What's causing these "bubble" shapes, I think they're stars out of focus???

Discussion of using SharpCap for Deep Sky Imaging
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psy1280
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What's causing these "bubble" shapes, I think they're stars out of focus???

Post by psy1280 » Sat May 16, 2020 5:47 pm

resize3.jpg
resize3.jpg (262.97 KiB) Viewed 386 times
I'm still getting used to my new ZWO294 MC Pro along with my Hyperstar. This was my second attempt imaging the Hercules Cluster. The normal image shows the stars not as pinpoint (however the Bahtinov mask was dead on). In the zoom image you see these "bubble" stars with a bright spot at about 1:00 O'Clock. Not sure if this is a focus issue and if so, if the Bahtinov mask shows perfect spikes...what else is there to do. I'm using a Celestron Evolution C8. Any suggestions or help...most appreciated. FYI: The photo above was a zoomed image in PhotoShop, it was taken with my iPhone from my computer screen. When I shot the cluster in SharpCap I was at 50% zoom...the stars were definitely not pinpoint.


Joe

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turfpit
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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by turfpit » Sat May 16, 2020 6:02 pm

Joe

Post a screenshot showing the Bahtinov mask pattern when you think you have focus. See viewtopic.php?f=18&t=1151 for an idea of what you are aiming for.

To focus use a single medium brightness star (not a bright one such as Arcturus or Vega). I am currently using stars such as Alkaid (for M51), Alphecca (for M13) and Dubhe for M81/82. If possible, use a star which is roughly the same altitude as your target.

Dave

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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by celkins » Sat May 16, 2020 11:43 pm

Joe,
your hyperstar need collimating - that’s what causes those bubble shapes.
You adjust it by making *small* adjustments to the push/pull pairs of screws around the base of the hyperstar.
Make sure that the rotation screws (the single ones) are finger-tight before starting this - this problem can also show if these are loose, letting the unit move.

It’s very useful to observe the image as you reach across the field of view, and use the shadow of your arm to work out which set of screws is closest. I find it easiest to center a reasonably bright star (mag.2-3), near overhead, then defocus, to produce a nice donut image - you’re aiming to have a perfect annulus, or as near as you can get. If there’s a noticeable asymmetry, then adjust the pair of screws either closest to, or opposite to the narrow section, using the arm shadow trick to work out which... small adjustments... always finish by double checking all screws are secure.

Once you’ve completed this, refocus with your mask, and you should be good to go. You might see some degree of mirror flop as you point to different parts of the sky, which should be no worse than a slight elongation of all stars in the same direction: if you want to be ultra-precise, then you could always correct for this, but I’ve never bothered...

Hope this helps,
Carl

psy1280
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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by psy1280 » Sun May 17, 2020 1:06 pm

Carl,
Your assessment makes perfect sense. This is the second time I've used Hyperstar and I've never done a proper collimation. One question: since the hyperstar and camera get removed after each session and then replaced the next time out, is the Hyperstar's collimation sensitive to this movement? Will I have to collimate often?

Thanks again,
Joe

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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by oopfan » Sun May 17, 2020 2:58 pm

Joe,

I don't dismantle my scope when a session is complete. I simply remove the scope from the mount and carefully set it aside, then I do the heavy lifting to get the mount indoors and set in a place away from foot traffic. Once there I reattach the scope and cover it with a sheet.

Brian

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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by celkins » Sun May 17, 2020 8:35 pm

Joe,
I don’t have to break down, but on the occasions when I have, it’s help collimation pretty well.

Happy to be able to share experience,
Carl

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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by psy1280 » Mon May 18, 2020 6:06 pm

I believe that Brian's suggestion to leave everything attached and cover the scope with a sheet when done for the night is probably the best way to ensure no inadvertent collimation issues, however, I'm totally neurotic and since my scope is set up in my living room and I enjoy gazing at it, having it covered with a sheet would be giving up that pleasure during the many cloudy nights. :-) Carl, thanks for the encouragement. Perhaps with being very careful I too can minimize collimation issues when dismantaling the Hyperstar/camera.

Joe

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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by oopfan » Mon May 18, 2020 6:13 pm

Joe,

You don't need to put a sheet over it. I do it to keep dust off the scope and to keep dust off the gears since they are exposed. You still might want to look into covering your corrector plate. Perhaps a he-man sized shower cap?

Brian

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Re: What's causing these "bubble" shapes, I think they're stars out of focus???

Post by psy1280 » Mon May 18, 2020 7:07 pm

I believe that Brian's suggestion to leave everything attached and cover the scope with a sheet when done for the night is probably the best way to ensure no inadvertent collimation issues, however, I'm totally neurotic and since my scope is set up in my living room and I enjoy gazing at it, having it covered with a sheet would be giving up that pleasure during the many cloudy nights. :-) Carl, thanks for the encouragement. Perhaps with being very careful I too can minimize collimation issues when dismantaling the Hyperstar/camera.

Joe

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