Flat Wizard & Bias Frame Process Issue

Somewhere to ask questions about the best way to use SharpCap
Forum rules


If you have a problem or question, please check the FAQ to see if it already has an answer : https://www.sharpcap.co.uk/sharpcap-faqs
Post Reply
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Flat Wizard & Bias Frame Process Issue

#1

Post by GaryS »

Hi

Several of you read the confusion I had when SC displayed "...capturing Flat Darks..." when it was/is really capturing Bias Frames to calibrate the Flats. I continue to be thrown-off a bit whenever I see this but I realize it actually means to say 'Bias'.

I noted today another puzzle when I realizde that SC was making Bias Frames (as part of the Flats process) without moving the filter wheel to the opaque position. I feel I must be missing something in my understanding of how SC preps the bias frames but doesn't capturing Bias frames require blocking all light to the sensor? What am I forgetting here?
Gary
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Flat Wizard & Bias Frame Process Issue

#2

Post by admin »

Hi Gary,

when you are making flat frames, cutting out the light doesn't really matter. A much shorter exposure (for instance 1ms for the bias against maybe 100ms for the flat) is sufficient.

Imagine it like this...

Flat with a no light bias:

(A) 100ms frame = bias level + 0.100 * [1s exposure brightness]
(B) bias frame = bias level

subtract (B) from (A) and you get the flat frame which is 0.100 * [1s exposure brightness] with the bias level removed. This will work nicely as a flat (providing you use darks when capturing...)

Flat with a short exposure 'bias'

(A) 100ms frame = bias level + 0.100 * [1s exposure brightness]
(B) 1ms frame = bias level + 0.001 * [1s exposure brightness]

subtract (B) from (A) this time and again the bias level cancels out, but you get 0.099 * [1s exposure brightness]. Once again this will work nicely as a flat (basically it is the flat above with all the values reduced by 1%).

The advantage of the second approach is that it doesn't require a filter wheel, or manually covering the scope during capture - it can just get on with the capture and work :)


For the bit about 'darkflat' vs 'bias', I think you are referring to the filenames appearing in the title bar during capture of the individual flat/dark frames - I keep meaning to hide those :)

cheers,

Robin
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Re: Flat Wizard & Bias Frame Process Issue

#3

Post by GaryS »

Thank you for that nice explanation Robin.

I guess I’m understanding that the method SC uses to create Bias frames is a tiny ‘cheat’ that allows SC to automate the Flat- making process. A 1% cheat doesn’t sound like a big price to pay for the convenience during EAA observing. None the less, the AP folks, including Guru’s like Charles Bracken, all declare Bias frames to be “no light” frames. Perhaps is just that with AP, all the calibration frames are being done separately and the benefit of 100% ‘no light’ Bias frames is no added effort.

It does seem though that, with your Sequencer and new ‘framing shot’ option, SC is becoming more tailored to AP… and yet, SC does not offer the individual Bias, Dark or Flat frames to the user - something the AP community would probably want(?). To folks like me doing variable star and exoplanet transit photometry, the sequencer and framing tools are very useful but the loss of even 1% effectiveness in the Flats would generally be considered unacceptable. If the use of SC’s Flat Darks does require the user to cover the OTA first, that might give comfort to those grasping for detection of 0.001 magnitudes.

Thank you for the response and all that you’ve done and for the observing and science community.
Gary
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Flat Wizard & Bias Frame Process Issue

#4

Post by admin »

Hi Gary,

my advice is always that if you are doing EAA then use Capture Flat/Capture Dark as they are great shortcuts. However, if you are doing traditional AP, it's much safer to capture all images (light, dark, flat, dark flat) in the sequencer and process them in PI or DSS or whatever your favourite tool is.

The reason is that there are too many subtleties about how the master dark/flat processing could happen to be able to ensure that whatever SharpCap does matches what the processing application expects. Also, if you use the master dark/flat created by SharpCap then there is all sorts of scope for user error - if you add bias frames into the processing application and it doesn't know your flat is a master flat, it might decide to use the bias frames to process the flat, which would account for the bias twice, etc.

cheers,

Robin
GaryS
Posts: 175
Joined: Sun Mar 31, 2019 5:18 pm
Location: Boston, USA/Jackson Hole, USA

Re: Flat Wizard & Bias Frame Process Issue

#5

Post by GaryS »

Oh, I didn’t realize the SC Sequencer can capture individual darks, bias and flat frames. Guess I need to dig into the manual on that. This would make SC a close equivalent to NINA - everything but the calibration itself - thank you.
Gary
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Flat Wizard & Bias Frame Process Issue

#6

Post by admin »

Hi Gary,

if you are using the sequence planner, then you do it in the capture list - see https://docs.sharpcap.co.uk/4.0/#The%20Capture%20List

If using the more powerful sequence editor then there is a step to set the frame type.

cheers,

Robin
Post Reply