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First broad band image with the AG optical FA12 Convergent with a 6200mm. Trying to trouble shoot a few things and will bounce this against the hive. I know my flat frame did a poor job, and I think I know why (flat panel was too small for the aperture). Seeing was quite poor that night. Two issues that I wonder if I can get some suggestions. 1) I am still new to spider veins and diffraction spikes (been imaging with SCTs/ Refractors). There is an extra long diffraction spike after stacking on the bright Hatysa. I know there are other issues with this image too. Is this due to a slightly misaligned spider vein blade? 2) There is an artifact that is on the edge of the right lower corner. Assuming it is a light leak of some sort?  |
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My personal opinion is that what you see in the lower right corner is not a light leak but I do not have a reliable answer what it might be - a light leak does not have such a defined shape. In that area around M42, aren´t there some smaller nebulae? And regarding those long spikes - I was told, that long and thin spikes come from thin spider arms. If they were misaligned, they were wider/double etc. But that is what I was told, maybe correct, maybe not - but it would make sense to me. CS Georg |
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Georg is right, misaligned vanes will cause double spikes. Those are absolutely perfectly aligned. Side question, how's the blue bloat on that scope? |
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| The "extra long" spike isn't actually from that star, Hatsya, but from another one off-field, curiously aligned as it were. A similar situation is what you see on the far lower right corner. The "blob" is a weird off-axis reflection of a bright stars in the corrector optics. I had few of mine with a custom focal reducer but I always attributed that to the poor coating of the latter. |
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Georg N. Nyman: Thanks George for saving me the trouble of messing with the spider veins. The reason I don't think the blob in the right corner is a real structure is because I have seen it in other frames, albeit on a different OTA and completely different imaging train that looked very similar. For example I have seen it in this frame (part of M31 with the C11 and 2600mm pro, Starizona 0.7x reducer). You can see it to the left and to the bottom right.  Brian Puhl: I still have very limited frames on the blue filter. I have been imaging mostly in narrowband so far. Can't make a judgement yet. From the limited time I am with it I haven't seen much in the way of bloat though. andrea tasselli: Thanks Andrea. That seems very plausible! I did look to see if there are any extra bright stars just outside that field of view in the direction of that spike and I am not sure I see any on Telescopius. I am stumped.  I sure hope the blob is not caused by poor corrector lens coating. It is very dirty though and I have not attempted cleaning it. |
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Asraf, Those "blobs" are caused by internal reflections off the internal baffles. When I first started imaging using a visually oriented Celestron SCT, I had tons of them. Consulting with the manufacturer, they told me to flock the baffle exiting the back of the scope. I ended up flocking all the internal surfaces and the loops disappeared. They occurred only with very bright stars, even if they were out of the field of view. Ron PS - Here is an example of what I was seeing and why I think you have the same issue. This image has a lot of those loops.  |
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Ron: That makes sense. My baffle is flocked, but I wonder if these reflections are coming from the interna focuser tube. It is the only thing that is shared between those two images. Thanks for the feedback! |
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| Let us know if that solves the issue. I’m pretty sure it will. |
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Your spikes are very nice. I need to correct the following that was miss-stated before. A) Unperfectly othogonal vanes will produced two crossing spikes per vane direction (i.e. if you have for vanes that form a cross, you will get two "X" shapes slighly missaligned. B) doubled but non-crossing spikes reflect poor focus of the image. See my old M45 combining a bit of both problems... https://www.astrobin.com/full/oe78vz/0/ CS |
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Your spikes are very nice. Thanks that makes sense. Do you have any suggestions what might be causing that appearance of a "long diffraction spike" in the image? |
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Ashraf AbuSara:Your spikes are very nice. @Ashraf AbuSara My understanding is that large vanes produce very intense and long spikes. I have 0.5mm vanes and yet my first capture of M42 with that scope presents very very long spikes on that star too. This is a very bright star. See for yourself : https://www.astrobin.com/fhk68n/ However, a more recent capture, with the same gears, exhibit more reasonable spikes. https://www.astrobin.com/otnl1r/ I suggest to work on the star processing to stretch less the low light to get rid of that. That is what I did on the second image. |
