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endlessky

Tecnosky 80/480 APO FPL53 Triplet OWL Series - First Light

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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.

M31.thumb.jpg.425894a7a191443a60af7bd223a36898.jpg

And here's a mosaic generated with the AberrationInspector script.

M31-MOSAIC.thumb.jpg.dc2fa06b2bc00c0ebbfcb599b382d84d.jpg

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:

1054450290_BackfocusDistance.jpg.5bddc3f705526a9640b713b220a24df7.jpg

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

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So, I took the rig out last night (October, 7th) for another session. I removed the L-Pro filter, to have less variables in the mix.

I also took four pictures of the same star field and same exposure time, rotating the camera (D5300) - using the telescope built in camera rotator. I started with the camera at 0°, then 90°, then 180° and finally 270°.

I even tried doing the same tests with the D90, and got pretty much the same results. So, it seems camera independent.

I feel I can rule out tilting, since the star shape stays pretty much the same in the corners, independently of camera rotation (for example, same top left corner for all for images present same star shape, and so forth for the other corners). The star shape is also more consistent, and they all look elongated in an orientation perpendicular to radial. To me it's quite possible that the camera sensor is a little too far from the flattener/reducer, meaning I should somehow shorten the distance.

I also don't think the filter had any fault in the way the first pictures turned out, since the star shapes in the pictures without filter and in the pictures with filter are pretty much identical.

I emailed the store where I bought it describing my concerns. I will see what they suggest to do.

I tried calling, but my call got redirected to another Tecnosky store and I spoke with somebody else that wasn't the person that have been following me for all these months. He suggested to try adding distance, first, and see if that works - since at the moment I have no way of removing distance, other than buying a low profile T2 Nikon ring.

We'll see how things develop from here onward.

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