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Old 20-04-2010, 10:04 AM
TrevorW
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A question of exposure time

Just a question

I tried capturing the nebula IC 4604 around the triple star system in Scorpius the other night and could get little of the blue nebula captured in 13 by 6 minute subs although the triple comes up well with big diffraction spikes

How do you determine as in this case the exposure time required to capture the dim nebula when I can't find reference as to its VB

or is it a case that the FOV was too tight with the 8"RC even using the FR

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Old 20-04-2010, 10:52 AM
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rogerg (Roger)
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I would ordinarily take a test shot and then look at the pixel brightness of the area I'm wanting to capture. I can then work out the multiple I need to achieve the pixel brightness I'm happy with (compromised with exposure time length) and so that multiple times by my test exposure time gives me how long to do each exposure for.

In reality I do all that in my head with a healthy dose of gut feel thrown in.

I'm not sure which nebula in particular you're referring to, to be able to comment about FOV sorry.

Roger.
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Old 20-04-2010, 01:14 PM
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Trev, i have taken some pics of IC4605 the same region in Scorpio, and i was looking at 10 minute subs minimum, for capturing anything to good effect. 20 mins would be better, and easy doable with no noise on the QHY8




ADDENDUM : Mate, i would say the FOV of the RC is way to tight, its too tight even with my 8" F5 scope, and i have found the only half decent way to capture it, is with a Canon lens to QHY8 adaptor, using a 55mm zoomed in.
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Old 20-04-2010, 01:26 PM
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For any extended object, lower the F-number, the better.
F5.6 is 4x better than F11: On the image the stars (point sources of light) will look the same, the nebuale will be 4x brighter.
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Old 20-04-2010, 01:27 PM
TrevorW
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I was thinking longer as that must be one dim nebula surrounding those stars
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Old 20-04-2010, 02:01 PM
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rogerg (Roger)
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Interesting.

I just googled the IC number and see it's the same as what I photographed on Saturday night. I'll have to make time tonight to post my image in the forums.

I used my un-modified 350D with Megrez @ F/5.95, using 3 minute exposures @ 800ISO. I have quite a nice display of the blue nebulosity in my image.

I guess your RC must be at F/8 or something? with 6 min shots I would've thought, based on what I got, you'd have results similar to mine.

Of course your FOV is much narrower than mine.
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Old 20-04-2010, 05:36 PM
TrevorW
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Thanks Roger like to see

maybe it's also the processing I'll spend a bit more time on it tonight

need to also redo flats to take into account the FR in the image train

Cheers
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Old 20-04-2010, 06:05 PM
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multiweb (Marc)
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Quote:
Originally Posted by TrevorW View Post
I tried capturing the nebula IC 4604 around the triple star system in Scorpius the other night and could get little of the blue nebula captured in 13 by 6 minute subs although the triple comes up well with big diffraction spikes

How do you determine as in this case the exposure time required to capture the dim nebula when I can't find reference as to its VB

or is it a case that the FOV was too tight with the 8"RC even using the FR

You just need to shoot a lot more exposures. If it's a dark site and you have very little sky glow you can go for long exposures. If it's light polluted then you have to compromise and shoot a lot of shorter exposures and try to get the best SNR. Stack a lot of them and eventually you'll get all those photons.
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Old 20-04-2010, 11:42 PM
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General rule of thumb that I am using now is that the ADU should be somewhere between 800 and 1000 as a minimum. This often means that the Luminence layer needs to be between 15 and 30 minutes as a such exposure.

To test your ADU get a program like CCE inspector and measure your first sub and then adjust to suit.
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Old 21-04-2010, 12:24 AM
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Are there any cheap alternatives to such programs as ccd inspector ? That work almost as well ?

Thanks.
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Old 21-04-2010, 09:21 AM
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Sorry I can't help there oz. Maybe someone else can suggest a program.
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Old 21-04-2010, 09:50 AM
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It seems there is none..
http://stargazerslounge.com/equipmen...inspector.html
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Old 21-04-2010, 11:37 AM
TrevorW
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You can get an ADU count using Maxim (which I use to capture) info window I assume this would provide the same info
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Old 21-04-2010, 12:22 PM
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Quote:
Originally Posted by Mighty_oz View Post
Are there any cheap alternatives to such programs as ccd inspector ? That work almost as well ?

Thanks.
open the image in Iris (free). Select an area with the mouse you wan't to measure. Right click and select stats( I think). This gives you the level in the area and the background level.
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Old 21-04-2010, 12:52 PM
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open the image in Iris (free). Select an area with the mouse you wan't to measure. Right click and select stats( I think). This gives you the level in the area and the background level.
Yes of course.. IRIS
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