Quick Deepskystacker question

[aqualand]

Hi all,

Previously i was using around 20 bias frames for the 1100d.  I then read a few posts that suggested I needed a 'few' more so recently generated a new bias library of 200.

Stacking with 20 frames wasn't much of a problem but now i'm thinking that once DSS has generated the master bias for the 200 after their first use, I can use that single master in future.

Is my reasoning correct?

Thanks

Garry

[wxsatuser]

If you use dark frames then you don't need bias as the bias is in the dark frames.

Bias may be needed if you want to make a scalable master dark.

Bias should be at the same ISO and temperature as the lights.

[aqualand]

I'm confused now as i thought bias weren't temp dependant? They are at the same iso though.

For info i'm not using darks as read that they can cause more problems with dslr as not being at a set temp so im trying dithering at the minute.

Thanks for the info.

[wxsatuser]

You will get a lot of conflicting ideas but bias and darks same iso and temp.

Flats it does matter.

The main thing is get lots of lights and preferably long exposures, the longer the better

[aqualand]

Great thanks, guess I need to start taking my bias at the end of the night.  At least they are quick to take.

Thanks again wxsatuser

[jgs001]

Garry, you are correct... once you have stacked the Master Bias the once, you do not need to stack each time. Just load the Master Bias that was created  into future stacks.

[dph1nm]

No, the bias does not have to be done at the same temperature. They probably don't have to be done at the same ISO,  but that might be a camera dependent statement, so I would stick to keeping the ISO the same as the lights.

NigelM

[lensman57]

Hi all,

Previously i was using around 20 bias frames for the 1100d.  I then read a few posts that suggested I needed a 'few' more so recently generated a new bias library of 200.

Stacking with 20 frames wasn't much of a problem but now i'm thinking that once DSS has generated the master bias for the 200 after their first use, I can use that single master in future.

Is my reasoning correct?

Thanks

Garry

I am not surprised that you are confused. But first is first. Yes you can use the master bias for future but it is best to do a master bias each session as it really does not take much effort.

I am not sure how many types of  calibration frames you are using at the moment. These are usually Bias, Flat, and Dark. To put your mind at rest Bias is always subtracted from all calibration frames and the lights. This is the only way to make sure that the best calibration is performed regardless of what type of frames are at hand. Most people with cooled CCDs do not bother to take Darks. If you did take Darks and no Bias your lights will be correctly Bias subtracted by the Darks but your Flats will not be correctly subtracted as you need Bias. You can ditch the Dark often with no ill effects but Bias is essential, this way your Flats will be Bias subtracted and a master Flat made, your lights will be Bias subtracted and then divided by Master Flat and then stacked to give you the Master Light frame so Darks play no role in this. Hope that find this useful. BTW a Master Bias made of 200 Bias frames is ideal, for flats you need a Minimum of 25 I do 50~100. The aim of all this is to prevent calibration frames inducing noise in the final stack so the more of them the cleaner the Master calibration frame.

A.G

[aqualand]

I generally take about 40 flats and ill start taking bias as part of my imaging session as they dont take long, not sure i'll take 200 per session though. No darks as dithering setup between apt and phd.

Thanks everyone for the input, all very helpfull.

Garry

[wxsatuser]

Bias frames from my 60Da, these have just been stretched slightly in PS.

Did ISO 400 to 3200, the first frames were taken from cold, sensor 15C.

Then I did the same ISOs but after doing 4 x 300sec darks to warm the sensor, sensor now 24C.

400@15C

400@24C

800@15C

800@24C

1600@15C

1600@24C

3200@15C

3200@24C

[almcl]

apologies for the hi-jack, but Mike where do you get the sensor temp info from? Is it built into the Canon exif somewhere, or something else entirely?

[kalasinman]

OK, so what I get from this is that for DSLR users, lights, darks, bias, and flats are the way to go. Interesting that wxsatuser has clearly shown evidence countering the instructions given in the help files for DSS regarding bias frames. Bias frames do not account for thermal noise, so for uncooled camera users, darks make sense.

[wxsatuser]

apologies for the hi-jack, but Mike where do you get the sensor temp info from? Is it built into the Canon exif somewhere, or something else entirely?

Yes, you just need something to view it.

I use EXIFLOG or Irfanview.......EXIFLOG free and Irfanview a donation if you want.

[wxsatuser]

OK, so what I get from this is that for DSLR users, lights, darks, bias, and flats are the way to go. Interesting that wxsatuser has clearly shown evidence countering the instructions given in the help files for DSS regarding bias frames. Bias frames do not account for thermal noise, so for uncooled camera users, darks make sense.

Must honest I don't know which is the best way to go.

Jerry Lodriguss in his book points out you don't need bias if you use darks.

You do need bias if you want to scale a master dark plus you will need them for flats.

He does say that bias ought to be done at the same ISO and temperature.

If anyone can prove one way or another what is correct I would go along with that.

Scott Rosen, who does incredible dslr images uses darks, flats and bias, so figure this is the way to go.

[dph1nm]

Bias frames from my 60Da, these have just been stretched slightly in PS.

So looking at those, I would say that the horizontal banding is different for every bias, but the vertical banding is similar (although maybe not identical) at the same ISO, but different at different ISOs.

If it wasn't for that vertical banding, you could probably replace bias frames by simply subtracting a constant value from every pixel (in fact when CCDs first appeared in professional astro that is what we did).

NigelM

[jgs001]

My process is typically to load a master bias, all the flats, and a single light (ignoring darks at this point) into the DSS master group, and stack. With bias and flats you need to set the black point to 0 in the raw/fits settings. Once complete, grab the master flat, and ditch the resulting image.

Then, unsetting the black point setting, load the lights, darks and master flat (but not bias), and stack.

For me, this proess produces the best results.If I don't do this, I get some very odd effects and a heavy green tinge to the entire image. I don't know whether this is down to the bias being subtracted from everything including the darks and this is causing a problem or what.

[lensman57]

My process is typically to load a master bias, all the flats, and a single light (ignoring darks at this point) into the DSS master group, and stack. With bias and flats you need to set the black point to 0 in the raw/fits settings. Once complete, grab the master flat, and ditch the resulting image.

Then, unsetting the black point setting, load the lights, darks and master flat (but not bias), and stack.

For me, this proess produces the best results.If I don't do this, I get some very odd effects and a heavy green tinge to the entire image. I don't know whether this is down to the bias being subtracted from everything including the darks and this is causing a problem or what.

This is fine, it does work. But what doesn't work is not taking Bias and relying on darks only in which case Flats can not be calibrated. You can ditch the Bias only if you take what is called dark Flats which are subtracted from  the normal Flats to remove thermal noise if the Flat duration is long, a sky Flat, and then the Darks will bias subtract the Lights. To me it is a lot easier just to take Bias, Flat, and Dark if need be. For DSLR imaging unless the temperature of the Darks can be guaranteed to be within a couple of degrees C of the Lights then more damage can be done by applying mismatched Darks. A cooled CCD such as an Atik 383L with the KAF 8300 sensor ( this one can benefit from having Darks applied beyond 900s subs ) can use Darks because of its very accurate setpoint cooling but not a DSLR with its varying sensor temp during imaging. Please have a look at the chart below which was compiled by Mr Garry Honnis.

Regards,

A.G