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Y'all, I have the Askar 400 FRA quint and it's amazing in my opinion. Looking to get a longer FL scope in a few months, but actually probably sooner, next paycheck or two if we're being honest, right? oh wait i have room on my credit card. Anyone here using the Askar 140 APO triplet? Wondering if, like the 400 FRA, it doesn't require you to screw around with backfocus. With my 400, you just attach the camera and you are good to go. Also, do you like the scope? -DL, aka IMR |
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| I have the 500 and do really like the scope. But the 140, being a triplet, is going to require all of the "normal" APO accessories such as a flattener and/or reducer. |
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| Thank you. That’s what I figured but could not clearly confirm on my own. |
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Interstellar Medium Rare: I have an Askar 120APO and love it - it's a very nice triplet. I just finished my first target with it. If you go check out the Tadpoles on my page, that was shot with the Askar APO and 1x flattener. To answer your question, yes, it requires a flattener or reducer, and back focus is 55mm. |
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As has been mentioned, the 140 APO is a triplet design that requires a field flattener, and thus, has a back focus requirement of 55mm +/- 0.5mm. You need to stay within the FRA or PHQ lineup of scopes from Askar/Sharpstar if you want to avoid the requirement for setting a precise backfocus distance. The 130PHQ or 107PHQ would be my pick for a few reasons. The 140mm APO is F/6.5, yielding a 980mm focal length. This reduces to 784mm F/5.6 with the 0.8x reducer (sold separately, $299) or you will need the prime focus flattener to have a flat field at the standard 980mm f/l (again, sold separately, $299) the 107PHQ is $100 more expensive than the 140 APO + whichever flattener you choose, has a 749mm focal length, 44mm corrected image circle and no backfocus requirement. There is a 0.7x reducer available ($359) to bring it down to 524mm at f/4.9, however, reports online basically state that the PHQ scopes perform MUCH better without a reducer than with one. the 130PHQ is $1000 more expensive than the 107PHQ, but gives you a focal length closer to the 140 APO. At f/7.7, the 130PHQ is 1000mm, again, has no backfocus requirements and an even larger 60mm image circle. The cons of the PHQ lineup is that they are all considerably slower than the FRA lineup, and the APO's when a reducer is used. For me, I'd happily go from F/6.5 to either F/7 or F/7.7 for an imaging rig... You just expose longer, or take more subs, not a massive deal... If it was a jump from F/2.8 to F/7 I'd be more concerned, but from 6.5 to 7~7.7, not a worry in the world... So... What's more important? You have 3 options here. COST: The cheapest is to buy the 140mm triplet and the flattener. but fiddle with backfocus distance, and any possible aberrations that still present themselves after correction in editing. BANG FOR YOUR BUCK: The 107PHQ is going to give you a big boost in focal length over your FRA400, and likely be sharper, while not requiring any backfocus distance fiddling. BEST: The 130PHQ will give you over double the current focal length you have with your FRA400, it will provide a noticable bump in aperture which will boost the overall resolving power of the rig substantially, and like the 107PHQ, provide a 'fiddle free' experience. |
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I just purchased the Askar 140APO and have posted my latest image using it. It isn't a petzval design so you will have to worry about backfocus but it isn't as bad as it sounds since its the standard 55mm with both the 1.0x and 0.8x flattener/reducer. I was able to get perfectly round stars on my first try with the standard 55mm distance with no need to adjust (I havent tried it with the 0.8x reducer yet, that is my next task). The chromatic aberration is very well controlled and is probably only noticeable if you're "pixel-peeping". For comparison the AT130EDT is similarly priced but lots of users have reported having high amount of CA with some samples of that scope. I didn't have any issues with the focuser and was able to easily attach the ZWO EAF. I also used the scope on my AM5 mount with a 10lb counterweight and didn't have any issues with guiding. It is fairly heavy at 24lbs but any other 140mm refractor will weigh about the same. I'll update this forum once I try out the 0.8x reducer but so far I would highly recommend the Askar 140APO. I don't think theres any comparable scope on the market that would provide a better "bang-for-the-buck" than the Askar 140APO if you're looking for a refractor with a 750-1000mm focal length. Also jonbosley on CN has reviewed it for visual use as well: https://www.cloudynights.com/topic/900932-askar-140-apo/page-4 |
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BANG FOR YOUR BUCK: The 107PHQ is going to give you a big boost in focal length over your FRA400, and likely be sharper, while not requiring any backfocus distance fiddling. Thank you so much. This is super helpful. I have recently heard that “at a certain point, focal length does not capture detail, aperture captures detail”. If that’s true that is one factor pushing me toward the 140APO. is that true? |
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Interstellar Medium Rare:BANG FOR YOUR BUCK: The 107PHQ is going to give you a big boost in focal length over your FRA400, and likely be sharper, while not requiring any backfocus distance fiddling. * The ability to resolve finer detail is a function of a few things. 1. Scope Aperture. Bigger scopes have better resolving power. A 90mm f/10 at 900mm can not resolve finer detail than a 200mm f/4.5 at 900mm could resolve. 2. Camera (more accurately, pixel size). Let's say you had a scope that had a 0.4 arc second diffraction Limited resolution, but the camera you pair it with results in a 1.8 arcsec per pixel resolution. Despite the scope being capable of 0.4", you'll never resolve detail finer than 1.8", because your camera pixels are individually seeing 1.8" of sky. 3. Seeing conditions. Same scenario as above, if you have a 0.4" diffraction Limited resolution, but you pair the scope with a camera that has TINY pixels and youre achieving say, 0.6 arcsec/pixel, but your local seeing averages at 2.6 arcsec, well.. you're not going to do better than 2.6" due to the muddy atmosphere. 4. This is the big one... guiding accuracy. let's say now you've created the perfect situation. You're in a location with 0.4" seeing, you've matched your scope and camera perfectly and you can resolve 0.4 arcsec/pixel.. you fire up PHD2, Get the rig guiding, but you're seeing 0.8" rms in Dec and 0.5" rms in RA, so a total of perhaps, 0.93" and peaks of maybe 1.4. Your images are now trash, because you can't achieve round stars, as the smallest stars will be resolving to 0.4", and you're dragging them 1.4" across 3.5 pixels. So you need to come to a happy medium. In my area, I know, best case scenario, I'm getting 1.8" seeing. I can guide to maybe 0.3" to 0.5", but no matter what I do, my conditions limit me to 1.8", so, I got a scope that can resolve just sub 1.2", and paired it with a camera that results in 2.3 arcsec per pixel. This means on the best possible night, I will get 2.3"/pixel, on an average night I will get 2.3"/pixel, something has to go horrifically wrong with my 0.3"~0.5" in order for it to negatively affect my 2.3" resolution limit. I'd love to get a 10" f/6 RC and chase galaxies. But the fact is, my local sky simply won't support the resolution, and all I will end up with is soft, blury looking images. I have the biggest scope I will buy given my location already, a 150mm f/2.8 at 420mm f/l newtonian, and on particularly windy nights I will use my 65mm apo which has a 416mm f/l, I can blend the data from those scopes incredibly easily... So, check your local sky conditions. look at your guiding logs and see what your guiding limitations are, then, based off the camera you have now, or the camera you plan to buy next, figure out the longest focal length that will put your arcsec/pixel resolution slightly worse than your local seeing average, and considerably lower than your average tracking accuracy. This will yield you the best possible resolving resolution for your setup and location. |
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| Thank you Alex. Very very helpful. |
