Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Flats for full disk.

  • Please log in to reply
7 replies to this topic

#1 davidpitre

davidpitre

    Aurora

  • *****
  • topic starter
  • Posts: 4,563
  • Joined: 10 May 2005
  • Loc: Central Texas

Posted 08 August 2020 - 12:21 PM

Can someone clarify for me that I can get good flats for full disk imaging using a cereal bag even though the blurred image of the sun does not fill the frame? I still have a perimeter of black sky using the bag. Does the entire flat frame need to be illuminated with diffuse light?

#2 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,656
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 08 August 2020 - 01:42 PM

Hi David,

 

A diffuser (opaque bag that scatters light) will diffuse light via scatter enough to iluminate the field beyond the hard edge of the disc. It needs to be placed a little farther away from the objective (such as using it on the lens hood) to ensure there's enough coverage to go past the limb of the disc. You do not need to fill the FOV with it. Only enough to cover the entire disc with a small overlap around the edge (enough overlap to not lose proms). Everything past that, no matter how wonky looking, is just void anyways. The diffuser will soak up a lot of transmission, so remember, the settings don't matter, the histogram does. So target something 65~72% or so as a histogram fill with the bag (any exposure time is fine, the histogram is all that matters); so it will take a long exposure time to get the histogram up.

 

Very best,


  • PhotonJohn likes this

#3 davidpitre

davidpitre

    Aurora

  • *****
  • topic starter
  • Posts: 4,563
  • Joined: 10 May 2005
  • Loc: Central Texas

Posted 08 August 2020 - 05:11 PM

Got it. 
So even if the histogram for my images were around 90, it’s ok to set the flat frames at 65-70? 
Thanks  as always Marty


Edited by davidpitre, 08 August 2020 - 05:14 PM.

  • MalVeauX likes this

#4 ASTROTRUCK

ASTROTRUCK

    Viking 1

  • -----
  • Posts: 602
  • Joined: 02 Jan 2010

Posted 09 August 2020 - 12:14 PM

how diffuse is diffuse - i am trying ziplock freezer bags multi layered held up to a energy saving light bulb - am adding layers until i see no shape of the light bulb -

is that too diffuse ?  likely that would block out the sunspots  but normally would then defocus - but if too diffuse likely would not need to defocus since dont see the sunspots -  so if no need to defocus the sunspots  is that too diffuse ?

 

is a layer of waxed paper too diffuse ?

 

tried firecapture artificial flats didnt work for me -  i use ic capture with imaging source cant work with fire capture.

 

need help with newton rings tilt adapters too pricey for me since its the pricey $800 one that works better



#5 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,656
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 09 August 2020 - 01:10 PM

Got it. 
So even if the histogram for my images were around 90, it’s ok to set the flat frames at 65-70? 
Thanks  as always Marty

Try to match histograms, roughly. Your histogram near 90% filled will likely clip data after you sharpen. I would back it down closer to 70~80%. When you make your flat, try to target that 65~70% area. If your flat and image's histograms are very different you might not get the flattening affect and get strange gradients and see dust motes, etc.

 

how diffuse is diffuse - i am trying ziplock freezer bags multi layered held up to a energy saving light bulb - am adding layers until i see no shape of the light bulb -

is that too diffuse ?  likely that would block out the sunspots  but normally would then defocus - but if too diffuse likely would not need to defocus since dont see the sunspots -  so if no need to defocus the sunspots  is that too diffuse ?

 

is a layer of waxed paper too diffuse ?

 

tried firecapture artificial flats didnt work for me -  i use ic capture with imaging source cant work with fire capture.

 

need help with newton rings tilt adapters too pricey for me since its the pricey $800 one that works better

It takes experimentation, I wish I could put a metric on it. I found the best results from an opaque cereal bag. I've tried produce bags from vegetables and plastic bags and trash bags without success. But a cereal bag did it perfectly.

 

Very best,



#6 ASTROTRUCK

ASTROTRUCK

    Viking 1

  • -----
  • Posts: 602
  • Joined: 02 Jan 2010

Posted 09 August 2020 - 02:42 PM

ok but just as a measurement  is the cereal bag as opaque as waxed paper or would waxed paper be too opaque ?

 

my full disk has the bright spot right in the middle so i need to reduce exposure but loose edge detail so will try bags.

 

power mate for sunspots  or prominences is a disaster so flats are next

 

 

thanks - next life i try basket weaving

 

STAY SAFE GUYS - THIS COVID  IS THE NEW MILLENNIUM - 



#7 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,656
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 09 August 2020 - 06:40 PM

ok but just as a measurement  is the cereal bag as opaque as waxed paper or would waxed paper be too opaque ?

 

my full disk has the bright spot right in the middle so i need to reduce exposure but loose edge detail so will try bags.

 

power mate for sunspots  or prominences is a disaster so flats are next

 

 

thanks - next life i try basket weaving

 

STAY SAFE GUYS - THIS COVID  IS THE NEW MILLENNIUM - 

I wish I could put a metric on it.... 

 

PST_FlatFrameCalibration_CerealBag.jpg

 

Cereal_Bag_Flat_03232019.jpg

 

(please ignore the gamma usage, it was just so screen shots would show higher contrast of real time view to make a screen shot show things easier)

 

Bag flat being created (diffused out beyond the limb, you can tell because its a gradient to the limb rather a sharp transmission, this is key that it's diffusing light instead of transmitting it, it's scattering); note the transmission is low, I'm doing exposures of several seconds long, the key is just matching the histogram in that range of 65~75% or so. Prefocus before putting the bag over the aperture. No need to touch focus after doing this. It should stay focused through the whole process and only defocus due to the bag.

 

Bag_Flat_01.jpg

 

The flat applied real time overlay; remember to adjust your exposure settings back down after you take the bag off.

 

Bag_Flat_02.jpg

 

Very best,


Edited by MalVeauX, 09 August 2020 - 06:42 PM.

  • PhotonJohn likes this

#8 ASTROTRUCK

ASTROTRUCK

    Viking 1

  • -----
  • Posts: 602
  • Joined: 02 Jan 2010

Posted 09 August 2020 - 11:21 PM

OK - sounds like cereal bag doubled is best available

 

i am not good at histograms so will take many flats to try decide which combo works best

 

thanks !!




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics