Yet another Flats question! :)

[CarlGreen]

Hello everyone, please don't laugh at this silly question, when doing imaging (Solar or Planetary or DSO - But this question mainly relates to Solar)
Lets say I am imaging a particular prominense on the Sun............
- If I have achieved focus and am happy with my framing, I then want to take Images (say a 30 second video) - I press the button and away it goes and captures the video.........Do I then take another 30 second video (or capture 200 frames ish) of a Flat (I will be using a breakfast cereal inner packet fixed to the front of my Scope ) to get the Flats needed (I would probably use the Sharpcap feature to capture the flats)
- If I then skew to a DIFFERENT prominance 9Or indeed start douing parts of the sun to ccreate a Mosaic) Do I have to take more flats?
I understand that I have to take them if I move the camera orienation (in the scope) but don't quite understand when I should be taking them.
Is it as simple as take flats for an imaging session once only unless you move the scope to a differnt location, or orientate the camera differently, but If I slewed to a different part of the sun (or indeed to a different planet (assuming it was night ) then the first set of flats will do?
Any pointers appreciated

Thanks and Clear Skies (Be that Day or Night!)

[admin]

Hi,

for solar flats, you should take them with the telescope pointing so that you are looking at the middle of the sun (sun filling frame without the flat cover attached). Usually they should be good for a whole session - until you move the camera relative to the telescope by taking it off/rotating it/making a big focus change. If you are using something like a Daystar Quark then the flat will correct for the kind of blotchiness you can sometimes get in the image - as the quark temperature changes the blotchiness may change requiring a new flat to be captured.

Personally, I capture my flats first using the SharpCap 'Capture Flat' option - maybe 30 to 50 frames, with bias frames subtracted. SharpCap can then auto apply this flat to every subsequent camera frame, so not only do you see the flat corrected image on screen, but it is also saved to file removing the need to flat process later.

cheers,

Robin

[CarlGreen]

Thanks Robin

"for solar flats, you should take them with the telescope pointing so that you are looking at the middle of the sun (sun filling frame without the flat cover attached). "

So Assuming I cannot get the whole disk in one shot, do I select a ROI that fills my screen with sun (effectievly zoom in on a bit so the whole capture area is filled with Sun) Am I interpretuing that correctly?
I would then put my Flat Cover on and then use the sharpcap Capture Flat option. .................
What is the Bias subtracted? Does using the Sharpcap flats tool do this automatically? Or is this another thing I need to do ?
(Thanks for your help. (Is there any you tube tutorials you know of on this?)

[admin]

Hi,

not sure about the procedure for a situation where the disc does not fill the frame - using an ROI will not work as it only gives you a flat frame for that ROI. I think in that case you would need to use something more opaque than transulcent film - a white T shirt or white piece of paper - to get a properly even illumination. That will, in turn, require longer exposures for the flats becauset the light will be dimmer, but even.

No need to worry about bias other than ticking the box when using SharpCap's capture flats tool - SharpCap will capture and subtract the flats, then record the bias level so that when the flat is used the same bias level is subtracted from the light frames. Just don't change the brightness/offset/black level control between capturing flats and capturing lights.

cheers,

Robin

[Jean-Francois]

Hello,

For more scrambling, you can use the same diffusor foil a second time after first layer.
Important is the distance between the two foils.
Larger distance produces more scrambling, but it reduces the light intensity.

You can experiment this with the computer monitor.
Place the foil directly on the monitor surface and you can continue to read some text on the monitor.
Move the foil away the monitor surface and you will remark that the contrast and details will be reduced or completely disappear.
Now you can add a second foil and test with different distances.
My starting guess is to have the foil distance equal the diameter of the telescope.

You can use a tube with both foil with in addition some aluminium or white paper on the inside wall of the tube.
That will redirect more photons to the second foil.

Alternative ... you build a 45° holder in front of your telescope tube and you place a white paper on it (ideally you can procure a Spectralon plate).
Rotate the mount so that the sun illuminates the white paper (note that the telescope may point to the ground).
You will need to experiment and compare the exposure time between both methods.

The "paper" method will destruct all the angle decency ... means you will be sure that the field will be illuminated homogenously.
But I guess with a longer exposure time as the foil method (but it depends on the foil diffuser and absorption characteristics).

Regards,
Jean-Francois