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CKemu

M101 - A Start, weird stars?

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This is a "RAW" stack of 6*720s images in the R channel using my Esprit 100ED and ATIK414ex, taken between 21:40 and 23:00 before the Moon started to blot the sky out. Hoping with several clear night forecast to build upon this with a set of 20 in each channel.

I'm not entirely sure what to make of the stars though, in this project and a few others the stars aren't "smooth", not sure if it's me being OCD about tiny details, or an actual problem. When shooting with my LX90 8" and this camera/filter config, the stars where smooth gradients, however with the smaller stars in this image, there are dints, or brightness changes seemingly in random places on the star.

Any ideas regarding this would be handy!

Edited by CKemu
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Hi CKemu.

At the risc of being called OCD as well... :

To me this lookes like a guiding effect together with some spheric abberation, which is clearest in the upper and lower left corners by the elongated stars and to a lesser degree at the right side as well, so it seems to be a combination of things: 
1  'rough' stars because of the guiding effect caused by not perfect PA or by PE 
2  Spheric abberation, particularly on the left side
3  2 being visible particularly on the left side, to me also means that the focal plane is not perpendicular to the optical axes

Never the less it is a nice image...

1  may be caused by rough guiding through too long guiding exposure time or wrong settings, or caused by the mounts PE
2  If using a flattener the distance between flattener and sensor is not optimal
3  Sagging of the focusser ?

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4 hours ago, Waldemar said:

Hi CKemu.

At the risc of being called OCD as well... :

To me this lookes like a guiding effect together with some spheric abberation, which is clearest in the upper and lower left corners by the elongated stars and to a lesser degree at the right side as well, so it seems to be a combination of things: 
1  'rough' stars because of the guiding effect caused by not perfect PA or by PE 
2  Spheric abberation, particularly on the left side
3  2 being visible particularly on the left side, to me also means that the focal plane is not perpendicular to the optical axes

Never the less it is a nice image...

1  may be caused by rough guiding through too long guiding exposure time or wrong settings, or caused by the mounts PE
2  If using a flattener the distance between flattener and sensor is not optimal
3  Sagging of the focusser ?

1: I shall have to check my guiding set up, it's off-axis using PHD2/ASI120mm for the camera. From my calculations I have a roughly 2.41" per pixel scale on the ATIK414ex and my RMS last night was 0.58-0.70", which being less than half I would say that is well within bounds, but there was a few rough patches, possibly caused by breeze or in one case a dog. Guiding updates where every 500ms - perhaps to short? Seeing interfering with the guide.

Graph looks a little rough, RA being noisy, with DEC showing more "pattern", though equally nothing that stands out over the entire duration of the guide.

2: No flattener, as I was under the impression my 414ex has a small sensor and wouldn't really be affected by the scopes edge distortion.

3: Possibly, it's something I shall have to investigate, M101 is pretty high, so will have to try various targets at different angles.

Thank you for your help, really appreciated, this "passion", keeps throwing new things at me to get obsessive about. Five years ago, I would have been supremely happy with this result and not questioned a thing!

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5 hours ago, CKemu said:

1: I shall have to check my guiding set up, it's off-axis using PHD2/ASI120mm for the camera. From my calculations I have a roughly 2.41" per pixel scale on the ATIK414ex and my RMS last night was 0.58-0.70", which being less than half I would say that is well within bounds, but there was a few rough patches, possibly caused by breeze or in one case a dog. Guiding updates where every 500ms - perhaps to short? Seeing interfering with the guide.

Graph looks a little rough, RA being noisy, with DEC showing more "pattern", though equally nothing that stands out over the entire duration of the guide.

2: No flattener, as I was under the impression my 414ex has a small sensor and wouldn't really be affected by the scopes edge distortion.

3: Possibly, it's something I shall have to investigate, M101 is pretty high, so will have to try various targets at different angles.

Thank you for your help, really appreciated, this "passion", keeps throwing new things at me to get obsessive about. Five years ago, I would have been supremely happy with this result and not questioned a thing!

I would say that you are not quite getting away without the flattner, in this instance and for now I would just crop slightly. 

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7 hours ago, CKemu said:

Thank you for your help, really appreciated, this "passion", keeps throwing new things at me to get obsessive about. Five years ago, I would have been supremely happy with this result and not questioned a thing!

You're most welcome. I know how it goes in the this hobby: if you were not a perfectionist to start with, you will become one...?

I agree with AdamJ, a flattener is almost indispensable and that will throw in some more chalenges...

Edited by Waldemar

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