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 Post subject: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 5:31 am 
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Thanks to Dean & Andy for providing a comprehensive tour of the pitfalls of deep-sky imaging, and how these obstacles may be overcome with lots of patience and enthusiasm.

By the way, as an illustration of Walker's Rule ("pain is proportional to the square of focal length"), I was talking with one Member who'd been struggling with his webcam on the back of a 10" f/10 SCT. Now whilst this combination is excellent for planetary imaging at high magnification, it's not really suited for the bigger and much fainter deep-sky objects, so I've lent him a little 80mm f/5 refractor to try instead.

With one-sixth of the focal length and half the focal-ratio, this will have several advantages:
  • 36x increase in field of view - so much easier to locate objects, and larger DSOs will actually fit on the sensor
  • 6x reduction in magnification, so tracking errors cause less much trailing, permitting longer exposures
  • halving the f/ratio means that the image is four times as bright - so one quarter of the exposure-time would be needed
Of course the image will only be one-sixth the size - but with a sensor just 1/4" across that's not such a disadvantage either!


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 Post subject: Re: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 5:58 am 
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I think that calibration frames will be covered in more detail at next month's workshop, but if you're going to practice in the meantime...

The principles are exactly the same for LX webcams and DSLRs.
You must use the same sensitivity ("gain" or "ISO" setting) as for your deep-sky images.
Bias (offset) frames: fit the lens-cap, set the shortest possible exposure-time, and take a "binary dozen" (= 16) of these.
Dark frames: keep the camera at the same temperature as for the deep-sky imaging session and use the same exposure-time. Fit the lens-cap and take 16 of these. Make sure that no stray light can get onto the sensor whilst taking them (e.g. from the camera viewfinder).
Flat-fields: keep the same optical configuration (aperture setting, filter positions, etc) as for your deep-sky images and take 16 pictures of an evenly-illuminated target (I've used a white projection-screen, or draped a white napkin over the lens and pointed it up at the daylight sky). Use your historgram display to choose a (short) exposure-time which does not saturate the sensor, since an over-exposed flat is worse than useless.

I demonstrated a flat-field with the webcam last night (using the painted wall as a target), which showed up lots of dust-specks on the sensor - although the FF division will reduce their detrimental effect on the final image, it's best to clean the sensor first!


Last edited by rwilkinson on Wed Apr 10, 2013 8:13 am, edited 1 time in total.

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 Post subject: Re: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 7:36 am 
thanks for adding some padding to what i went over last night Ross

I follow the same process for calibration files:
1. take object shots
2. do the darks
- put the lens cap on ( I always throw a coat over the setup too to ensure no light can get in)
- dont touch the camera settings
- keep setup at same temperature (put in garage or leave outside if you're sure the local pixies wont run off with it!)
- same number of shots (or as Dave R suggested 15-20 max)
- same length of exposures used to capture your object shots
3. do the offsets
- cap still on
- change the setting to TV move the apeture to the fastest setting i.e. 1/4000
- do the same number of shots (as above 15-20) these are very quick
4. do the flats
- setup outside (point at a uniform piece of sky)
- cap off
- change camera setting to P
- under expose by 1 (on the cannon 1100D & 450D press the AV button & scroll the wheel down until the pointer moves from 0 to -1)
- same number of shots as for darks & offsets (twist to this one - take 5 shots then rotate the camera 90degrees and repeat until the camera has rotated 360degress - Gerald tells me it removes any gradient in the flats)
- fastest shutter speed
Some people

Thats my process anyway - as with all things in this game - there's more than 1 way to skin cats.

Cheers
Andy


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 Post subject: Re: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 1:17 pm 
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Just to add to last night's excellent introduction - Andy mentioned flats should be shot (of the daytime sky) at the "P" setting (not the "fastest" BTW) with 1 stop under exposure but was unsure why. Just to explain.
The "P" setting would normally expose correctly for flats. However, with modded cameras the exposure meter under-estimates the amount of light getting through - it doesn't know you've removed the filter. So without setting under-exposure, the standard "P" setting would burn out the flats. For example, the BAS modded 300D is probably best set to under-expose 2 stops to get the flats correctly exposed.


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 Post subject: "Extreme" flats
PostPosted: Wed Apr 10, 2013 2:14 pm 
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At the weekend I was helping Carl to get some flats with his imaging system (C11 SCT with focal reducer and Canon DSLR), which has severe vignetting around the edges of the image. We found that even with -2 stops compensation in Auto mode the centre of the flat-field was still saturating - we thought that the camera's meter was affected by the weighting of the "dark" areas around the edges?
So we ended up using the camera's Histogram display to help us manually set the shutter-speed fast enough that we weren't hitting the "white" level at the right-hand side.


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 Post subject: Re: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 2:53 pm 
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If any of you have been inspired by last night's workshop to try wide-field imaging, here's an idea for mounting your camera and lens straight onto an EQ5 mount "dovetail" bracket, without needing to "piggy-back" it onto a 'scope:
Attachment:
File comment: Another astro-accessory from the woodwork shop!
newbracket.jpg
newbracket.jpg [ 184.85 KiB | Viewed 6108 times ]

This arrangement ensures getting "North at the top" of your images too! :D


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 Post subject: Re: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 5:43 pm 
Thanks for that Dave
are you saying that I need to adjust that under exposure by even more as my 450D had been modified?

As I said last night, I don't need to know the science behind shampoo, I just know I need to use it to wash my hair!

Ross, you're like an extra in the A team - there's nothing you can't make for astronomy using any old stuff - you should enter a team for scrap heap challenge. I suppose it highlights what Dave said last night about having to make your own stuff. No question i'd never have survived!!!
Cheers
Andy


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 Post subject: Re: Deep-sky imaging workshop
PostPosted: Wed Apr 10, 2013 6:18 pm 
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Simple attempt at mounting my Canon, needs some more work to keep North at the top
Attachment:
SDC10147 small.jpg
SDC10147 small.jpg [ 105.62 KiB | Viewed 6101 times ]


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