Darks, Flats & Bias question

[ArmyAirForce]

I've seen many images on the forum where people state the equipment they use, followed by X number of Darks, Flats and Bias frames. The quantities of each are always the same so far as I've seen, ie 15 Darks, 15 Flats and 15 Bias etc. Do they always have to be equal for a good image, or does more or less of one type affect the final processed image?

Thanks.

[johnrt]

No, they don't need to be equal. The same rules as shooting lights applies when making calibration masters - you want to build up enough images to rid them of random noise and get the noise readout from the camera in the case of darks & bias.

Darks are temperature matched to the lights and should be of equal exposure time - I'd shoot upwards of 20 to combine to a master dark.

Bias is the minimum exposure time your camera will allow and are not temperature matched. I think I shot 100 to make my last bias master frame.

Flats are to correct uneven illumination and remove dust bunnies and imperfections, exposure time is dependant on the camera sensitivity, I shoot around 50 to make my masters.

Hope that helps.

[ArmyAirForce]

Cheers. I knew what they all did and have been tinkering with them since I got my scope in February, but wasn't sure if equal numbers was important.

Now what I don't know is Dark Flats. What are they and what do they do?

[johnrt]

Dark flat is a dark frame of equal exposure time to a flat frame - you can then subtract from the flat frame to reduce noise in the flat before you calibrate your lights. I'm not sure if they're widely used, I don't make them!

[ArmyAirForce]

Thanks.

[ianaiken]

Hi Stephen, when I was imaging with my Canon 40D I took 100 Bias (probably over kill) but I only needed to take these every 6 - 12 months. You probably only need dark, flat and bias and just throw these into DSS. I tried using dark flats and found that it made no difference with my dSLR (and with my CCDs). I used to aim for 40 subs for each on my dSLR and for my CCD I aim for ~20 each (I don't use darks now on CCD just flats and bias)

[ArmyAirForce]

I wish there was an easy way to do darks. My Nikon D50 can only do up to 30 second exposures using its own memory, then the bulb setting for longer. I don't think there are any plug in gadgets to control the shutter for this camera. As a result, all my images and darks are timed with the stopwatch on my phone! Last thing I want to do at the end of the night is take 40 x 2 minute exposures by hand!

[ianaiken]

Ouch! Nikon do have remote capture software available but its pricey and I believe there are some cheaper software options available too, but I don't think any of them support the D50. Cheapest option is to invest in a second hand modified Canon. It would make sense the amount of images your banging out

[rfdesigner]

Dark flat is a dark frame of equal exposure time to a flat frame - you can then subtract from the flat frame to reduce noise in the flat before you calibrate your lights. I'm not sure if they're widely used, I don't make them!

Almost.. it's not really noise you're trying to correct here.

The flat frame needs to be 'zero-corrected'.

If you just divide by the flat frame, there will be a slight error. Imagine just for a moment that you have horrible vignetting so the corner of your flat has a signal level of 100 and the centre has a value of 10000. Now assume that a dark frame equal in length to the light frame would have a level of 100.

So in the corner of your flat you have a value of 200 and in the centre you have 10100.

When you flat field you'd get an error of 1% in the middle and an error of 50% out at the edge.

By subtracting the bias frame from the flat the result is just the sensitivity of the camera.

Hope this helps

Derek

[ArmyAirForce]

....... Cheapest option is to invest in a second hand modified Canon.....

I know, but the piggy bank is a bit low right now, and there's a guide scope etc to buy yet too!!

[ianaiken]

Actually this looks promising http://www.diyphotobits.com/download-diyphotobitscom-camera-control/ if you can connect up to a PC.

[ArmyAirForce]

I've just downloaded the program, but it won't let me set the exposure time, other than up to 30 seconds like in the camera memory. There's no 'Bulb' setting or option to set a longer exposure. :-(

[ianaiken]

not good. This next statement won't be of any use, but the classified section in this forum has a modified Canon 40D for sale. A 40D is a very nice camera and it's environmentally sealed.

Edit: Cause I'm tired and I don't make sense to non mackems

[Shibby]

When I used a D70 for my astrophotography, I purchased an AM3 intervalometer, one of these: http://www.astrointervalometers.com I don't know if Javier still makes them, but you might consider contacting him.

It has 3 uses

- Automated image runs of any exposure length

- Allows true RAW images to be captured. Without one of these, the older Nikon cameras apply a nasty, "star-eater" median filter, even in "raw" mode.

- Replaces the battery!

Of course, a Canon DSLR is preferable but this is cheaper...

[ArmyAirForce]

I've been looking at this too - http://www.opticstar.com/Run/Astronomy/Astro-Accessories-Imagers-Shoestring.asp?p=0_10_5_0_5_21

[ArmyAirForce]

.....

Edit: Cause I'm tired and I don't make sense to non mackems

I lived most of my life in Mackem land, but was born a Sand Dancer.

[ollypenrice]

I don't use DSS and can only contribute on the subject of darks for flats. All you need is a master bias. Tell your stacking software that your rmaster bias is a dark-for-flats and you'll be fine. You do need to do this, though, or your flats will not correct correctly...

Olly

[SteveA]

I wish there was an easy way to do darks. My Nikon D50 can only do up to 30 second exposures using its own memory, then the bulb setting for longer. I don't think there are any plug in gadgets to control the shutter for this camera. As a result, all my images and darks are timed with the stopwatch on my phone! Last thing I want to do at the end of the night is take 40 x 2 minute exposures by hand!

You don't need to take your darks immediately following an imaging session....

What a lot of people do is compile a "library" of darks at various exposure times and then use these to calibrate lights of the same duration. The darks library only needs updating every few months or so...if that. Admitedly though this is mainly aimed at the cooled CCD users where its realatively easy to duplicate the sensor temperature, but I would have thought that it would be possible to adopt a similar strategy with a DSLR as long as you could address the temperature issue for when you create your darks.

[ArmyAirForce]

I have the Shoestring DSUSB-IR interface for my camera and laptop now. Taking multiple long exposures is no longer a problem.