a larger field, still a hit on dynamic range, and slight image quality drop due to larger off-axis field) The trade-offs come down to choices of imaging time, dynamic range, mount cost, field size, and corresponding resolution. Bottom line is a fine RASA imaging system can be built that can match other seeing limited systems for small seeing limited scales, but the size/weight profile will drive a bigger mount, and you may find it necessary to do filter work and accept a dynamic range hit due to available imaging chips. There is no "physics" reason why a RASA can not perform well for seeing limited galaxy and nebula work excepting for the reality that the available imaging chips are dynamic range limited by 12bit ADCs. This stresses nebula work more than galaxy work. The rest is just $$ (mount performance). On the plus side, bigger aperture and lower f/ratio in the smallest platescale means more photons faster. I hope this helps others who might be thinking about their own choices. CS |
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Doug Summers: *** but a smaller pixel is collecting less photons than a larger pixel. 🤔 that’s the physics I’m talking about. you’re talking about speeding up a scope and then cramming it into a tiny pixel to reach the same resolution. Effectively not only have you slowed it back down, youre limited by the camera hardware now. Lower dynamic range and well capacity. Not to mention all the other problems that come with the rasa, poor star shapes, hard to focus, can’t use filter wheels without taking up precious aperture. Those using OSC are trading resolution and thus potential “speed” because 50-75% of the light is lost on pixel area not being used. We’re already talking a small pixel now divide that by 4 again. Even lower capacity and dynamic range. The rasa is potentially a fine instrument for wide field in the right hands. I can’t dispute that, though I much prefer the epsilon in every aspect. Too many times have I said eww looking at rasa images. every telescope design has its place, to refute that is an error. I’m done talking on this subject because it’s like beating a dead horse. I’ll continue on getting better images and better at processing with the equipment I use and know how to use. I certainly take all theory and credentials with a grain of salt. Nothing can replace actual in the field results with sharp eyes. |
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| Still, you can't put an EP in a RASA isn't it. Big limitation if you ask me. Nor barlow, afaik. And seriously, would you want to put a big DSLR in there? And wouldn't be so sure of the wisdom of comparing scopes with different apertures either... |
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Christian Großmann: Attending a star party is a great idea! I also like to search different combinations of equipment on Astrobin to see what others are able to with them. It's how I decided on pretty much all the equipment I use. |
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Christian Großmann: Indeed, switching the OTA can easily turn into a purchase of a whole new setup. That's why it's important to gather experience with the equipment you have and see if and where you might want to change something. If you change equipment too often, you'll likely be busy tuning and fixing things instead of producing quality data. (unless you enjoy fiddling a lot with your equipment). Björn |
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I was/am in a similar situation. I'm also considering looking at some higher focal lengths, and the C8 seems interesting. Mainly also because it would allow for the hyperstar addition to turn it into an F2 light gathering monster. Anyway, also using the 183MM and MC, so pixel size would be a thing. Now, what I do wonder, and never have been properly able to find. What's wrong with over sampling? in easy terms. Undersampling I get, the stars and details being smaller than the pixels of the camera, so instead of detail, your star only fills 2x2 pixels, when that star or detail could/should have filled 10x10 pixels and things turn blocky and less detailed, which.. is obviously.. bad.. But what's the exact issue though with oversampling? This is how I see it; in my mind, (and I'm probably wrong) the same detail of a nebula is being spread out of more and more pixels when over sampling, which would mean more detail but this obviously depends on your seeing conditions and no matter how many pixels you throw at it, it won't necessarily get more detailed if your seeing conditions only a certain arc seconds thingie.. But aren't you then just getting as much details as possible as your seeing at that moment allows you to get? Even if it would take you a bit longer to collect those details. (Possibly with more unnecessary noise than if a camera had larger pixels... but.. still.. as much detail as possible at that time?) So...in my mind, it's perhaps not ideal, but is it.. actually... bad? I never quite understood this, so if someone please can say if I'm on the right track here. or completely wrong. Either way. So far I'm not sure on the C8. I already have an Esprit 80 at 400mm. So the hyperstar conversion wouldn't be a huge change. So spending the extra 1000 or so euro to get a hyperstar conversion, seems a bit pointless if I have the Esprit going, as nice as F2 would be. And to use it as a longer focal length, I'd want to get the reducer at the very least. Meaning you're already reducing it from the 2000 to about 1200mm... But then I'm thinking, why not an RC or Newtonian with similar 1200-ish focal lengths, F4-F6, usually a lot cheaper. Anyway, it would also mean I'd upgrade the camera. And all of a sudden I'm basically setting up an entire new rig.. While not being bored yet of all the current targets I get to shoot. So, holding off on the longer focal length for now. And parts of the discussion in this topic has helped with that decision, so thanks everyone! |
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But what's the exact issue though with oversampling? Per es, there’s nothing wrong with oversampling from a mathematical point of view. (Oversampling means to collect data with a higher resolution than necessary to reconstruct the original data. The limits are determined through aperture and/or seeing whichever leads to the larger spread of a point). From a practical point of view, there are some issues: 1. the size of the sensor is limited. If you oversample by factor 10, your FOV has been unnecessarily decreased by ten. 2. Since the same signal is spread out over several pixels, the signal per pixel per time unit is decreasing accordingly. However, thermal and read noise don’t care about that and therefore, signal to noise ratio per pixel decreases. You can recover some SNR by statistically transforming the data (that’s what binning in CMOS is essentially doing) but that’s exactly what one should avoid by matching OTA and sensor specs. CS, Björn |
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I was/am in a similar situation. Basically, I also think there is nothing really wrong with oversampling, as long as you downsize your image. Oversampling and seeing basically means, that the image will look blurry, isn't it? The details you try to get by increasing the resolution in a "perfect system" will not be there because of the air movement and because your system isn't perfect at all. I think, all of this is mangagable if you don't go into extremes. I really realize the difference in the image sharpness going from my 70mm f/5 Flatfield APO to the 8" f/4 Newton to the 10" f/5 Newton. With every increase of the focal length (with the same 183MM sensor), the image loses sharpness and the stars get much bigger and are not as pleasant as they are on the 350mm APO. That, in fact, is the main reason why I wrote this first message here. Christian |
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Ok, I've let this topic run it's course (no updates here in a day or two), so I hope the OP got his money's worth on the original question posed. I'd now like to come back and address some pretty outlandish assertions made about RASAs by a couple of people (but primarily 1 person). That person issued a challenge. The claim was made repeatedly that RASAs have poor star shapes, and can't match resolution / details of RCs of equivalent size. Further, is was said that RASAs are only good for wide field work (and only in "the right hands"). The person said (and I quote): "I for one would love to be proven wrong, I will sell everything and own a bunch of 8" RASAs" (if it is the case that an 8" RASA can collect the same detail as an RC with 1 arcsec platescale in seeing of 2-3 arcsecs). He further stated "Long story short, big pixels and long focal length = perferable when it comes to resolving detail regardless of sky quality". So, the challenge was accepted. We started by coarse comparison of IC1396 images. This was done only to get a first look at the question of resolution (not signal or image processing or anything else). The comparison was admittedly poor except for the stars (as I didn't have a good IC1396 image). No agreement. Next, comparision was done (unilaterally) of my old 25 minute NGC891 against his 6+ hour image just released days ago. I balked at that and asked for a better comparison target. I then compared M81+M82 images, and IC 342 from our galleries. We couldn't agree in those cases either (different scales didn't help). When I stated that it was too bad that an Iris nebula or Cocoon Nebula image wasn't available (these were some of my latest images), I was informed that I had missed those images in his gallery and I should "look again". I did so, and now we all can look together and form conclusions about the results. It turns out that these two images are cleanly able to resolve what math already suggests; that two equally sized imaging systems with equivalent imaging platescales are going to be capable of achieving similar resolution and quality in a seeing limited regime. Is anyone going to be surprised by this? I'm now going to make the bold statement that not only does the RASA match the resolution of the compared Newtonian and RC, it exceeds them. Does the math support this claim? Yes. The platescale of a RASA 11 + ASI183mcPro is 0.799 arcsecs/pixel. The compared Newtonian has an imaging platescale of 0.95 arcsecs/pixel. The RASA should outperform in best seeing. We have two images. One (Iris Nebula) is not drizzled by the Newtonian, but is drizzled by the RASA. The second example is drizzled 2x by both telescopes. Ok, here we go: Example 1: Iris Nebula. I've cut out the "hole" in the Iris and matched scales so folks can see the "poor star shapes" of the RASA. Can you guess which image is the RASA image and which is the Newtonian image? Look closely. Ignore the nebula entirely and look at the stars in the hole. Look at shapes and for close double stars that can be used as resolution star split tests. Guess what: the math doesn't lie! The RASA's resolution outperforms the Newtonian. The image on the left is the RASA image. The right side image stars are square, and doubles aren't easily split (relative to the left image). Are you surprised? The RASA's 0.399 (drizzled) capability can support Nyquist sampling down to 1.2 arcsecs of seeing (i.e. round stars). The Newt image on the right can only achieve Nyquist sampling at 2.85 arcsecs or higher (hence the undersampled blocky stars in better seeing). Ok, so maybe not fair to compare a drizzled RASA image to an undrizzled Newt image? Let's go to object #2 (Cocoon).  Example 2: Cocoon Nebula. Both images in this case are drizzled x2. The RASA is still 0.399 arcsecs platescale drizzled. The RC in this case is stated to be at 0.59 arcsecs drizzled. I've taken only the core of the nebula and scaled it to better match each side (I did this two different ways, one by upscaling one image to match the 2nd, and then again by downscaling the 2nd image to match the 1st image). In both cases, the results don't matter to the comparison. Now for the eye test. Look first at the smallest star shapes and sizes in both images. Also look at small double star separations near the core. The RASA (right image this time) again has higher resolution (just as the math would suggest). Now factor that the RASA image is only 3hrs 20mins vs the RC image of 4hrs 20mins. 25% less time in the RASA image, better resolution, and 2-5? times the signal? I'd argue the details and dynamic range are better, but the scope of this discussion is just resolution and star quality. So, are you convinced yet? If not, I suggest going to the galleries of us both and look *not* at post-processing prowess (clearly I'm learning), but at star quality and resolution in the full scale images. In my case, I'd recommend staying in the top 1/2 of my gallery, as the bottom half is almost exclusively 30 minute test frames taken in my 1st year in the hobby. Those are all being reworked now. The top half has mostly 2-4 hour integrations.  Hopefully this exercise will help dispel the myths being widely circulated about RASA resolution and star quality. RASAs can do narrow field work just fine as long as they are paired with a camera that is matched to the desired platescale for Nyquist sampled images against expected seeing. Their stars can be fine too if the mount behaves. Math is math, physics is physics, and seeing actual images is (hopefully) believing. Myths and trash talk without support on the other hand is unworthy of any of us. I'm not arguing that Matthew should now eat crow and honor his commitment to sell all his gear to buy those RASAs like he said he would. I do think however that a good compromise would be to refrain from talking trash about other peoples equipment when not supported by direct experience, math, or actual evidence to back up assertions made. Finally, to the other person's claim about RASA being a good paper weight, I'll just note that nobody should take a camera off an old telescope and put it on a different telescope without some thought. Garbage in = garbage out as they say. All imaging setups deserve some thought, or a paperweight result is probably well deserved. With that, peace and CS to all. Doug S. |
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You still comparing my olds pics to yours? That cocoon I did in bortle 5 with a color camera and an RC8 last summer, I now live in bortle 2 and do mono, I would definitely get a better result now lol. I dont even use either of those two telescopes. Compare your RASA11 to a C11. Youre comparing processed data of different scopes a different resolutions. You're not very smart are you? You will not achieve the same resolution with a RASA11 as you will with a C11. What are you arguing? You started comparing your pics to mine. Get out of here. I wanted to compare equal aperture telescopes of different focal lengths all other variables being similar. The rasa stars are aweful, I stand by that to the end. Compare your data to my EDGEHD if anything. I challenge you, to get an image better than this with your RASA 11 against my EDGE925 My resolution scale is .33" First pic binned 2x second pic binned 3x in software. Are you up for that challenge!? Otherwise, sit down.   |
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| I gave quotes on your assertions. I did comparisons you yourself agreed to have me make (Iris and Cocoon). Now folks can draw their own conclusions. In the end, the assertions you made were provably false. So, be nice and let folks look and draw their own conclusions. We've both expressed our opinion(s). Others can decide for themselves about whether the assertions were real or just trash talk. Cheers, Doug |
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Doug Summers: Go start collecting data. I'm collecting data right now. Let talky more walky. |
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| You're just moving the goal posts now that your assertion(s) didn't work out. I don't have a mono 183 to do that test, but if I did, I'd have no problem taking up the challenge assuming the platescale on your 925 is equal to or greater than 0.799. Seeing limited regime, platescale, and mount performance are all that's going to matter for a resolution test of these two apertures. Bottom line is that it isn't going to be the OTA that will differentiate the result. |
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Doug Summers: AM I? And I quote myself : "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"." You changed the goal post by comparing your 11" RASA to my 8" RC. Dont make excuses for using OSC thats your fault. I guarantee you, I will not get my results with a hyperstar on my scope, in fact I dont even get comparable results with the reducer. A RASA was made for widefield imaging, it's not even close to diffraction limited, it's a compromised design. |
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| I made it clear I had a RASA 11" from the start. Doesn't matter whether 8 or 11 though. Both operate in the seeing limited regime. We're not talking about diffraction limited resolution here. Also never made any "excuse" for OSC. I like OSC and accept the limitations vs the benefits. Just can't do the "moved goalpost" test you now want to do, as I don't own that equipment. You'll need to find someone else for that (but I'm sure the result isn't going to be different from this earlier comparison if you do find someone to do that test). |
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Doug Summers: Even seeing limited your RASA11 is simply not going to operate at the same resolution of a EDGE11 unless you're severely seeing limited which is rare. Believe it or not the optimal sample rate is actually a lot more than people think. 3.5x is a good middle range but 4-5 is not bad either. |
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Matthew Proulx: Here we go with unfounded claims again. Here's the spot diagram for the RASA11. While it's true the RASA was conceived for wide field, that doesn't mean it can't do narrow field as you've claimed. The 183 constrains the off-axis distance to the first 3 rows due to chip size. Do you want to elaborate on what you see as technically compromised in the spot diagram for us all to consider? Do you have a spot diagram for your other preferred systems so we can compare?  |
