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A few questions about flats

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#1 Whichwayisnorth

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Posted 30 January 2013 - 06:19 PM

I am using MaximDL and I had a few questions about taking flats.

I figure while my mount is at the manufacturer I would get ahead of the game and take some indoor darks, bias and flats.

Darks and bias are no problems. I took sets of 30 at each of the commmon temps I will be using -10 and -20 and I took them at 2min 5min 10min 15min.

Now that have moved on to flats I took a big sheet of movie screen material, I illuminated it as even as I could as I don't see streaks or shadows. I put two layers of this white translucent plastic garbage bag plastic over the end of my OTA. I set up my imaging train, put my telscope up on a foldup table, pointed at the screen.

I have a QSI 683 with LRGB O3 HA N2 S2 filters.

For the LRGB I am typically at 1-1.4 seconds to get an average of about 33000. When I move on to my O3-S2 filters I am finding that to get over 20K I need to do at least 60 seconds. What am I doing wrong?

Also Should I Bin x2 the RGB or leave then at x1? And if I do bin them should I move the gain down to LOW. I assume I should since I think I read that on QSI's site.

Thanks for any tips you could provide.

#2 rigel123

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Posted 30 January 2013 - 08:38 PM

Depending upon the Bandwidth of your NB filters I'm not surprised at 60 second exposures for your flats particularly if you can't adjust the brightness of the light you are using. Is this simply practice for taking flats, because if you tear down your setup and try to use those flats for imaging later after you have set up again you could have some issues with dust motes being in different positions on your flats from your lights.

#3 zerro1

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Posted 30 January 2013 - 10:10 PM

If that camera uses a shutter

You might want to watch that duration and maybe go to a 3 or 4 second on the LRGB flats. You could capture the shadow of the shutter if the shutter isn't fast enough. I encountered that on my QHY9 early on, just sayin..

#4 CounterWeight

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Posted 30 January 2013 - 11:47 PM

Shutter effect is when the light region is out of round, and you'll be able to see it easily. I'm curious - is it that long for all the NB filters? Wonder if the light source is maybe not a very broad band? or the reflective surface?

(not to derail anything but Happy Birthday Robert!)

#5 Whichwayisnorth

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Posted 31 January 2013 - 11:43 AM

Yah all of the filters are 3nm astrodon.

I noticed that specifically the R & B channels I do see a straight line cut off along the top. But these are at 4 seconds or so. In fact, the longer I expose the more pronounced it is. Actually more on the red and the blue. I wonder if it is the OAG pick off prism? Any thoughts on that?

To get at around 30k area I have my flats at the following durations.

L Bin1 high gain 1.2sec
R Bin2 low gain 4sec
G Bin2 low gain 1.9sec
B Bin2 low gain 5.2sec
03 Bin1 high gain 180sec
HA Bin1 high gain 160sec
S2 Bin1 high gain 220sec
N2 Bin1 high gain 180sec

All of them seem fine except Red and Blue, which are both 4sec or more.

GREEN:
Posted Image

BLUE:
Posted Image

RED:
Posted Image

#6 Whichwayisnorth

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Posted 31 January 2013 - 11:46 AM

Shutter effect is when the light region is out of round, and you'll be able to see it easily. I'm curious - is it that long for all the NB filters? Wonder if the light source is maybe not a very broad band? or the reflective surface?

(not to derail anything but Happy Birthday Robert!)


Yah I am going to experiment with some new ideas here. You are probably right about not having a broad band spectrum. I am just trying to avoid spending a lot on a ready made light box. I need to build my own. I have up in the attic two aquarium light boxes that I bought once by accident. I wonder if those bulbs, which are for live plants, will offer a broad enough spectrum?

#7 Mike Wiles

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Posted 31 January 2013 - 11:55 AM

For the LRGB I am typically at 1-1.4 seconds to get an average of about 33000. When I move on to my O3-S2 filters I am finding that to get over 20K I need to do at least 60 seconds. What am I doing wrong?



I have an EL panel that I use to capture flats in a very similar setup (ST-8300m and Astrodon LRGB & 3nm NB filters). I can set the brightness of panel pretty low and have no trouble taking flats of 2 to 3 seconds through the LRGB filters. For narrowband, even with the lamp turned up to 100% exposures to get a max ADU of 35000 are in the neighborhood of 90 seconds. It's a bit of an annoyance, but nothing that can't be fixed by just shooting sky flats with the narrowband filters.

As I'm planning to remotely locate the telescope at some point I'm still hunting for a better solution that doesn't involve sky flats for narrowband filters....or extremely long exposures with an EL panel.

Mike

#8 RedLionNJ

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Posted 31 January 2013 - 12:14 PM

And not to try to add to any confusion - but are you 100% certain you're at exactly the same focus position you will be when taking light frames? Not so much of an issue if nothing major moves (i.e. you have a fixed primary mirror), but in some situations it can appreciably change the outcome of the flats.

Grant

#9 Whichwayisnorth

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Posted 31 January 2013 - 01:09 PM

And not to try to add to any confusion - but are you 100% certain you're at exactly the same focus position you will be when taking light frames? Not so much of an issue if nothing major moves (i.e. you have a fixed primary mirror), but in some situations it can appreciably change the outcome of the flats.

Grant


Truthfully no. I had it all set up then tore it all down to bring it inside. No telling how the focus was affected.

Will the focus change the requirements for exposure duration to obtain a nice medium or are you refering to the things the flats are supposed to take out?

Right now I am just trying to get the durations down. What I will probably have to do is set up the mount and get focus tonight then without breaking it down move it inside.

#10 Jared

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Posted 31 January 2013 - 01:35 PM

The focus won't change the requirements for exposure duration (or at least not materially--technically focal length and therefore focal ratio shift very slightly with different focus), but the amount of vignetting in your system will change significantly with focus. For most scopes, as long as your focuser is within a millimeter or two of infinity you are probably fine, but much outside that and the vignetting will change enough that the flats won't work well.

As others have mentioned, you really can't take flats beforehand and expect them to work properly later (unless you have a permanent setup--and even then it's possible for dust motes to shift and change).

#11 Jared

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Posted 31 January 2013 - 01:50 PM

Based on the sample images you posted, it looks like the pick-off prism is intruding into the light path--that's why you see the horizontal line. It's not likely to be a shutter artifact since SBIG cameras all have constant illumination shutters--all parts of the CCD should receive the same amount of light regardless of the exposure duration. This problem is more an issue with leaf shutters.

In order to determine whether you have a problem with the pick-off prism, you need to re-test at infinity focus. It is possible that the vignetting will go away when the camera is focused properly. Then take your actual flats on the night you are going to image.

#12 Whichwayisnorth

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Posted 31 January 2013 - 02:22 PM

Thank you Jared. LRGB are the easy ones to do under twilight skies. The exposures are so low I have time to grab a bunch of each on each filter. The problem is the NB filters require such long exposures there is really no way to get enough of them. At least that is the issue I am running into. What I am tempted to do is dust off the filters, put my camera in a big ziplock bag, then tonight get into focus and take a bunch of LRGB flats. Then bring it inside, without touching the train or the focus, and try a lightbox for the NB filters. Then try to use those flats for the next time out. About the only way I can think of to have them all.

#13 Mike Wiles

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Posted 31 January 2013 - 02:49 PM

The problem is the NB filters require such long exposures there is really no way to get enough of them. At least that is the issue I am running into.


to get a bunch of NB flats, I have a piece of translucent white plastic that I put over the end of the tube which ensures an evenly illuminated field. Then, I point near the zenith...and while there's no direct sunlight hitting the telescope, I'll take flat frames during the last few minutes of daylight. If I start about 10 minutes before sunset, the sky itself is plenty bright to knock out a bunch of NB flats before it sets...then you can carry on as usual for LRGB flats. Sunlight has more than enough signal for narrowband flats. EL panels and light boxes....not so much.

Mike






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