Dark frames

[gonzostar]

Hi 

This may sound a silly question, and is! How is it best to collect dark frames, and how many? I have a canon 450D DSLR camera attatched to a celestron 8" SCT (using a focal reducer)  on a AVX mount. 

I was planning to collect several 2min images of the andromeda galaxy

Ta

Dean

[ollypenrice]

In your case the key thing is to get the darks at the same temp as the image. You also need to exclude all light and I don't find lens caps do this, so off the scope may be best.

What you should certainly do is dither the scope, manually if necessary. That is, shift it by at least a dozen pixels between exposures. This has a prodigious effect on most forms of DSLR noise.

Olly

[gnomus]

I am a beginner too, so take my advice with a pinch of whatever takes your fancy.  I normally aim for around 25 dark frames.  As Olly says, if you can dither your light frames, then do so.  When your stacking software aligns all your dithered images the fixed noise is not stacked on top of itself (because it has been moved a dozen or so pixels).  As such it tends to get 'dialled down' somewhat in the final image (or at least that is how I understand it).  

When I was using my Canon, I would control the camera via APT -  http://www.ideiki.com/astro/ it is a no brainer cost-wise.  The program allows the option of dithering.  There is also something called Backyard EOS.  This gets much praise but I have never tried it so cannot comment.

[gonzostar]

Thanks guys for your feed back. 25 dark frames for how long is your exposure time for your light frames. Also what are flat frames? Never heard the term dither before Must give that a try.

Ta

 Dean

[Bizibilder]

If you are taking 2 minute lights then you should record the ISO and the temperature (nearest 5 degrees will do) as well.

Flat frames (more or less essential) are taken while your imaging rig is still set up and without touching anything including the focus.  You need an even light source - some wait till daylight and take their flats with a t-shirt over the end of the scope (or take flats first before it goes dark) or use a light panel (I use one of these: http://www.amazon.co.uk/gp/product/B00J3NRAV2/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A23HHFRIW02ZZW ) and simply point the scope to the zenith and put the panel over the end of the scope). If you set your DSLR to Av mode it will set the correct exposure time for you.  Take about 25-40 flats.  You must take new flats at every session as they are a record of your optical train, including any vignetting and dust bunnies etc.

Darks:  These need to be at the same ISO and temperature (within a few degrees) as your lights and the same exposure time.  I take mine in the garage or observatory on cloudy nights so as not to waste valuable imaging time.  Make sure the camera has the cap on and that no light can leak in - if necessary put the whole thing inside a closed cardboard box as well!  I use a timer for darks but there is no reason why you can't use your imaging computer.  Take about 40 and keep them as you can build up a library of darks and reuse them.  I have to admit that I often find that darks actually add to the noise in my own DSLR images and so I often process without them.

Bias:  If you want them are easy - again same ISO as your lights but this time set the exposure to the shortest value that you can (in my case 1/4000 sec).  Temperature is unimportant and you can run off maybe 40-60.  These too can be kept for future use but are so quick to do that I often take a new set.  I think I'm right in saying that if you don't use darks you really do need to use bias frames.  However, if you forget to take them you can use an old set successfully.

[gnomus]

Thanks guys for your feed back. 25 dark frames for how long is your exposure time for your light frames. Also what are flat frames? Never heard the term dither before Must give that a try.

Ta

 Dean

I believe that you can dither 'manually', that is using the handset, but I've never tried that and it sounds like a right royal ......

Ideally, you should take darks as near to the time of your lights as possible, with same ISO and exposure time. If you do them later, you cannot be sure that the temperature in your garage is the same as it was out in the cold. It almost certainly isn't. (One of the advantages of a cooled CCD camera is that you can do them anytime with the chip at the same temperature as you set it on the night.). You can take the camera off the scope and put the lens cap on (as Olly suggests). I would then spend some time doing visual observing whilst the darks run their course.

Whisper: Some folks think that if you dither you might not need to do darks at all!!!

I think that Flats are more important than darks. The advice you have received already is sound.

Bias frames can be done any time. Shoot off 100 or so frames using the fastest speed possible. Do them once and they can be reused over and over - essentially they measure the base signal that your camera produces.

On a typical night, therefore, your workflow might be:

1) Shoot your lights

2) Shoot your flat frames (again 25 or so should do)

3) Shoot your dark frames with camera off scope and body cap on.

4) Take bias frames whenever is convenient

There is a book that everyone recommends called Making Every Photon Count. Its available from FLO and the author regularly visits this forum (and seems to be a good guy).

Good luck

Steve

[pshanahan]

Hi folks,

Just asking, do you then use deep sky stacker? To put all of the frames together?

Thanks

Shanahan

[gnomus]

Hi folks,

Just asking, do you then use deep sky stacker? To put all of the frames together?

Thanks

Shanahan

There are several ways. DSS is one of many programs, though it's not the one I use and I cannot comment on it. Most programs seem to allow a reasonable trial period so you can try a few out. You can do it just with Photoshop I believe, but how you go about this I'm not sure.

[gonzostar]

If you are taking 2 minute lights then you should record the ISO and the temperature (nearest 5 degrees will do) as well.

Flat frames (more or less essential) are taken while your imaging rig is still set up and without touching anything including the focus.  You need an even light source - some wait till daylight and take their flats with a t-shirt over the end of the scope (or take flats first before it goes dark) or use a light panel (I use one of these: http://www.amazon.co.uk/gp/product/B00J3NRAV2/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A23HHFRIW02ZZW ) and simply point the scope to the zenith and put the panel over the end of the scope). If you set your DSLR to Av mode it will set the correct exposure time for you.  Take about 25-40 flats.  You must take new flats at every session as they are a record of your optical train, including any vignetting and dust bunnies etc.

Darks:  These need to be at the same ISO and temperature (within a few degrees) as your lights and the same exposure time.  I take mine in the garage or observatory on cloudy nights so as not to waste valuable imaging time.  Make sure the camera has the cap on and that no light can leak in - if necessary put the whole thing inside a closed cardboard box as well!  I use a timer for darks but there is no reason why you can't use your imaging computer.  Take about 40 and keep them as you can build up a library of darks and reuse them.  I have to admit that I often find that darks actually add to the noise in my own DSLR images and so I often process without them.

Bias:  If you want them are easy - again same ISO as your lights but this time set the exposure to the shortest value that you can (in my case 1/4000 sec).  Temperature is unimportant and you can run off maybe 40-60.  These too can be kept for future use but are so quick to do that I often take a new set.  I think I'm right in saying that if you don't use darks you really do need to use bias frames.  However, if you forget to take them you can use an old set successfully.

Very imforative thanks for your information. With flat frames do you delibratley put the scope out of focus and with the same iso as lights?

[gonzostar]

There are several ways. DSS is one of many programs, though it's not the one I use and I cannot comment on it. Most programs seem to allow a reasonable trial period so you can try a few out. You can do it just with Photoshop I believe, but how you go about this I'm not sure.

Which photshop do you use?

[gonzostar]

I believe that you can dither 'manually', that is using the handset, but I've never tried that and it sounds like a right royal ......

Ideally, you should take darks as near to the time of your lights as possible, with same ISO and exposure time. If you do them later, you cannot be sure that the temperature in your garage is the same as it was out in the cold. It almost certainly isn't. (One of the advantages of a cooled CCD camera is that you can do them anytime with the chip at the same temperature as you set it on the night.). You can take the camera off the scope and put the lens cap on (as Olly suggests). I would then spend some time doing visual observing whilst the darks run their course.

Whisper: Some folks think that if you dither you might not need to do darks at all!!!

I think that Flats are more important than darks. The advice you have received already is sound.

Bias frames can be done any time. Shoot off 100 or so frames using the fastest speed possible. Do them once and they can be reused over and over - essentially they measure the base signal that your camera produces.

On a typical night, therefore, your workflow might be:

1) Shoot your lights

2) Shoot your flat frames (again 25 or so should do)

3) Shoot your dark frames with camera off scope and body cap on.

4) Take bias frames whenever is convenient

There is a book that everyone recommends called Making Every Photon Count. Its available from FLO and the author regularly visits this forum (and seems to be a good guy).

Good luck

Steve

Thanks Steve. Very good information. I have copied your todo list on the board in the shed!

[gnomus]

Very imforative thanks for your information. With flat frames do you delibratley put the scope out of focus and with the same iso as lights?

Do not put the scope out of focus. It is imperative that the flats are taken with the imaging train set exactly as you took the lights. That is why it is best to do them straight away.

The 'flats-out-of-focus' technique is one that is used when shooting solar images. How else would you get a solar flat?

Oh... And just to be absolutely clear, you need specialised filters before taking solar images... Please don't point your scope, camera, eyes, or anything else at the sun. I know you already know this....

[gnomus]

Which photshop do you use?

I wouldn't suggest trying to do this in Photoshop, I was only saying that I think this is possible. Try the various options. Some are quite cheap to buy after the trial period is over. I started out playing with Nebulosity, but now use PixInsight (which is one of the dearer options). Plenty of people use DSS and would be able to help if that is what you have.

[gonzostar]

Hi folks,

Just asking, do you then use deep sky stacker? To put all of the frames together?

Thanks

Shanahan

Yes i am usig DSS. However reading the above posts it sounds like i need to do some flats and more darks! Good at getting light frames though Can yo do the darks just before you set the scope up on that evening. Also is it worth doing darks at the end of the evening to?

[Bizibilder]

Do the darks any time you like as there should ne no light entering the camera.  All you need to do is make sure that you use darks that have been taken at the same exposure time,ISO and temperature as the lights you have taken.

Use DSS for stacking - just put all your frames in and run it with the default settings to start with - there are several tutorials around on the forum and the web.  Photoshop (or the freeware GIMP) are really for processing a stacked image rather than for doing the stacking itself.

[pshanahan]

Roger,

A question from me. After 30 second exposures it's all bulb. So, what if you are off by a few seconds in a 2 minute exp?

Respectfully,

Shanahan

[gnomus]

Roger,

A question from me. After 30 second exposures it's all bulb. So, what if you are off by a few seconds in a 2 minute exp?

Respectfully,

Shanahan

I don't suppose that it does, but most folks will control their cameras either with software or an 'intervalometer' (which you can get for a few quid from places like from eBay).  Might as well get it right if it's no effort to do so.  Having to time each exposure yourself seems to me to be making things harder than they need to be.

[Bizibilder]

You may also find that the stacking software will refuse to stack images within a batch if they have different exposure times - When I first started out I did take longer subs subs manually (with a remote shutter release) but it quickly becomes a real pain and I soon bought a timer!

[gonzostar]

Also what setting are you using in the DSLR camera, do i need to shoot in raw, jpeg or combo?

[PeterCPC]

You want RAW images for stacking but you might also want jpgs to be able to see what each frame is like if you can't view RAWs easily on your computer. So RAW+L maybe.

Peter

[gonzostar]

Okay will try that. How do you download pics onto this website. I would like to show people where i am going wrong! 

[PeterCPC]

At the bottom right you see "More Reply Options" - click that and you can select jpgs pr pngs to upload. You can't upload TIFF files or RAW files.

Peter

[ollypenrice]

Dithering manually is not a pain at all and is likely to be far more effective than using badly temperature matched darks. Take a few subs under autoguidng. Stop the guider, nudge the mount using the handset just a bit in one direction, re-launch the autoguider and take a few more subs. (There will be no need to recalibrate the autoguider. Nothing has changed.) A few subs later repeat the prcocess but nudge in a different direction. And so on. We could all get terribly excited about the precise maths of dithering but, on the offchance that you live in the real world, just give this bearable method a whizz and see what you think...

Olly

[gnomus]

Presumably you would need to choose a different guide star using this method, else you would simply end up back in the same position?

[gonzostar]

Dithering manually is not a pain at all and is likely to be far more effective than using badly temperature matched darks. Take a few subs under autoguidng. Stop the guider, nudge the mount using the handset just a bit in one direction, re-launch the autoguider and take a few more subs. (There will be no need to recalibrate the autoguider. Nothing has changed.) A few subs later repeat the prcocess but nudge in a different direction. And so on. We could all get terribly excited about the precise maths of dithering but, on the offchance that you live in the real world, just give this bearable method a whizz and see what you think...

Olly

Ok so if i wanted to take a picture of M31. How many darks, lights would i take at 1, 2 minute exposures? When you sat nudge the mount with handset what is a nudge?