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I will preface this by saying that I have an 8-inch Edge HD scope, which I love (the OTA at least). This might sound like a silly question but is there much of a benefit to upgrading to a 9.25- or 11-inch Edge HD scope with respect to long exposure deep space astrophotography? I am obviously aware that there are weight and cost considerations. I understand that aperture is king for visual and planetary imaging but for long exposures the limiting factor for image resolution will almost always be the seeing conditions so, in my mind at least, additional aperture wouldn't really add anything in this respect? Are there are other benefits to DSO imaging that I may not have considered? Any thoughts would be much appreciated. Cheers, Rob |
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It does indeed. But, as we know, there's no free lunch....Of course, as you go larger so does the price. AND the weight - you may need to invest in a beefier mount and maybe counterweights. Also, I beleive that the focal reducer (if you use one) may not fit the larger scope(s) and you'd need to purchase that. I started out with a C8 ages ago and that was my scope for a long time. I did eventually move up to a 9.25 (bought two of them actually - used). Sold one of them and still have the other. Within the past few years, I moved up to an Edge HD11 and really love it. I've been around this hobby for probably close to 40 years (started wtih film!). In that time I've also accumulated other scopes. Before I got the 11, I purchased a Sky Watcher Quattro 250P 10" newtonian. Love that thing. And the price is far less than an SCT. Again, keep in mind the weight of new equipment and if it will not overburden your current mount. Hope this helps Alex |
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It really depends upon what you plan on imaging. I have an Edge 11 inch SCT and a 130mm refractor. There are many deep space objects that are quite large (mostly nebulae) for which I only use the 130mm refractor. When I am imaging a galaxy or a planetary nebula, I almost always use the 11 inch SCT since they are generally quite small. If I am imaging a galaxy group, I revert back to the wider field of view refractor. As others have said, your mount has to be capable of handling the additional load. Roger |
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If you’re thinking of upgrading then go for the 11 over the 9.25. The extra aperture is worth having. You are correct that there won’t be any gain in resolution, but you will collect more light from the object in a given amount of time. But also consider, if you are keeping the same camera, the image scale will get larger, so your sampling will get finer. Cheers, Ian |
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I understand that aperture is king for visual and planetary imaging but for long exposures the limiting factor for image resolution will almost always be the seeing conditions so, in my mind at least, additional aperture wouldn't really add anything in this respect? Are there are other benefits to DSO imaging that I may not have considered? Aperture is absolutely king in DSO imaging too. The speed of your system (how much total time it takes to achieve a given SNR) at a given image scale is directly proportional to the area of the aperture. Speed is king in imaging and so aperture is king. If you took images with each and sampled them at the same image scale, the 9.25" would be 34% faster than that 8". The 11" would be 89%. Now, it's true that if you don't hold the resolution constant by using the same pixel size and all you care about is SNR, they would have the same speed because they have the same f-ratio. And furthermore, the resolution of all of them at f/10 with the usual camera suspects is wasted, but you can downsample the data all day long. With a 3.76um native pixel, a 3x binned 8" and a 4x binned 11" would have about the same effective (and decently sampled) image resolution and the above 89% speed advantage holds. Of course, astro is full of trade-offs: the 11" would have only 53% of the 8" FOV, but that wouldn't matter for objects that fit both. |
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I agree that EdgeHD11 is great OTA, but for resolution large refractor can compete with Here is my recent EdgeHD11 image, M16, HEA43 / EdgeHD11 / 6200MM(Bin2) / EvoGuide50+174 0.55 arcsec/pixel, this is equal to 1400mm f/5 + ASI1600MM pixel resolution and FOV. https://www.astrobin.com/tplosa/ When I compare to Askar 130PHQ, also M16 at same day, resolution difference is next to nothing. https://www.astrobin.com/tqg18l/B/ I will go to Askar 151PHQ and I bet 151PHQ's resolution can be better than EdgeHD11... |
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When I compare to Askar 130PHQ, also M16 at same day, resolution difference is next to nothing. I don't know. I see a noticeable resolution difference in favor of the Edge (which is probably not even fully realized at that pixel scale) but it's also hard to compare two independently, fully processed images. Plus if you downsampled the Edge further to have close to the same image scale, the Edge system would be nearly 5x faster at getting you a 1.1" image if that's satisfactory to you. Speaking of, how did you get a 1.1" image out of a 130/1000 with a 3.76um pixel? At native pixel size, the image scale should be 0.78" and a 2x2, 1.56". |
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I agree that EdgeHD11 is great OTA, but for resolution large refractor can compete with Hi, I see that EdgeHD11 still gives higher resolution than 130OTA. Am I wrong? |
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Jackie: Nope. I concur. It's quite striking actually. |
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Agree with the comments above. One thing I think of often when choosing an OTA but I do not hear discussed enough in my option is field of view. The C8 has a focal length of 2032mm. The C11 has 2800. Both can be reduced with a focal reducer but the same point applies. When I choose my OTA I try to select a range of focal lengths so that my FOVs are different. I currently own a 405mm refractor, a 600mm RASA 11, and a C11 EdgeHD at 2800 that I use with a focal reducer at 1960mm. This gives me different FOVs to optimize for a given target. I intend one day to get one more OTA in the 1200mm range to fill that FOV hole. So consider your different FOV and the targets you will get when considering a new OTA. Eddie Hunnell Longmont, CO |
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Jackie: Hi sorry for confusion, the results are processed by BXT so sometimes I can see very good result Here is two raw Ha image, taken at same clear and calm night. 130PHQ(1000/7.7/3.76um) is 140% resampled to match EdgeHD11 Bin2(2800/10/7.52). Just my opinion or Just I thought, EdgeHD11 280mm vs 130PHQ Askar : almost equal resolution (tested another day, 130PHQ Askar vs TS 200mm f/6 newtonian + 533MM without corrector = without any glass lens : 130PHQ wins) in short, I wanted Rob to remember that there is another way, large refractor. of course having both is great fun for astrophotography  |
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Roman Pearah:I understand that aperture is king for visual and planetary imaging but for long exposures the limiting factor for image resolution will almost always be the seeing conditions so, in my mind at least, additional aperture wouldn't really add anything in this respect? Are there are other benefits to DSO imaging that I may not have considered? *** Thanks Roman, appreciate the detailed response. Note sure I completely understand your explanation and from looking at other articles and forum posts there appears to be a lot of confusion over this. Focal ratio determines how fast or slow a scope is and the focal ratio of both the Edge HD 8 and Edge HD 11 is the same? Granted the Edge HD 11 has a different focal length and so it will have a different pixel scale that can be traded off for imaging time (through binning), which is what you were suggesting if I'm not mistaken? Light gathering ability is proportional to the square of the aperture area and this typically a metric referred to in visual astronomy. Aperture alone doesn't solely determine the flux hitting the sensor so I'm not clear on how that equates to total integration time for imaging? Apologies if I have misinterpreted. *** |
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I have to agree that the difference in resolution between an 11" SCT and a 130 mm refractor (for DSO imaging) is not enough to argue about. The 11" SCT really distinguishes itself with resolving lunar and planetary details by lucky imaging, though. If you don't care about lunar and planetary imaging, then I'd say to go for "ease of use" first and choose a telescope that will reliably produce results at your given location without a lot of babysitting and maintenance. For example, my C11 EdgeHD made a lot of sense when I lived in western Texas and had plenty of clear nights with dry air. It is unbearable to use it here in NY where the weather is humid and rainy. When I do get a clear night, I have to stay up all night and fiddle with dew shields, a hairdryer, and various issues related to the mirror locks and temperature effects, which have prevented me from buying an autofocuser. My 100 mm refractor provides enough resolution for most DSOs, has a huge field of view, has better dew/frost resistance, and has an autofocuser. It does not tolerate cold weather well, though. Match your telescope to your climate and make your life as easy as possible. For me, the second consideration is field of view. The small FOV of the C11 EdgeHD is more than just annoying if you live in a climate that doesn't provide many clear nights. Even when I get a nice image with the SCT, I could have imaged 25x the field of view with the refractor in the same time, with just a little loss of resolution (or sometimes no loss when the seeing is poor). I mainly choose the C11 EdgeHD when I have dry weather and good seeing, and I want to image a small planetary nebula or globular cluster... which is once or twice a year. |
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Jackie: So the left is from 130OTA and the right is from C11, right? I think that the difference is still there. Thanks for sharing these images. I got a C925 and an APO140. In general, for deep sky imaging, C925 is kind of more powerful than APO140 in terms of resolution  |
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You will never beat a refreactor for ease of use and reliability over an SCT. I own a WO GT71, ES 102 FCD100 CF, ES 127 FCD100, a C8, a RASA 8 and an Edge 11. The majority of my DSO images are taken with the WO GT71 and the ES127. Once in a while the RASA. Planatery C8 and EDGE 11. In the RARE case i want to get in tight on a DSO (<3% of time) i will use the Edge 11. The RASA and Edge have gone back to Celestron for repairs and alignment at least 4 times each over the years. I have colimated them too many times to remeber. So if you want easy of use and reliability go with a big refractor. In fact ASKAR just can out with a 185 for an exclllent price. I may be giving up my RASA 8 and C8 for that. SCTs need care and love and TIME. I dont have too much of the last. And dont get me wrong. I love my SCTs, and especially my EDGE 11, but since i live in New England, where the weather is stingy with clear nights, i always reach for my refractors. Choose wisely... |
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Jackie: What was the FWHM in arcsec of your EdgeHD Ha frame vs your Askar 130phq? |
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Ashraf AbuSara: Yes we should talk with numbers, EdgeHD11 stacked, without BXT : 206 x 7.52um / 2800mm = 0.55as/px, 5.3px = 2.93arcsec 130PHQ stacked, without BXT : 206 x 3.76um / (1000mm x1.4) = 0.55as/px, 5.94px = 3.29arcsec -> EdgeHD11 can be 12% better than 130PHQ  This test was done June 2023, at one night, EdgeHD11 on HAE43 + 130PHQ on AM5 (both PHD2 shows around 0.5"). So "Seeing" can be same on both OTA. now I have 151PHQ. I tested 130PHQ vs 151PHQ, and 151PHQ was 15% better than 130PHQ. AFAIK 151PHQ has almost same optical resolution as EdgeHD11, and far more easy to handle. |
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AFAIK 151PHQ has almost same optical resolution as EdgeHD11, and far more easy to handle. No, it has not. It has 279.4/152 = 1.83x the resolution of the refractor, assuming both operating at their resolution limit. Weight being about the same but the C11 being the more compact of the two handling should actually being comparable. Note: I had a C11. |
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Ashraf AbuSara: Have you tried Bin1 with your EdgeHD? Not exactly an apples to apples comparison when it comes to resolution if the EdgeHD is Bin2 and the Askar PHQ130 is Bin1, because you are deliberately sacrificing resolution for SNR in case of the EdgeHD. Maybe you would be too oversampled for your seeing conditions so hard to know. The assessment / recommendations highly depends on your seeing conditions. For example from my backyard here close to coastal Texas, I routinely can have imaging conditions that support a resolution under 2" in the Ha frame. A FWHM of above 2.5" is below average for my location. It is not that the EdgeHd has the same optical resolution as the 151Phq or just 15% worse than the 130PHQ, its just your seeing conditions do not support a better resolution. This is all assuming that the EdgeHD has good collimation and that thermally it has been managed well (either with insulation or good cooldown), and guiding error has been below the image scale posted for the EdgeHD to fully utilize the resolution. I don't have Askar PHQ 130 but I had an AT130EDT and it never matched the resolution of my standard C11 with a starizona reducer under any conditions. Maybe the quality of that refractor is inferior to the 130PHQ, but that was my experience with a 130mm refractor. Note that the C11 was acquired hand for a fraction of what the 130phq costs. I don't have a direct apples to apples comparison (yet) against the AT130EDT, but I rarely imaged with it during the summer because I enjoyed the additional resolution that came from the C11. I typically used the standard C11 at Bin1 with 2600mm pro and a 0.7x reducer. C11 optics are slightly slower than the official specs so the FL was 2065mm and the resulting image scale image scale was 0.3755"/px. I am oversampled but didn't feel the need to Bin2. Here are a couple of stacked Ha frames for different targets during the past summer with the C11. This was all done on the AM5 harmonic drive btw.   |
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Ive considered making the move myself, from an 8 to maybe 11. The 8 is limited by my seeing conditions 99% of the time, so the 11"s extra possible resolution and smaller image scale wont gain me any extra detail. As far as SNR is concerned, both systems will be the same F stop, so i expect extended sources of light to recieve the same illumination per area. The difference will come down to image scale and binning between them. The 8 gives me a slightly larger scale, and the larger edges smaller. If im always binning 2x2 the 8 givws me around 1.1 ar sec, which is just a bit under sampled. The 11" is preferable at bin 2 as I could still bin2x2 and sample typical seeing well. Where as on a 1x1 bin, both scopes will oversample seeing. Not worth the switch in my opinion. Im considereding an Espirit 120 to complement the egde8 (and whatever RC I upgrade to from it), to provide a faster, wider FOV scope. |
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To me it is worth it to go with a larger longer focal length scope only if I can match the camera to the focal length of the scope to achieve a reasonable and useful image scale for the application. For example I use a cooled IMX432 on my longer focal length scopes. With the large 9um pixels there is far more signal to each pixel compared to the more common cameras, and the overall arc second per pixel resolution is still within the seeing conditions of my location. With the C925 and reducer I sample around 1.25" which is wonderful for my location, and the 9um pixel camera has about 5.75x the area per pixel as a 3.75um pixel camera does. That's roughly 5x the amount of signal per pixel just by choosing an appropriately matched camera to the telescope and the use. The added resolution of a large telescope is not necessarily something that I can take full advantage of any my location of generally poor seeing conditions. In other instances where I wish to split and measure double stars with a long focal length telescope I will use a smaller pixel scale camera and very high frame rates, then apply planetary imaging techniques. It's all about what I plan to do with the telescope, which camera I choose for that telescope. All of that is to say, if you are using one of the popular 3.75um pixel cameras for deep sky imaging with a larger scope, your seeing conditions are average, and the atmosphere is limiting most of the time - the extra resolution that a larger aperture f7 scope will provide you with might not be realized. So, the next best thing to do would be to use a larger pixel camera in order to take advantage of the extra light from the larger aperture, while maintaining the sampling rate of a smaller system; not gaining resolution in the image but instead gaining signal per pixel. |
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andrea tasselli: One more thing to consider other than Aperture, Central Aperture obscuration. I tried to plot Airy Disk Pattern by MATLAB, center obscuration causes blighter diffraction ring = worse PSF = less contrast on (long exposure) AstroPhotography. My conclusion, we can argue for century about SCT vs. Refractor.  |
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| This entire arguement of 130 vs 11 is shaky- literally, due to dependent variables such as seeing. Resolution is completely dependent on seeing conditions in the case of the 11HD, whereas the 130 is going to almost constantly have less field average distortion compared to the 11HD. Comparing any <150mm Refractor to an EdgeHD or any reflector for that matter, is like comparing a pistol to a sniper rifle. The pistol works for almost anything you need to shoot, but if you have a specific target, the right time, the right location, and the right conditions, the sniper is much more effective. |
