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Old 03-08-2015, 06:42 PM
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dark frame focus?

do dark frames need to be done at the prime focus if lights have been done that way and flats?
i know temperature is important but is focus?
pat
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Old 03-08-2015, 07:24 PM
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Atmos (Colin)
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Focus doesn't matter at all because there shouldn't be any photons hitting your CCD at all. Temperature and exposure time is all that matters.
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Old 03-08-2015, 07:50 PM
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thanks colin, so theoeitically i could do darks indoors the next day as long as the temperature is close to the same?
if you have a ccd and no light box, would you have to stoke up the camera the next daytime with the same light frame temp and focus as the previous night?
pat
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Old 03-08-2015, 09:01 PM
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Yep, as long as temperature and exposure time are the same it doesn't need to be connected to the telescope at all.
Are you referring to the flats for the second part or still the darks?
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Old 03-08-2015, 09:20 PM
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flats
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Old 03-08-2015, 09:46 PM
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Theoretically flats should be taken around 5-10 seconds at the same focus, orientation and temperature as your lights.
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Old 03-08-2015, 10:28 PM
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thank you colin, that does make it much harder to do all in one night then
a light box will be a necessity if to be done properly
pat
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Old 04-08-2015, 01:40 AM
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Quote:
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theoretically flats should be taken around 5-10 seconds at the same focus, orientation and temperature as your lights.
iso?
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Old 04-08-2015, 05:14 AM
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When I setup outside I can usually get within about 0.5 a rotation of focus on my refractor by eye which is probably going to be close enough if you're not wanting to do as astrometric studies.

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iso?
Should also be the same if using a DSLR.
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Old 04-08-2015, 08:46 AM
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ah yes i forgot about the iso, same as lights and put it on shutter priority?
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Old 05-08-2015, 03:01 PM
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When i do darks and bias i simply screw on the ccd cap and fire away in my office simply connected to the computer. Flats i do at the end of the night while scope still set up with a lightbox. I go to bed and get it all in the morning. I cover the light box and scope with a sleeping bag to make sure no stray dawn light ruins the exposures.
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Old 05-08-2015, 03:38 PM
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do dark frames need to be done at the prime focus if lights have been done that way and flats?
i know temperature is important but is focus?
pat
Darks can be shot with the camera even off the scope, so are bias frames. All you need is to make sure it's dark and there is no stray light. Most cameras these days have a regulated TEC so you don't have to shoot on site at the time you do your light subs. I have a library of 40x20min darks to make a master dark because I never shoot subs longer than 20min and you can scale the darks down to calibrate shorter light subs. It's a good idea to reshoot your darks every 6 months or so.

For light flats, I use an EL panel and I shoot them in the dark so there is no daylight. You need to be in focus accurately to shoot flats. I usually shoot 80 of them to make a master. Subs duration varies from 1s up to 8s for NB or even 20s for IR at long FL. All depends on the scope and filter. I also shoot dark flats of the same duration at the same temperature to make a master bias that I subtract to the flats so they don't over correct. I always shoot flats and bias after every imaging session. Unless the camera hasn't moved or rotated if I image for a few nights.
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Old 05-08-2015, 05:52 PM
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ta lewis and marc
ok so i can do the darks anytime just hook the ccd into the computer, fire up the TEC and voila get darks, no focus
lewis i thought i may need a lightbox for this to do properly
............... so flats need same (ccd?) orientation, precise focus TEC on?
lewis do you leave your TEC on til morn?
marc how do you scale darks down from 20 mins to match a different exposure, this is great!
pat
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Old 05-08-2015, 06:25 PM
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ta lewis and marc
ok so i can do the darks anytime just hook the ccd into the computer, fire up the TEC and voila get darks, no focus
lewis i thought i may need a lightbox for this to do properly
............... so flats need same (ccd?) orientation, precise focus TEC on?
lewis do you leave your TEC on til morn?
marc how do you scale darks down from 20 mins to match a different exposure, this is great!
pat
For the darks, as long as temperature, ISO and exposure time are the same as the lights, it doesn't matter whether they are connected to the telescope or not. There is of course ways of scaling darks for shorter exposures BUT there is the chance that this method can actually introduce more noise so you are better off just capturing the darks correctly. PixInsight does the calculations to scale it down, it is not something that you do manually.

With the flats they should be at the same orientation as the lights because what you are trying to correct for is the flat illumination error of the telescope to the CCD sensor. The larger the sensor the more important that this becomes on most telescopes as it is usually beginning to encroach on the corrected imaging circle.
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Old 05-08-2015, 07:57 PM
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For the darks, as long as temperature, ISO and exposure time are the same as the lights, it doesn't matter whether they are connected to the telescope or not. There is of course ways of scaling darks for shorter exposures BUT there is the chance that this method can actually introduce more noise so you are better off just capturing the darks correctly. PixInsight does the calculations to scale it down, it is not something that you do manually.

With the flats they should be at the same orientation as the lights because what you are trying to correct for is the flat illumination error of the telescope to the CCD sensor. The larger the sensor the more important that this becomes on most telescopes as it is usually beginning to encroach on the corrected imaging circle.
the flats explanation makes perfect sense thanks colin
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Old 06-08-2015, 10:02 AM
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ok so i can do the darks anytime just hook the ccd into the computer, fire up the TEC and voila get darks, no focus
Yes you don't even need a scope. Cap the camera facing down and let it shoot all night. I do that in my living room indoors over a couple of nights.

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marc how do you scale darks down from 20 mins to match a different exposure, this is great!
I use CCD Stack to calibrate all my subs and it does that automatically. I'd say every other software has a way of achieving the same thing. You can scale a 20min dark down to calibrate any sub up to 20min exposure but you cannot calibrate a 30min sub with a 20min dark because in a 30min sub it is likely more hot pixels and defects will appear than in a 20min exposure. So there will be things in a 30min dark that are not in a 20min dark if that makes sense.
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Old 06-08-2015, 02:15 PM
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thanks marc, if you want to take the flats the next day do you have to reboot and cool your ccd? or if you had a lightbox as lewis has said and did them the same night, is it wise to leave your ccd tec cooler on all night, i would have thought not?
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Old 06-08-2015, 02:17 PM
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thanks marc, if you want to take the flats the next day do you have to reboot and cool your ccd? or if you had a lightbox as lewis has said and did them the same night, is it wise to leave your ccd tec cooler on all night, i would have thought not?
pat
It's the temperature at the time that is important, that's all. So you are right, you don't need to leave the cooler on when you're not using it.
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Old 06-08-2015, 02:36 PM
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thanks marc, if you want to take the flats the next day do you have to reboot and cool your ccd? or if you had a lightbox as lewis has said and did them the same night, is it wise to leave your ccd tec cooler on all night, i would have thought not?
pat
I always take the flats on the night. If I image for a few nights I have sometime taken them the second or third night. As long as you make sure your scope is in focus and you haven't moved the camera at all you'll be ok. Makes sure you shoot your dark flats (bias) as well at the same time. Cover the scope and shot your bias.

If I don't use the camera I cool it down slowly to 0 then turn the TEC off. Never power off the camera when it's at -20c. I also ramp it down to 10,0,-10 then -20c when I start. Always did that. I never leave the TEC running for no reason. When I'm done with the camera I also always open it, vent it then store it in dry sealed pelican case.
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Old 08-08-2015, 06:44 PM
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Flats should be taken at the lowest native ISO of your camera. They do not have to be identical to the lights or darks.

H
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