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WIP M101 - Scope Test


DMouse
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Hi, here is last nights shot at M101. This is a luminance run only and was to test to see if the central Halo was still there, and it is, grrrr. I have a sneaky suspicion it is coming from the edge of the secondary mirror. I have pretty much painted everything else matt black, so far. Anyway, Here is the Luminance run.

Subs: 10x 300sec, 5x 420sec

Processed in Pixinsight. BIAS, DARKS and FLATS applied.

Cheers

Paul

M101_2019_04_28.jpg

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Is it your flat's causing the problem, as usually they would have removed any unevenness in illumination provided you didn't alter anything between taking the lights and doing the flats. Bias and darks can be done 'off scope'?

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On 30/04/2019 at 15:56, fwm891 said:

Is it your flat's causing the problem, as usually they would have removed any unevenness in illumination provided you didn't alter anything between taking the lights and doing the flats. Bias and darks can be done 'off scope'?

The central halo is a lot worse without the flats. I have contacted skywatcher and they are looking into it. I took some more flats as a test today with my Canon T2i, and the halo is still there, so it's not the ATIK camera and filter wheel. I have a feeling its either the coma corrector(Skywatcher f4 version) or a reflection off the secondary mirror. It comes with black sides and back so I didn't check it for any other issues. 

Stack_L_No_Flats_DBE_Example_1950.jpg

Stack_L_Flats_DBE_Example_1950.jpg

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Am I reading your description correctly: You've shot the lights with the Atik 383 and your flats with the Canon? If so that won't work as camera format and orientation remain the same for matching flats/lights...

I'm watching how this progresses. I still feel that your flat densities need more tuning. Stars are tight to the corners so corrector spacing looks good.

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No, The canon was to see if the halo was visible with this camera as well and therefore ruling out that it was a problem with the ATIK camera. I'm not quite that new to this hobby ;) The flats for the ATIK and the Canon both show the halo.

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It seems to me that your flats are doing a decent job, but not perfect. Most dust motes are taken care of. But it looks like there are some residuals left of the strongest motes in the upper left corner, near the upper right corner/edge, and one going from the left galaxy to the left edge. To see if the calibration process works, you can calibrate single flat exposures with the master flat. (I would take the first, middle, and last flat exposures.) This should give perfectly calibrated "flat" images. If you still see residuals, your calibration process isn't doing its job.

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9 hours ago, wimvb said:

It seems to me that your flats are doing a decent job, but not perfect. Most dust motes are taken care of. But it looks like there are some residuals left of the strongest motes in the upper left corner, near the upper right corner/edge, and one going from the left galaxy to the left edge. To see if the calibration process works, you can calibrate single flat exposures with the master flat. (I would take the first, middle, and last flat exposures.) This should give perfectly calibrated "flat" images. If you still see residuals, your calibration process isn't doing its job.

Thanks for the idea. Just tried calibrating some of the Flats subs  with the Master Flat. This worked really well. Maybe a hint of a left right gradient, but only if I really stretch the image. The dust motes and halo are not evident.

A_Flat_LUM_6s_-25_1x1_0.02546_c.jpg

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The gradient in the flat calibrated light looks embossed, which means that whatever caused the ring in the middle is shifted between the flat frame and the light frame. Did you change focus by any chance? Or may focus have shifted between the light frame and the flat frame?

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How about posting som of your unedited lights, flats, bias and darks some it's easier for people to help at finding the problem?

I'm not sure about what could cause the halo, but for the problem with flats not fixing it perfectly my guess would be that the scope just isn't stiff enough so one of the mirrors moves a little bit from the imaging position to the position you take the flats.
A pretty large dither should fix problems like this if you dither every 1-3 frames. It can take up a lot of time to dither that often, but can at least give you some usable results.

A way to test if the scope is stiff enough is to collimate the scope pointing straight east, then leave the collimator in and slew so it's pointing straight west, if the laser is missaligned you found the reason for the flats not doing their job 100%

Edited by Xplode
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On 29/04/2019 at 22:29, DMouse said:

Hi, here is last nights shot at M101. This is a luminance run only and was to test to see if the central Halo was still there, and it is, grrrr. I have a sneaky suspicion it is coming from the edge of the secondary mirror. I have pretty much painted everything else matt black, so far. Anyway, Here is the Luminance run.

 

What is the make / type of your filters?

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