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Old 19-08-2015, 12:01 PM
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graham.hobart (Graham stevens)
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curious about a couple of issues

Hello,
I have three things that have been bugging me and I don't know the reason for...

1/ taking bias frames the other day with the newt capped. A towel over the end and black tape over the finder of the DSLR I noticed a very subtle grey area top of the frame that looks like a light leak (it was daylight) but I don't know where its coming from.
I will check dark frames and bias frames done at night for this at the weekend.

2/ using a light box for flats on my SV 60 (+ 1000D) scope a get a nice thin peak on the histogram in APT, using another light box with my 60Da on the newt I get a double peaked histogram in all the histo's, why?
3/ what's an average exposure length for iso 800 light box flats?
Using APT I tend to let it set the exposure using AV and the flats setting, but if I do it manually I have to use minuscule i.e 0.01 sec exposures to get the histogram looking right, and still I am not convinced either of them are correcting properly

Cheers
Curious Graham
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Old 19-08-2015, 12:45 PM
LewisM
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Why not use the Newt's end cap AND a towel? Would be even more foolproof of stopping errant light. Even a black towel during the day will not be very effective, considering the weave (small holes everywhere). Even doubled over you could get errant photons. best to use a solid cover (plastic end cap, cardboard taped on etc) THEN a dark towel.
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Old 19-08-2015, 01:09 PM
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graham.hobart (Graham stevens)
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curious

Lewis, I did.

Cap on end, dark towel doubled over and placed over the end cap. Multiple layers of tape over the camera finder plus a bag.
it's not fool proof but then I don't know what would be doing this in daylight. Obviously awaiting further constructive advice....
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Old 19-08-2015, 01:30 PM
glend (Glen)
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Why not just shoot darks and offsets with the DSLR off the scope and lense cap on? You gain nothing by shooting them on the scope as far as I can tell.
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Old 19-08-2015, 01:34 PM
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Quote:
Originally Posted by glend View Post
Why not just shoot darks and offsets with the DSLR off the scope and lense cap on? You gain nothing by shooting them on the scope as far as I can tell.
+1 - also shoot in the dark not daytime. All cameras have light leaks to some extent.
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Old 19-08-2015, 01:38 PM
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graham.hobart (Graham stevens)
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curious

I do mostly, the basic question was where is the light leak? just curious. If it is just that camera bodies are leaking around cable points etc I was surprised a bias frame would show it up on such a short exposure. Saying that, if there is no remedy then I will shoot at night time. I only did it in the day as I was doing some flats with a light box so had it all set up.
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Old 19-08-2015, 01:42 PM
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Quote:
Originally Posted by graham.hobart View Post
I do mostly, the basic question was where is the light leak? just curious. If it is just that camera bodies are leaking around cable points etc I was surprised a bias frame would show it up on such a short exposure. Saying that, if there is no remedy then I will shoot at night time. I only did it in the day as I was doing some flats with a light box so had it all set up.
It would be very hard to find where it comes from. Even capped my QHY8 had light leaks from the sides and back. I had to use electrical tape. Usually the wires from the fan going inside, holes in the casting, etc... I assume a DSLR would have plenty of little gaps everywhere too.
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Old 19-08-2015, 02:33 PM
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Fair enough.

What about the double peaks on the histo with the 60Da?
what's that all about? Is that just a facet of its differing sensitivity as compared to the 100OD?
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Old 19-08-2015, 02:49 PM
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Does it show up if you do liveview and crank up the ISO?
If so you could do that and then move your hand/piece of card/cloth around the scope/camera and see if you can find a spot where it changes...?
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Old 19-08-2015, 04:41 PM
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Quote:
Originally Posted by graham.hobart View Post
Fair enough.

What about the double peaks on the histo with the 60Da?
what's that all about? Is that just a facet of its differing sensitivity as compared to the 100OD?
hi Graham. you will get three peaks in the histogram, one for each colour. How many of the three peaks are distinguishable depends on the spectral distribution of the light source and the relative sensitivity of the camera in the three Bayer filter bands. Depending on the camera and the source you may be able to distinguish only one peak (ie all three colour peaks overlay), two peaks (two colour peaks overlay) or all three peaks.

Last edited by Shiraz; 19-08-2015 at 08:33 PM.
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Old 19-08-2015, 07:13 PM
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Shoot your flats at ISO-100.

Use Live View Exposure Simulation (if your camera has it), enable the histogram overlay, adjust your exposure until the histogram peaks 1/3rd to 1/2-way across the X-axis. Done.

Once the flats are done, also take the flat darks, at the same exposure duration.

H
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Old 20-08-2015, 07:37 AM
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curious

thanks Ray/H
I always thought you did the flats at same ISO as subs? I will try this.
The APT histo splits the colours so you get three histograms for RGB, is that what you mean Ray or are you meaning the peaks on each histogram?
If so I assume the 1000D is a one peak and the 60Da 2 peak overlay.
Cheers for clearing that up.
Thanks all
Graham
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Old 20-08-2015, 09:17 AM
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Flats, Darks and Bias should all be taken at the same ISO as what you're imaging in. Having the same ISO is just for pixel to pixel variations, won't effect vignetting and what not.
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Old 20-08-2015, 09:19 AM
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From all the literature I've read over the last ten years, it has always stated to shoot the flat lights/darks at the lowest native ISO of your camera. For Canon's, this is ISO-100.

Worked for me.

H
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Old 20-08-2015, 09:35 AM
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Quote:
Originally Posted by graham.hobart View Post
thanks Ray/H

The APT histo splits the colours so you get three histograms for RGB, is that what you mean Ray or are you meaning the peaks on each histogram?
If so I assume the 1000D is a one peak and the 60Da 2 peak overlay.
Cheers for clearing that up.
Thanks all
Graham
I was assuming that you were looking at the histogram for the raw without any deBayer or splitting - then there will be one histogram peak for each colour (possibly with some overlap). If you are getting two histogram peaks in one of the colours after splitting the raw, I have no idea why. It means that there are two distinct regions in the frame with different average brightness.

regards ray
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Old 20-08-2015, 12:05 PM
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curious

Hi Ray, you may be onto something- see my first point re? light leak. Maybe there is a light leak producing a second source which is producing the double peak on the histograms.
H + Atmos ..
so which is it?
ISO 100 or at the ISO I shoot at - 800 or rarely 1600?
I have to say recently for some reason my flats have sucked badly so I'm inclined to give ISO 100 a go and see if it is any better.
cheers
Graham
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Old 20-08-2015, 02:01 PM
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Graham,

If you do a search for my posts in the deep space forum, you'll see DSLR images that I've made over the years. Each has been flat-fielded using ISO-100.

H
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Old 20-08-2015, 03:37 PM
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For technical accuracy across the entire image the same ISO needs to be used.

If all you're wanting to do is flat field out dust bunnies and vignetting, it doesn't matter what ISO you use as they are an artefact of both the telescope and any bits in between.
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Old 22-08-2015, 03:18 PM
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I am not sure that is correct Colin. ISO is simply amp gain before the ADC. Higher ISO should not make the unevenness of the illumination any worse. That comes from the optics and sensor and dust. So far as I know ISO does not affect the unevenness of the illumination although higher ISO does reduce dynamic range. Most Deep Sky objects though have low dynamic range compared to a terrestrial shot with bright sun and shadows in the same image.

Flats are merely a shot of the unevenness of the illumination across the field from optics and sensor and removal of dust artifacts. So I am not sure why higher ISO would affect that as long as Humi says a decent histogram exposure is done (otherwise your flat would over/under correct).

Darks may be different though. ISO probably should be taken into account as dark current would also be amplified by the higher ISO.

Additionally ISO is not done the same way all the way to the top of the range. There is a point where the amp is maxed yet that is not the highest ISO. At that point there is something done in the ADC (analogue to digital converter) to create an artificial ISO boost. That level is often know by DSLR imagers. For example on the Nikon D800e its ISO1600.

Greg.
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Old 22-08-2015, 11:57 PM
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That is what I was getting at to an extent. If all you want to do is remove issues caused by the optics then the ISO doesn't matter. This includes dust bunnies, vignetting and optical light fall off.

For technical images where you're wanting to do astrometric studies you do want to keep the ISO the same so as to not potentially introduce any inconsistencies. Flats are not just a map of what has already been mentioned but physical pixel of pixel sensitivities irrespective of light fall of. This is a part of the sensor and therefore changes in ISO have the potential to change stellar magnitudes. Not by much but potentially enough to reduce accuracy when trying to get millimagnitude accuracy.
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