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Hi, I have been looking at the Askar M54 OAG but I come to a back focus distance of 57mm. My current setup is below (from camera to telescope). I am curious to find out how people have solved this. Thanks! ASI294MC Pro (6.5mm) T2-tilter (11mm) ZWO-EFW 2"x7 (20mm) M54M-M48F (2mm) T2-M48 (16.5mm)  |
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I think about the best way to solve it is by getting the matching filter drawer, or a matching OAG for your filter wheel. When I switched to a filter wheel, I bought a matching OAG for it and ditched the Askar OAG unfortunately.
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I think about the best way to solve it is by getting the matching filter drawer, or a matching OAG for your filter wheel. When I switched to a filter wheel, I bought a matching OAG for it and ditched the Askar OAG unfortunately. I wanted to avoid the ZWO OAG as it has a small prism (8x8mm). The ZWO OAG-L (12x12mm) does not work with this camera… 😵💫 |
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if you subtract 1/3rd of the camera window and filter thickness from your backfocus, you'll probably find out you have something close to 55mm, I'm guessing 55.8 but now that I'm thinking of it, I'm not exactly sure if you're supposed to add or subtract  , either way, I know it is 1/3rd of thickness for every piece of glass that light travels throughsomeone else will have to confirm I used the Askar OAG with an EFW, tilt plate and 2600mm pro on a coma corrector that requires 55mm, and my stars were okay |
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Oscar: If you're dead set on the Askar OAG, this is probably perfectly fine, most backfocus numbers are just estimates, and filter thicknesses do add to the required backfocus by 1/3 of the thickness. About the only time I can think of backfocus being critical sub 1mm is with a Rasa or hyperstar, so you should be fine even at 58mm, should being the key word there. The ZWO oag should be fine though at 8mm, especially on a small sensor like the 294. The larger prisms really only help out with high focal length and large sensors, like an SCT and a 2600 size sensor. I went with the Player One Phoenix Wheel and their OAG-max, works really well for a 7x2 wheel. I use Touptek cameras so they didn't match up correctly and I ended up drilling out some of the threaded holes. The most important thing to me was that the prism line up correctly on the sensor long edge so I didn't have a shadow anywhere. Have you looked at the Touptek OAG? I can't remember if it lines up correctly with the ZWO EFW or not. |
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Have you looked at the Touptek OAG? This is becoming quite the puzzle with the added difficulty that I live in South Africa and we have very limited brands/items available... The Touptek is 17.5mm vs ZWO 16.5 and Askar 19.5mm  My future expansion will include the ASI2600mm and re-using my SCT again, so I am looking to get a bigger prism from the start (trying to avoid buying three OAG  ). I am using the ASIAIR so I am kinda stuck with ZWO cameras for now.Thus far I only found the Askar OAG on stock in South Africa, so I might need to postpone this upgrade to my overseas trip. |
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Do you need the tilter? That would free up a lot of backfocus. If you do need tilt adjustment there are these thin shims that could work: https://www.firstlightoptics.com/adapters/astdym_m48_tilt_adjuster.html |
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I bought the Askar OAG, seems that it would be a great OAG for just 2mm more (57mm back focus). But maybe a stupid question, I am mounting the OAG on my ZWO EFW 7x2". The mount that goes on the camera side, then goes on the EFW and has these three lips to position the OAG with respect to the camera sensor. But for some reason, the lip is positioned at the side closest to the free space... This prevents me from attaching the OAG because there is no spacers between the OAG and the EFW. Anyone an idea how to solve this? Sorry for the poor description, maybe the photos might explain the position of the screw-on mount. Thanks!   |
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I bought the Askar OAG, seems that it would be a great OAG for just 2mm more (57mm back focus). same happened with me I cut a spacer ring out of aluminum foil (the foil was folded into about 4 layers before cutting into a spacer) and put one between the EFW and the 3 lip adapter that you show in the photo it was just enough for me to have the 3 lip adapter rotated slightly in one direction, so I could then use the OAG |
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Mine is similar, but I dont have the tilter. So asi1600mm (6.5mm), efw (20mm), askar m54 oag (19.5mm), with a 10mm m54 extension tube.. that gives me 56mm (+1mm to account for filter thickness). I would probably ditch the M54 extension tube and put a M42 tube between the EFW and OAG if I needed more space there. |
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Oscar: Did yours end up at the same place? On the photo, the connector was not tightened yet, just at the end of the thread. Think it will move a few more mm when tightened. This makes me wonder how much spacers I need to add to position the OAG prism away from the sensor, messing up the 55mm back focus completely... It really looked like a brilliant setup that the Askar OAG has and being flexible with lining out on the camera sensor, but this is just useless... Very disappointed with this. I am still hoping I am doing something completely wrong! |
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Oscar: well, yes I think so, but I tried tightening it as much as possible, and obviously, it still wasn't enough, since I had to resort to cutting out an aluminum spacer it's easy, just use one sheet of foil, then two, then three, etc, until you have the correct position I just think the Askar OAG was made to be used with the Askar filter drawer more than with any other configuration, but that doesn't mean it can't be used with an EFW - I did it, and I had a tilt plate, and I got good stars I still recommend, people who buy an EFW should get the ZWO OAG, not the Askar - the ZWO also has the added benefit of being able to be bolted to the EFW I like the bolting, because, well, the Askar OAG can be wiggled; I was once getting tilt in my system and found out my OAG was tilted by 1-2mm, all because of that 3 lip attachment design which doesn't guarantee totally flat contact between the OAG and EFW; I had to push the OAG against the EFW and use pliers to really tighten the 3 lip screws, to finally get a perfect flat contact (I think maybe this whole tilt problem was because of the foil I added, but, it serves it's purpose pretty good  , I guess it would be better if you use some solid metal, thin spacers)another thing, be careful with the prism, don't touch the screw that tightens the prism into place and also don't force the stalk into the OAG; I tightened the screw (because I thought the prism was too wiggly; it actually fell out at least once, thankfully, not when the OAG was attached to the image train), and I forced the stalk into the OAG and cracked the whole dang prism, but I still sometimes get guiding numbers down to 0.27" total RMS, so I'm still pretty happy with it  |
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Oscar: The reason I went for the Askar was that the ZWO OAG-L does not work with combination of the EFW and a colour camera... I have the ASI294MC +EFW and then you need to use the ZWO OAG with the 8x8mm prism. When I then want to upgrade to the ASI2600MM, I would have needed to buy the OAG-L again... But this still seems to be a better option than the Askar, which doesn't really work. Perhaps that is also why there are no reviews of the Askar OAG & EFW combinations I should have known
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