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

More Flat questions

  • Please log in to reply
7 replies to this topic

#1 arrowspace90

arrowspace90

    Viking 1

  • -----
  • topic starter
  • Posts: 579
  • Joined: 15 May 2009
  • Loc: United States

Posted 13 August 2020 - 12:10 PM

From what I read, ask any of the top AP guys what might be wrong with your images and they will say, "flat field errors".  I read this again and again.

 

So, one thing that a very respected guy said goes something like, "your light flats should be at least sever seconds in exposure time".

 

Well, I can't get that with my affordable LED light pad.  Some people of course spend hundreds of dollars on a flat light.  But I know darn well that "most" people 

use a cheap, LED tracing pad.  

 

When I use mine on the lowest setting, depending on which filter I use, even with a thick white sweatshirt under the LED, I have to get down to well less than 1 second exposure to put the historgram at the middle of the range.  

 

Why is it good to have exposures of several seconds?  I am not arguing the point, but the source didn't explain why. (Ha, he did say that a problem data set someone sent him had "darks" that were no longer good after 6 months in the library because the guy's camera had somehow changed).

 

So are guys able to, or do they even try, to get those flats up to several seconds?  How many shirts should I pile on?  Can a cheap LED pad be further dimmed?

BTW, I image with a CMOS camera and a RASA 8.



#2 Dynan

Dynan

    Skylab

  • *****
  • Posts: 4,490
  • Joined: 11 Mar 2018
  • Loc: NOLA

Posted 13 August 2020 - 12:22 PM

I use plain printer paper between my El Cheapo light panel. I use two sheets, set on lowest light and shoot 3.3 second flats. Statistics land right around half maximum...that's what's important.

 

https://www.cloudyni...sue/?p=10409868



#3 rhart426

rhart426

    Vostok 1

  • -----
  • Posts: 150
  • Joined: 16 Sep 2017

Posted 13 August 2020 - 12:40 PM

It's best to have exposures above 1-2 seconds because the shutter action can interfere with capturing the true flat field.

 

I recommend building your own rather than appropriating a panel that's built for some other purpose.  I can post details on my build if interested.


  • arrowspace90 likes this

#4 arrowspace90

arrowspace90

    Viking 1

  • -----
  • topic starter
  • Posts: 579
  • Joined: 15 May 2009
  • Loc: United States

Posted 13 August 2020 - 01:19 PM

I use plain printer paper between my El Cheapo light panel. I use two sheets, set on lowest light and shoot 3.3 second flats. Statistics land right around half maximum...that's what's important.

 

https://www.cloudyni...sue/?p=10409868

I guess I could try paper.  It would have to be pretty large sheets to cover the dew shield of the RASA 8.  And perhaps the RASA at F2 is a factor in the really short exposure times?


  • Dynan likes this

#5 jdupton

jdupton

    Mercury-Atlas

  • *****
  • Posts: 2,820
  • Joined: 21 Nov 2010
  • Loc: Central Texas, USA

Posted 13 August 2020 - 01:22 PM

arrowspace90,

 

   It really depends on the particular camera you are using. There might be no issue with flat exposures of less than a couple of seconds. The following are some of the reasons it may matter, though:

  • Camera Shutter -- if the camera has a mechanical shutter, exposures of less than two or three seconds may show the effects of "shutter sweep." This is where the opening and closing of the shutter may allow some parts of the sensor to be exposed longer than other areas due to the motion of the mechanical shutter itself.
     
  • Camera Timing -- many (or most?) cameras use a different timing method for short exposures. At some exposure length, like 1 or 2 seconds, the camera switches from internal to external timing. At exposures less than the switch-over point, the camera electronics do the timing. This can vary from camera to camera of the same model.

    At exposures longer than the switch-over point, the device driver on the PC does the timing. Since most common PC operating systems we use are not built for real time operations, the length of a PC timed exposure can vary based on other things the PC is doing. If the exposure happens during high CPU load due to other programs, the exposure may be longer than intended. In lightly loaded CPU periods, the exposure could be shorter than other times. This inconsistency is hard to account for.

    The variance in exposures is not usually much but can lead to minor mismatches between the Flats and any Flat-Dark frames taken.
     
  • Camera / Sensor Response -- many camera sensors have been shown to be fairly non-linear at short exposure times. This effect is small and can be additive to the effects of timing switch-over as described above. As an example here, the Sony IMX294 sensor shows a higher rate of Amp Glow growth for exposures of less than 1 second. Other sensors show different but still troublesome anomalies at very short exposure times.

   None of the above are show-stoppers if you must shoot your Flats with very short exposures times. The most troublesome one is the effect of a mechanical shutter. You should examine each frame carefully and throw away those that show an obvious moving shutter shadow. The other effects result in inconsistencies from frame to frame and can usually be overcome by taking more frames (Flat and Flat-Dark) than might otherwise be needed. By averaging a higher number of frames the subtle effects can be lessened.

 

   For the most part on nearly all cameras, shooting for three seconds or longer can avoid these effects. If your flat panel is too bright, you can use it after carefully looking for these effects or you can add a layer (or two) of solar filter material (such as used to darken windows sold at home improvement or automotive parts stores) over the panel to dim it down a little.

 

 

John


Edited by jdupton, 13 August 2020 - 01:27 PM.

  • arrowspace90 likes this

#6 arrowspace90

arrowspace90

    Viking 1

  • -----
  • topic starter
  • Posts: 579
  • Joined: 15 May 2009
  • Loc: United States

Posted 13 August 2020 - 02:17 PM

My ASI533 has a "rolling shutter".  Is that mechanical?



#7 t-ara-fan

t-ara-fan

    Surveyor 1

  • -----
  • Posts: 1,521
  • Joined: 20 Sep 2017
  • Loc: 50° 13' N

Posted 13 August 2020 - 02:35 PM

My ASI533 has a "rolling shutter".  Is that mechanical?

No.



#8 cmooney91

cmooney91

    Messenger

  • *****
  • Posts: 424
  • Joined: 18 Sep 2017
  • Loc: Massachusetts

Posted 13 August 2020 - 02:58 PM

I use  dark car window tint film with a couple layers of diffuse fabric in between it to dim my LED panel. The only issue is it gives a slight color cast. 

As the panel gets dimmer it is important to chase down any light leeks into the OTA since they will have a higher effect on the low powered flat light.

 

Put a small wide angle C-mount lens on the camera in the focuser, Cap the scope in a bright room, and take an exposure, you might be shocked!  My Newt leaks like a sieve around the mirror and around the focuser tube.  Here are my light leaks.

 

Turn the lights off and go hunting with a flashlight with the camera running to find specific trouble areas.

 

bonus: A wide angle camera lens in the focuser is also an handy tool for quick mirror alignment for rough collimation. You can turn on the on-screen cross-hairs +concentric circles to help quick alignment. 


Edited by cmooney91, 13 August 2020 - 03:30 PM.

  • Dynan likes this


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