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Old 10-03-2020, 09:17 PM
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Larger stars out of focus

Hi all

I have been using a bahtinov mask to focus my scope but sometimes when i have larger stars in my photo they turn out like the one in this shot . Shot was taken with zwo 294 pro with optilong l enhance 5 minutes with 120 gain . Any advice more than welcome .
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Old 10-03-2020, 10:22 PM
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mental4astro (Alexander)
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Hey Kuz,

The first thought that comes to my mind is an internal reflection in the optical train for the star to be bloated and flared/distorted.

The other smaller stars look to be nice and tight. No tracking issues that I can notice.

I could be mistaken though. Some one else could offer other suggestions or reasons.

Alex.
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Old 10-03-2020, 11:45 PM
Ukastronomer (Jeremy)
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Are they OOF or over exposed
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Old 12-03-2020, 12:40 PM
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Looks good with Nebulosity and round stars. Maybe just a lot of water vapour in the air or a little over processed.
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Old 12-03-2020, 01:39 PM
Xeteth (David)
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Is this a single sub? If not can you post a single sub with just an auto stretch?
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Old 12-03-2020, 06:22 PM
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This is a screen shot I took when I took my first sub. It was taken in a bortle 5 sky' . I have attached a horse head with same issue
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Old 12-03-2020, 06:34 PM
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mental4astro (Alexander)
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Kuz, your Horse Head pic is very telling. I suspect it is internal reflection. Reason being the distortion is repeated with all bright stars across the field, and the pics being taken on different nights the dew and moisture levels would all have been different, but the distortion is consistent.

Any of your adapters have any shiny surfaces? Any black paint on surfaces that's been damaged exposing bright metal?

I can't see how over exposure could be responsible for such a distinct shape of distortion. The disortion is very uniform across the field. I could be wrong.

Alex.
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Old 13-03-2020, 05:28 AM
vin_ap (Caleb)
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Hi Kuz,

I have seen this problem occur once before when I was using some borrowed cameras to test. When looking at your listing I assume that you are using the Esprit 80 with the ASI294MC Pro. If this is right the pixel ratio is 2.39 which could lead to Over exposure especially with bright stars. Using a reducer field flattener could make it worse.

Hope this helps, I may be completely wrong but the only time I have seen subs similar to this is when I had the pixel ratio above 2

- Vin
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Old 13-03-2020, 08:00 AM
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I think its both, internal reflections exacerbated by over exposure, best to try lower gain or shorter exposures and see if that looks better.
This might help.

https://www.atik-cameras.com/news/ru...to-break-them/

Rick
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Old 13-03-2020, 09:05 AM
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mental4astro (Alexander)
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As this discussion was developing, I was beginning to suspect it could also be a case of both, Rick

Internal reflections can be very crafty. They can be difficult to control, originate in the most unlikely of places, and the $$$ of the gear is no guarantee either.

As for the over exposure, this is something that has to be learned to understand consequences, be able to identify these aberrations when they show up & come up with solutions to mitigate them. AND the multitude of variations thrown up by using different gear.

Great thread, Kuz!

Alex.
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Old 13-03-2020, 05:44 PM
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This is one of my 600 second dark subs taken the day before . I streached it a bit and have noticed a light spike on the right side of the frame is this normal . It was completely dark before I streached it . . Also attached is a screen shot of my focus. Before subs where taken

Thanks
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Old 13-03-2020, 09:51 PM
Xeteth (David)
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Are you taking those darks with the camera attached to the scope? If so, try removing the camera, covering it and running the dark again. Don think that artifact should be there... It's possible you have a light leak or something on your OTA?

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Old 14-03-2020, 07:14 AM
vin_ap (Caleb)
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Hi Kuz,

That looks like amp glow to me, it should calibrate out when you process the image.

https://astronomy-imaging-camera.com...-amp-glow.html

I suspect two things, one as I mentioned before pixel ratio the other is that there may be a slight flex in your image train maybe where the filters are attached. Keeping your setting on the camera, could you try a 30sec sub of the horse head nebula without filters and post it ?

Regards,

Vin

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Old 14-03-2020, 11:43 AM
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Will do. i hopefully will be able to get some subs on Sunday night

Thanks
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Old 14-03-2020, 11:47 AM
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That is defintely Amp Glow associated with the 294, Darks will eliminate this in post process. As for the odd star patterns turn down your gain and as Alex said check for internal reflections, I have the same camera and don't have the star bloat issue
Besides there is no real need for a 600 second exposure, you can halve it and double the number of subs instead.
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Old 28-03-2020, 02:49 PM
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Hi Vin 1 x 30 second sub taken without filter and same gain setting which was 120. Thanks
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Old 09-05-2020, 03:15 PM
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I think I have finally found the cause of my issue with the shape of my larger stars . The old not enough o ring trick .
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Old 14-05-2020, 06:46 AM
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Well I'll be!

There are a couple of my scopes I'll be having a close look at. I seem to recall an objective or corrector plate having a similar uneven edge. I had noticed an unexplained diffraction spike on one of my scopes that doesn't have a spider, so now I have a possible source.

Out of your pain, Kuz, this is a really good example of why a field stops & baffle rings need to be perfectly smooth and kink free, and focuser drawtubes not cut into the optical train.

I wonder now how many Newts have unexplained diffraction spikes or flares due to the drawtube? And have the scope's owner tortured not knowing the source. Especially with so many Newts needing to have the focuser rucked in just about all the way in to achieve focus with a camera. I have noticed the shadow of a focuser draw tube with one of my Newts (a solid tube OTA) when the focuser is rucked in al the way. Time for a closer look...

Excellent investigative work, Kuz! One to certainly keep in the memory bank of problem solving.

Alex.

Last edited by mental4astro; 14-05-2020 at 06:59 AM.
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Old 09-07-2020, 04:25 PM
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New scope on the way

Thought I would post the outcome of the distorted larger stars issue, the lens buffer o- ring was to small and was causing all larger stars to turn out distorted. Skywatch have come to party with a brand new scope.

Thanks
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Old 09-07-2020, 08:46 PM
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Good to hear of a good outcome, I was reading through and going to post some ovbservations on the ASI294 from my experience. First, I could not see how the cam could cause your artifacts (Hopefully now resolved with a new scope) Second, gain 120, offset 30 seems to be about the sweet spot for the 294. and third, that glow on your 600 second dark is normal for the ASI294. It is inherent in the sensor and all of them display it. It should calibrate out of your lights with a good master dark. A bit of a time investment for 600 second subs but I try to make my master darks with 100 dark frames.

Last one, I dont know about via the filter as it is more or less a multi narrowband and reduces transmission, but without an NB filter, 600 seconds is a sure recipe for bloated, saturated white stars. I did an image of the trifid and lagoon recently and used lots of 120 second subs, most of the time I use 300 seconds as a reasonable compromise between getting faint detail and saturating brighter stars.
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