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Current equipment is: Explorer 150pds, EQ5 pro, Canon 600D (stock), and the EvoGuige 50ED and 120mm mini for guiding. I'm looking at getting the camera astromodifed (~£100) and also a coma corrector (~£130). I only have the budget for one right now, so I was wondering if anyone had any advice on which to purchase first?
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:
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?
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 couldn't find a topic on Coma Correctors, sorry if I missed it, if so please move this question to the right place.
I'm looking at installing the F4 Aplanatic coma corrector into my Orion 200 astrograph (Focal length 806.9mm) and I'm sorting out the spacers needed for the back focus to my CCD camera.
It states a back focus of 55mm (which seems standard) however I think thats for a 1000mm focal length, I think I need to reduce it slightly to 53.66mm (seen this number somewhere but cant find it now).
I know I need to be as accurate as possible to get the best out of my set-up, can anyone help me work out the exact back focus length I need before I order a load of spacers?
By Gary Shaw
I just acquired a 200mm F4 newtonian from TS Optics which I'll use primarily for Electronic Assisted Observing with several ZWO cmos cameras.
My challenge is that my current scope is an F3.6 schmidt newtonian so I have no experience with coma correctors or spacers/adaptors in the optical train. TS tells me that I need 55mm from the coma corrector (TSGPU) to the sensor. They provided 40 mm of various adaptors and spaces and I guess I get the remaining 15mm from the camera housing itself - as shown in the attached image. The threads on several of the adaptors(see notes) are bad so those pieces need to be replaced but, once that's fixed, I have four questions for this community:
1. ...is this odd assortment of parts and pieces typically how one goes about achieving the required 55mm dimension from the Coma Corrector to the camera sensor? Just seems that there must be a simpler, and sturdier way to do this without having to use 4-5 separate parts.
2. Once all the adaptors add up to the correct dimension and are attached to the Coma Corrector, where does one locate the whole assembly(with coma corrector) in the focuser draw tube? Is it a matter of trial and error?
3. If one did not care about viewing peripheral stars looking a bit like comets and didn't, therefore, use the coma corrector, would any of these adaptor/spacers be needed to bring the camera to focus so one could observe using, say, Sharpcap 3.2 Pro or other software?
4. If I were to use a filter or two, where would these best be located in the lineup and do they work with the various threads and adaptors?
Apologies for my lack of experience and thank you for any help you can provide me in understanding how to observe and image with coma correctors