Dark frames question

[ultramol]

Is it adequate to just put the cover on my 200p when taking dark frames. I ask because when I left it running taking the dark frames slowly shot by shot the burning image of andromeda started to appear in them. Should I cover my set up with a blanket or somethig ? Thanks in advance.

[jambouk]

If there is light leaking into the frame, it's no good. The camera doesn't need to be in the scope for a dark frame, so you could take it off and put a lens cap on or the whole camera inside a box or bag. But make sure you've done your flats before moving the camera, and make sure your darks are done at the same ambient temperature as the imaging run... And some might say make sure you do your darks when the sensor is at the same temperature as it was during the inaging run... Nothing in astro-imaging is easy is it!

James

[ultramol]

Thanks James, I also think I have issues with my intervalometer and mirror lock up on my 350D. I used the self timer on the camera as learned on the net and it took 30 1min subs, with mirror lock up, no bother at all. But when I set it off doing the darks it threw a wobbler for want of a better term. Sometimes the mirror locked up other times it didnt, the shutter opened but never closed, it just seemed to go haywire. I think my camera is on its last legs....

[jambouk]

I've only ever used an external intervalometer; you can get wired ones for most cameras on ebay for about £10. Also, i've never bothered with mirror lock up either other than experimenting.

So I can't offer any help.

Have you tried turning the internal intervalometer off and connecting the camera to the laptop and runnung EOS utilities?

James

[ultramol]

I use windows 7 and canon does not support drivers for 350D and W7...Think they are trying to get me to buy a new camera....

[philherbert]

Hi,

Just to say that I have a 350D and Windows 7 and the drivers do work. The caveat is that you have to run 32-bit Windows 7 O/S. I'm using APT and it works fine.

Phil

[ultramol]

Thats interesting Phil, I got onto canon and they said there is no drivers for W7 and a 350D, think they were telling me porky pies....

[ollypenrice]

I cannot take darks on the scope even with a refractor which has a fully sealed tube (Newts have lots more light leaks) and sealed electric filterwheel. Light does get onto the chip somehow. It is absolutely vital that no light whatever get onto your chip and that means taking off the camera - so do your flats first.

Olly

[graemlourens]

Hi.

Similar Problem arose in another thread: http://stargazerslounge.com/topic/254791-light-pollution-from-synscan-handset/

And my experience with an ED80 - you can not make reliable darks with cam on the scope: http://stargazerslounge.com/topic/241143-light-leak-in-skywatcher-evostar-80ed/)

As long as you're not messing up any flat libraries, and the angle of the cam is irrelevant for you (or else remember how you put it on last time) take it off, cap it and make darks.

Regards, Graem

[dph1nm]

I cannot take darks on the scope even with a refractor which has a fully sealed tube (Newts have lots more light leaks) and sealed electric filterwheel. Light does get onto the chip somehow. It is absolutely vital that no light whatever get onto your chip and that means taking off the camera - so do your flats first.

So actually, you could make the argument that in fact you do want the light leaks on the dark, on the grounds that they will be in the lights as well, and so should be subtracted off! This really applies to light leaks coming from somewhere other than the cap on the front aperture of course, and you would probably have to be pointing at the same alt/az as your lights were taken at, otherwise the leaks might be different, so I am not necessarily recommending it ...

NigelM

[jambouk]

If ypu are doing darks in the pitch black, the same as when you capture lights, then yes, do them outside. The issue is many people do darks at twilight when the ambient lighting is different.

James

[cjdawson]

Thanks James, I also think I have issues with my intervalometer and mirror lock up on my 350D. I used the self timer on the camera as learned on the net and it took 30 1min subs, with mirror lock up, no bother at all. But when I set it off doing the darks it threw a wobbler for want of a better term. Sometimes the mirror locked up other times it didnt, the shutter opened but never closed, it just seemed to go haywire. I think my camera is on its last legs....

If the camera's internal timer isn't activated then the camera will appear to go crazy with you use mirror lock with an intervalometer.

Without the intervalometer, the camera normally requires 2 presses of the shutter release.  First press will lock the mirrow, second press will take the photo.

The intervalometer simulates one long press.

So, to avoid the need for the second press, you need to turn on the camera's internal 2 second timer.

That way, you only need one press.  That one press with activate the mirror lock and start the 2 second timer.  When the 2 seconds is up, the camera will activate the shutter and take the photo.  The Intervalometer will then take over and when the button is released, the shutter will close and drop the mirror.

You probably turned off the 2 second timer without realising, so yes, it appears to go crazy.

[ollypenrice]

So actually, you could make the argument that in fact you do want the light leaks on the dark, on the grounds that they will be in the lights as well, and so should be subtracted off! This really applies to light leaks coming from somewhere other than the cap on the front aperture of course, and you would probably have to be pointing at the same alt/az as your lights were taken at, otherwise the leaks might be different, so I am not necessarily recommending it ...

NigelM

If ypu are doing darks in the pitch black, the same as when you capture lights, then yes, do them outside. The issue is many people do darks at twilight when the ambient lighting is different.

James

Not convinced. Darks are not flats. Darks should deal with camera noise and nothing else. If you want to trace and correct imperfections in the light path then take flats in the same circumstances as the lights, but taking darks with some kind of random light leakage not necessarily applicable at the time of the image capture will introduce nothing but noise. Besides, think of the calibration routines used by the stacking software. Darks are subtracted and flats are divided. If you start trying to mix the roles of darks and flats you are going to get into a royal mathematical mess - or so it seems to me.

Take proper darks. Take proper flats. Take lights, darks and flats with no light leakage onto the chip. Then you know what you're doing.

Olly

[ultramol]

I have it sussed now. Because I have to use the 2 second timer on my camera to get the intervalometer to work there is a light on the camera that flashes whilst the timer is on, a very bright light. I wrapped a CLEAN pair of my black jockey shorts around the camera where the T2 adapter goes on the camera and enters the focuser tube. My darks then are truly dark and when used with my lights there is no noise at all on my pictures. This has only worked once as the clouds are back but I am hopeful blacking out the timer light may help.

[jambouk]

There are certain heart sink moments when you see an email arrive, seeing an email from SGL saying "Olly Penrice has quited a post you made" is one of those moments! Like the school master shouting your name

I think I am OK though, as I think what Olly has said supports what I said... I'll probably get another email from SGL shortly and another heart sink moment

James

[jambouk]

Ultramol, were they clean?

[ollypenrice]

I have it sussed now. Because I have to use the 2 second timer on my camera to get the intervalometer to work there is a light on the camera that flashes whilst the timer is on, a very bright light. I wrapped a CLEAN pair of my black jockey shorts around the camera where the T2 adapter goes on the camera and enters the focuser tube. My darks then are truly dark and when used with my lights there is no noise at all on my pictures. This has only worked once as the clouds are back but I am hopeful blacking out the timer light may help.

There was a very long and acrimonious thread on an American forum which concluded that, while black jockey shorts were more light-proof than white ones, soiled shorts were more opaque than clean ones. 

More seriously, do not under-estimate your camera's remarkable ability to record any light landing upon it. I would strongly advise you not to believe that boxer shorts or lens caps will allow you to take meaningful darks. They won't. You need to get that chip very seriously light proofed with a proper cover. That means off the telescope.

Olly

[gnomus]

There was a very long and acrimonious thread on an American forum which concluded that, while black jockey shorts were more light-proof than white ones, soiled shorts were more opaque than clean ones. 

More seriously, do not under-estimate your camera's remarkable ability to record any light landing upon it. I would strongly advise you not to believe that boxer shorts or lens caps will allow you to take meaningful darks. They won't. You need to get that chip very seriously light proofed with a proper cover. That means off the telescope.

Olly

Your posts are normally so comprehensive, so I am disappointed to note that you make no mention of string underwear theory.

[ultramol]

Ultramol, were they clean

ish.................

[ultramol]

I cannot take darks on the scope even with a refractor which has a fully sealed tube (Newts have lots more light leaks) and sealed electric filterwheel. Light does get onto the chip somehow. It is absolutely vital that no light whatever get onto your chip and that means taking off the camera - so do your flats first.

Olly

Hi olly, if I take the camera off and screw a dust cap on the lens opening then start firing off the darks maybe under a cover will that be ok ? and do you need mirror lock up when doing darks or can I omit that to speed the process up, i.e. does camera shake affect darks ?.......Geoff

[Pompey Monkey]

Hi olly, if I take the camera off and screw a dust cap on the lens opening then start firing off the darks maybe under a cover will that be ok ? and do you need mirror lock up when doing darks or can I omit that to speed the process up, i.e. does camera shake affect darks ?.......Geoff

I'm afraid that I'm not Olly, but camera shakes will not affect the darks (unless the pixels are also accelerometers! - just kidding!). Mirror lock-up is irrelevant for the same reason.

Is it OK to use a dust cap and cover the camera? Well yes, if that prevents any light whatsoever from reaching the sensor. The purpose of darks is to capture the build-up of noise from the camera itself, which can then be subtracted from the lights of the same duration.

It is also important to have the darks taken at the same (or close as possible to) temperature as the lights. This is because the noise from the camera is temperature dependent.

HTH.

[gnomus]

Hi olly, if I take the camera off and screw a dust cap on the lens opening then start firing off the darks maybe under a cover will that be ok ? and do you need mirror lock up when doing darks or can I omit that to speed the process up, i.e. does camera shake affect darks ?.......Geoff

I think that using a dust cap is the way most folks do this.  And no you don't need to worry about camera shake.

[ultramol]

Thats great. I can now remove the camera and get on with taking everything back in the house whilst my darks are being taken. I think I will make a dedicated dark box. Many thanks for your input.