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MikeBY

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Everything posted by MikeBY

  1. Which I noted in my comment. The 2600mm is APS-C, so you probably haven't run in to this issue full force yet. Going to an OAG the size of the image circle and drop- off of light intensity outside the area of the APS-C sensor becomes important. Mentioning the issue in terms of full frame support might be considered a bit off topic, but it's relevant tovmention because the OP is asking sbout the larger OAG. One consideration is supporting full frame devices.
  2. Well, if you want to be consistent getting the best performance it's problematic technically speaking. But so is being out of focus. So if we're careful it is ok. We just have to be mindful about what it's doing. Most SCTs, are quite tolerant about backfocus. They work across a fairly wide range without much impact. That is until we add a focal reducer/flattener. At that point we have keep the backfocus at a set point to get the expected results. The EdgeHD has the corrector built in so we are stuck needing to keep backfocus at 146mm. If we stray too far off that there's an impact. If we're designing an optical train for one, sure a microfocuser in front of the OAG that has sub-micron steps is great. They do take up backfocus and we also need it to behave as if it has a fixed length so that we stay at the right backfocus. That last part is on the us. It's up to the us to get a very close pre-focus with the scope and watch the microfocuser position to be sure we aren't shifted too far.
  3. You cannot put a focuser between the reducer or telescope and camera. You mess up the backfocus..
  4. You have two issues to deal with. 1st is that the back focus requirement for the EdgeHD. It's 146 mm. That's the same with or without the 0.7x reducer! The 2nd is image circle, especially if you contemplate going to a full size sensor. The 2600mm is APS-C, so you probably haven't run in to this issue full force yet Here's the problem with EdgeHD scopes and full frame sensors: The image circle is only 42mm. With the 0.7x reducer for 9.25 and larger the image circle is maintained . EdgeHD 800 it shrinks to under 30mm with the 0.7x reducer! Although you might be tempted to use a larger diameter OAG because you've got a 2" EFW. You will run into trouble picking off light with the prism and getting focused. Any OAG larger than 48 mm will likely NOT work. It's a problem no-The EdgeHD scopes cannot fill a full size sensor. Buy the over priced and smallish M42/M48 diameter Celestron. OAG? It might be too small I'd go with ZWO OAG-L (NOT the OAG-L 68 Version). Why? Because you have the large filter wheel. And it went cause as much vignetting. It's a true 48mm OAG. It's kind of a toss up though under these circumstances getting the back focus to 146 mm will require extra spacers with either. Look at the guide port. The helical focuser is about 37 to 45 mm high. Move it to mid travel. Depending on which camera and how you attach it (i suggest threading the guide camera on) calculate how much the camera. adds or subtracts from the focuser length. For example a ASI178 adds 12.5 mm using the 1 1/4 nosepiece. It adds only 5 mm threaded on to the top of a helical focuser. .An Asi174 mini might slip down Into the top of the focuser a bit so it's a negative number (sensor is closer then the top of the focuser.) Attach the main camera to the OAG as a visual aid and adjust prism position to get prism taper in the image circle as far as possible while not interfering with the main camera sensor light path. Measure optical path from the middle of the OAG (1/2 of the thickness of the OAG is used) to the main camera sensor. Compare to length from guide sensor to the point on the prism where the 45° cut starts. The distances need to match. The focuser gives you +/= 4 mm. Adjust so they are equal. Next measure from main camera sensor to the edge of rear glass (is either 0.7x reducer or at back of edgeHD scope). . This Includes full thickness of OAG. Add spacers as needed between OAG and 0.7x reducer. To get to 146 mm. .
  5. Keep the helical focuser. Remove the nosepiece and I/R filter from the ASI178. Thread the filter into the internal threads directly in front of the sensor. Remove the 1 1/4 nosepiece thumb screws on the helical focuser. Thread the camera all the way on to the focuser and check orientation to the OAG prism. It should line up! If it does not, get a very thin m42 threaded stop ring and thread it all the way on to the helical focuser then thread on the camera and back the camera off only enough to get it oriented with the prism. Tighten the stop ring against the camera to secure it to the focuser. Set the helical focuser to mid point (+4 mm). Note, the back focus of the ASI178 is split. The sensor is set 5 mm behind the front surface of the sensor window. The depth of the M42 threaded nose of the camera is 7.50 mm. If the helical focuser's M42 threads are 7.5mm or more and the camera threads completely on, the minimum backfocus of the helical focuser plus camera is 37 +5 + = 42 mm. Maximum focus distance is 45 +5 = 50mm. If a stop ring is used or the focuser threads are not deep enough you will need to measure this distance: The minimum depth of the focuser from the OAG frame to the front most edge of the camera and add 12.5 mm to get the this distance and add 8mm more to get maximum (8mm helical focuser range) The last adjustment is the prism depth. It looks like the prism tube is fully extended. (6+14+6=26 mm). Focus distance (6+14 +0) = 20 mm. Focus range *fully threaded camera on helical focuser* is 62 to 70 mm ( you can march focus with the main camera at 65 mm) However, the prism looks a bit low and may interfere with main sensor. Adjust it Loosen 2mm set screw(s) on the OAG frame that lock the prism in place. This may be in on the camera side or may be on the OAG frame edge. BE CAREFUL. the prism carrier will be loose and can easily fall out. Do NOT TOUCH the prism. Surface! Move the prism further away from center by 3 mm (shaft measurement now 11mm). This puts the center of the focus range right at 65 mm. Focus main camera 1st. Then guide camera. Check for vignetting on both the main camera and guide camera. (During day) Main camera vignetting, move prism away from center and refocus guide cam. Guide camera vignetting, move prism towards center. Refocus Guide cam. Tips: Be sure prism is perpendicular to the focal plane when adjusting length. Prism may extend into guide camera. if adjusted to high (large main sensors) the top of prism may hit filter or sensor. BE CAREFUL. . If you have tilt adjustments, take test shots and adjust out any tilt. Tilt set screws can also be used as fine adjustment of Guide camera backfocus avoiding the 42mm stop ring. Similarly some OAGs have tilt/stop screws to adjust the stop position of the helical focuser where it threads on to OAG . A note about your prism backfocus measurement point. The image seen by the guider is refracted 90° by the 45° prism surface. The measurement point for focus is the plane of the image, which is the closest point to the camera where the 45° surface starts, not the midpoint of the slope. Be sure everything is tight and lock the helical focuser. You should not need to adjust OAG focus again! Worth the effort.. If you switch to a DSLR the flange to sensor distance can vary slightly. Cannon EF is 44mm sensor to flange plus 11mm Flange to T2 adapter = 55 mm. Mirrorless is only 17.5 mm. to flange. However, if your buy the Cannon mirrorless camera T adapter it adds the 17.5 mm back in (it's 37.5 mm) to maintain the industry 55 mm standard.
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