central bright area...vignetting?

Discussion of using SharpCap for Deep Sky Imaging
psy1280
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Re: central bright area...vignetting?

#31

Post by psy1280 »

Great advice....
I shall begin the experimenting Brian. I'll keep you posted.

And yes, I always keep my gain at 120 for all

Maybe you can help with this. I was just trying to re-do my dark library and something strange is going on. The frame count is not being registered at the bottom and the ETA keep shifting...getting longer and longer. Any idea??? I submitted this question in the forums...very strange.

Stay cool...
psy1280
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Re: central bright area...vignetting?

#32

Post by psy1280 »

Oops! I was using the wrong output format when capturing darks, everything's working now.
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oopfan
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Re: central bright area...vignetting?

#33

Post by oopfan »

Joe,

Can you share some snapshots of your rig when you are taking Flats?

Thanks,
Brian
psy1280
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Re: central bright area...vignetting?

#34

Post by psy1280 »

Morning Brian,
Since I haven't been able to get a cloud free night and the 10 day forecast looks horrible, let me at least describe my flats at f/2:

1. I have the hyperstar + ASI239 attached to the corrector plate of my OTA
2. I have the dew shield covering the above
3. I then place a white tee shirt (held in place with an embroidery ring) on the front of the dew shield (pointing straight up)
4. I the place a artist illuminated sketching panel (which has three layers of automobile sun screen film taped to it to dim it sufficiently)

That's it. By the way I was asking a friend about the mysterious central "vignette" (?) and he had an interesting comment. He asked, since I was using Hyperstar, how I had the rear opening of the OTA covered. I had placed one of those white inserts that come with various astro purchases to protect them while shipping. It is not quite opaque, and clearly this would sound suspicious, however, I'm not aware of any light source coming from the rear of my OTA. Nevertheless, I shall replace this white plastic cap, with a covered eyepiece next time out. Interesting possibility, huh?

Hope your skies are more forgiving up north,
Joe
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oopfan
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Re: central bright area...vignetting?

#35

Post by oopfan »

Hi Joe,

It's great to hear that you are using a dew shield.

Let me ask you about the "artist illuminated sketching panel". I was under the impression that you were taking Sky Flats. Are you certain that there are no hot or cold spots? The T-shirt should help diffuse it.

I think that your friend is onto something! Why tempt Fate? Definitely replace the white insert with a black one.

Making progress!

Brian
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turfpit
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Re: central bright area...vignetting?

#36

Post by turfpit »

Joe

My Celestron C8 has a cap which screws on at the eyepiece end of the scope instead of the visual back. I think using the opaque cap will be beneficial when imaging with the hyperstar.

Dave
psy1280
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Re: central bright area...vignetting?

#37

Post by psy1280 »

Thanks Brian ( & Dave),

I'm pretty sure my flat procedure is correct, i.e., a white tee shirt held taunt in place by the embroidery ring, which is placed over the dew shield. I have some layers of dark film (used in cars for sun protection) taped to my light panel...the lighting seems even on the view screen. At this point all I need is an opportunity to verify what we've been discussing in this thread.

Also, for the first time I'm going to be trying plate solving. I'll be using CPWI to connect my Sharpcap, focuser, and Stellarium. Considering my history, I'm certain I'll be scratching my head afterwards, LOL
One thing I'm still unclear about is whether one needs to do a star alignment (I could do this easily with Starsense through CPWI) if you're plate solving. I've been told a star alignment isn't necessary, i.e., you select a star in Stellarium, go to Sharpcap and do a plate solve, then synch. If that's about right, it sounds like magic. However, like I said, for me...well, I'm not expecting magic!

Thanks for any comments,
Joe
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oopfan
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Re: central bright area...vignetting?

#38

Post by oopfan »

Joe,

Why wait for a clear night to take Flats? You've got a light panel. Do it inside even if it's raining outside!

Brian
psy1280
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Re: central bright area...vignetting?

#39

Post by psy1280 »

Brian,

Not exactly sure why you’re suggesting taking indoor flats. I mentioned that visually, my flats looked even, i.e., nothing out of the ordinary compared to the many flats I’ve taken in the past. I guess what I’m saying is that unless I apply my flat to an actual image how else can I notice vignetting? Can you, in fact decipher vignetting by looking at a flat?

Thanks for keeping me going with this issue :D

Joe
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oopfan
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Re: central bright area...vignetting?

#40

Post by oopfan »

Joe,

Since you have a light panel there is no difference between an indoor flat and an outdoor flat. Flats alone tell you a lot. It will reveal dust, dirt, and fingerprints on the sensor window and filter. It also shows vignetting.

Capture 50 Flats with whatever filter you normally use for deep-sky imaging. Use the same gain you used for Darks and Bias from last week. That was Gain 120, right? Don't forget to specify the Target type = FLAT. And then upload to Google Drive like you did last week. From those 50 FITS files, I'll make a Master Flat, and then post it here on the Forum.

EDIT: The only type of frame that requires you to be outside at night is a LIGHT frame. Every other frame type can be done indoors, unless you take Sky Flats.

Brian
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