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I believe I've landed in stump-land with a new imaging setup I'm putting together. In a nutshell: dialling in proper backfocus for my reducer causes the focuser to lose inward travel. In depth: I used the classic "look at the stars in the corners and add shims until they no longer radiate" and I'm very close to getting it perfect (<1mm). Trouble is, as I've increased the distance between sensor and reducer, the focuser (situated between scope and reducer) has had to move ever more inward to achieve focus. This to the point that I now have no more inward travel available and cannot achieve proper focus. I'd massively appreciate any ideas, thoughts, opinions or guidance. _ Image train is: FS-60CB ⇢ CAA (TKA21200) ⇢ Reducer (TKA20580B) ⇢ Adapter to M48 ⇢ OAG ⇢ Filter Slider ⇢ Shims ⇢ Camera

Just purchased a demo Rasa and tried to use for the 1st time last night. i have a ZWO EAF , using ASIAIR plus and at the front, the camera adapter connected to a ZWO filter draw and then ASI 533mc Pro camera. I coul not get focus, just these rings. Any ideas where I need to start correcting?

Hello all, I bought relatively recent a SCT 0.63 reducer/flattener for my C6. I'm planning to use it with the ASI1600 camera, but it seems that I can't figure out the proper spacing distance between the reducer and the sensor. That or there's another problem with the scope that I can't figure out. The first distance that I tried was ~105mm. I achieved this by using a SCT adapter to T2 which is 50mm + a Baader VariLock set to ~28mm + the filterwheel 20mm + 6.5mm the distance from the sensor to the camera thread. + the filter width, it should be ~105mm. I understood that the closer the sensor is placed to the flattener, the better the field is flattened, but the reduction is weaker. I didn't mind that. I then tried to use an M48 (has also a T2 thread inside and deeper) to T2 adapter which was about 15-20mm. In both cases the stars were not in focus away from the center, with an out of focus elongation towards the edges (I don't know which word describes this). The following M13 is with the Baader VariLock and the Crescent was with the shorter adapter. I didn't have time to test without adapter at all and now it's going to stay cloudy for a while. Don't mind the quality, the M13 is unguided with ~20min (6s subs), the Crescent is ~20min (1min subs), guided. Very bad seeing. Did anyone use a C6 with a 0.63 reducer/flattener and APS-C or 4/3 sensor with success? Or does somebody know which is the correct backfocus distance and how to get rid of the distorted stars towards the edges, but still using the same scope + flattener? Many thanks and clear skies, Alex

Celestron's EdgeHD product line features an integrated field flattener. As with all flatteners, these produce optimal results when the imaging plane is a specific distance from the flattener. In the EdgeHD whitepaper Celestron describe the optimal backfocus of 133.35mm (5.25") for the 8" model, and 146.05 (5.75") for the 9.25", 11" and 14" models. They suggest that the image plane should be placed within 0.5mm of this distance. When putting together an imaging train it can be quite hard to determine the actual backfocus. You could add together all the optical lengths quoted by manufacturers, you could get calipers and actually measure each part or even try to measure the entire thing (although it can be quite hard to figure out where to measure from. At some point, you have to trust some manufacturer spec (unless you fancy risking your sensor). Once all this is done you might, however, find that things vary ever so slightly; everything from the tightness of threads to the T-ring not quite giving exactly 55mm. How do you work out if you've done it all correctly? In a table in the whitepaper (page 13), focal lengths are given for each OTA (for example 2125mm for the 8" model). Hypothetically then, it should be possible to measure whether or not you're at optimal backfocus by plate solving for your image scale. In the same table, an image scale is given for a sensor with pixel size 6.4 micron but you can use a calculator (such at the astronomy.tools one) to work out the expected image scale for your particular sensor. This does require that your image is as close to perfect focus as possible. Putting all this into practice. I used my calipers to try to get the image train as close to 133.35mm as I could and then plate solved some resulting data taken with a DSLR with 5 micron pixels. From my calculations, if I'm in focus at the correct spacing, I should have an image scale of 0.485"/pixel. However, my astrometry.net solves gave an image scale of 0.495"/pixel. Working backwards, this indicates that I was at a focal length of 2083mm, quite a way inside 2125mm. Although I can't find a reference I've read that, for an SCT, the focal length changes by approximately 3mm for each 1mm of backfocus, this implies that my sensor is 14mm too close! Now, I'm no expert with calipers but I feel like I couldn't have been more than a few mm out, and if anything I thought I was too far. I suppose I could have been a bit out of focus but surely not ~10mm. Is there a mistake in my logic of aiming for 2125mm focal length?

I have already posted my first astrophotographic session report in the telescope review thread: Tecnosky 80/480 APO FPL53 Triplet OWL Series - Review. But since that is more of a general review/diary of my experience with the new telescope, I feel some of the issues I am having are being buried and they will probably get more visibility if I post them - in a more synthetic version - in a dedicated thread. So, a few nights ago (October, the 5th) I took out my new telescope for its first light. All the photos have been taken with the 0.8x flattener/reducer and the Optolong L-Pro 2" filter attached to the reducer. The camera is an astromodified Nikon D5300. The only processing the following pictures have consists in this: - AutomaticBackgroundExtractor - ColorCalibration - Stretch Here we have a 90s shot of M31. And here's a mosaic generated with the AberrationInspector script. What I do like: - tightest, smallest, roundest stars I have gotten since I started doing astrophotography at the end of January. Obviously comparing it to what I have been achieving with a kit 70-300mm zoom lens, these can't be anything else but better by orders of magnitude What I don't like: - star shape not consistent in all areas of the image - residual chromatic aberration, especially on stars that are not round: there's clearly some red and blue edges visible I didn't expect this from an apochromatic refractor, but maybe it's just because the stars are kinda "smeared", so not all light is focused at the same spot? I don't see this around the center of the image (or, at least, the problem is less pronounced). Maybe I have some tilting in my imaging train/sensor? I have been doing some reasoning about it and it seems like a combination of tilting and/or backfocus spacing. According to the following image about backfocus spacing: if the stars are elongated radially, the sensor is too close, if they are elongated tangentially, the sensor is too far. But to me it seems I have a little bit of both: in the top right corner, for example, the stars look radially elongated, in the bottom right, they look tangentially elongated. Top left they look tangentially elongated, bottom left also, but a little less. Seems like there has to be some tilting as well, otherwise they would all have a symmetric shape on all corners, correct? How do I determine - is there even a way - if the issue is due to tilting only, backfocus only, or the combination of the two? Is there a sure proof way of checking for tilting? Like, rotating the camera and taking pictures with, say, the camera at 0°, 90°, 270° and 360°? If there's tilting, the pattern of the star shapes should follow the camera, correct? I also tried splitting the channels in R, G, and B components, doing a star alignment of the blue and red channels with the green as a reference, and recombining the channels. The blue and red edges become a lot less evident, which is good, but obviously the star shapes remain the same. In my Telescopius gallery you can also find two other images, Capella and Capella Mosaic showing pretty much the same issues. Also, one issue with the guide camera: ZWO ASI 224MC. When attached to the guide scope (Artesky UltraGuide 60mm f/4), I can't seem to get a "sharp" focus, I even tried on the Moon, and the best I got was a soft lunar disc, with some major features visible, mainly by change of color/brightness (the maria, for example), but no details. The image still seemed blurred/bloated. Is it because of lack of IR blocking filter? I tried the same camera attached to the main refractor, with the L-Pro filter (which blocks UV and IR, as well) and I could focus perfectly. Do I need an IR block filter for guiding or even if the stars appear a little soft, the camera guides just fine? Matteo

Hi all, I tried my takahashi reducer 0.73x (CR version) for the first time with my FSQ85. I can see a distortion problem but I'm struggling to understand the reasons. you can see it happens before the meridian flip and also after, in the exact same ccd area (QHY268M APS-C sensor). I'm pretty sure the reducer 0.73 should be able to cover an APS-C sensor without issues. The right part of the image looks like my sensor is too close to the reducer, with stars pointing towards the centre. but why the problem isn't mirrored on the left part? single exposure 60s, some zoom-in.... (full pictures attached) upper right corner lower right corner upper left lower left thanks for any input!

hi , have anyone used the QHY294C Pro ? 1. What is it's back focus distance? does any adapter (M42-M48) and spacer include in the box like the zwo 294 version for the backfocus maintenance? i will attach it to my GT81 . 2. does it come with the power adapter ? 3. any user review of this camera?

https://www.astroshop.eu/telescopes/tecnosky-apochromatic-refractor-ap-70-478-quadruplet-flat-field-ota/p,57327 How to know this? I have read that with 65/420 scopes you cannot use 2" diagonals because there isn't enough backfocus.

I have an Orion 8" astrograph f/3.9. I'm looking to purchase a Baader 2" Multi Purpose Coma Corrector - MK III, I'm currently imaging with a DSLR which the backfocus should be fine. My question pertains to looking into the future. If I purchase this coma corrector now, which says it has a backfocus of 55mm, and later upgrade to a different camera, say a ZWO ASI178MM, will I still be able to use this coma corrector with that camera when the ZWO camera says it needs a backfocus of 17.5mm. How does the backfocus of the coma corrector relate to what the camera wants for backfocus. Hopefully I've worded my question well enough. Any help would be appreciated. Thanks in advance.