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Flat not flatting

Edoardo Zanoni
in General
Hi Adam! Thank you for all your courses and precious informations... I'm amazed by the work you've done!

I'm having an hard time with my flats and I think this could be a good "office time", if I can say it myself
I've read and read and read, watched yours and other videos, searched everywhere and tried everything in my power to make my images works.

I'm imaging from my balcony so I've access only to up to 65 degree and usually I take my images in a range of 30 to 60 degrees in altitude, which makes my images not that good, but I try none the less untill I can caccess better skies; having problems with my calibration frames makes these "bad" photos even harder to process.

This is my setup:
  • Sky: Bortle 7 (18.58 from LightPollutionMap)
  • Camera:
    • Main: ZWO ASI294 MC-Pro
    • Guide: ZWO ASI 120 MC-S
  • Telescope:
    • Main: SkyWatcher Explorer 200/1000 PDS
      • Mirror mask on the primary
      • CNC Spider Vane
      • Artesky Focuser UltraLight 2" V3
    • Guide: Omegon Microspeed Guidescope 60/240
  • ​​Autofocuser: Primaluce Lab SestoSenso 2
  • Mount: SkyWatcher EQ6-R
  • Filter: IDAS LPS D3
  • Coma corrector: Askar Sharpstar 1.0x
  •  Computer: Odroid N2 with Armbian
  •  Software:
    • KStars/Ekos + INDI
    • PHD2
    • PixInsight
  • Extras:
    • OCAL Collimator
    • Geoptik Flat Field
I've tried eveything in my power to make the flats right.
No matter which ADU, method or pray I've made changes by much the final result: the vignetting looks overcorrected and the motes don't get removed.
Everything looks correct, other have tried with my data (Google Drive) and had the same problem and could not understand.

Since I've made these shots I've completely disasembled my scope, fllocked it, painted black the more reflective parts, checked every nook and cranny for stray light (none found), but the result is the same; I've also changed my exposure time from 180s to 30s without success.

I'm not fond of asking you like this, but I've tried everything and the only thing that could help me is The Expert.

Whatever the result, thank you very much for doing what you do and improving the AP hobby fo all of us!

Regards,
Edoardo

Comments

  • BlockHeadBlockHead
    Hi Edoardo,

    In Horizons I have a video on Night Sky Flats that might be helpful to you. You do not have Horizons though... 
    I did make this information available through my Patreon account. 

    This is the video:

    This is my patreon page:

    So..it would cost you $10... and you would have access to some Horizons content including this Night Sky Flats video. 

    I think this is a good place to start. If you pursue it, you will need to read the articles in which I explain how to access Patreon content. Basically there is a code I make available in these articles.
    Then you can just add the subscription.

    -the Blockhead
  • Edoardo Zanoni
    edited January 2024
    Hi Adam!

    I'm still doing the Fundamentals, so I will go for the Patreon at the moment.
    I hope it will help, not for the money (we are into astrophotography) but for the amount of frustration out of this problem.

    As always, your help is always appreciated. Thank you Adam!

    Regards,
    Edoardo
  • BlockHeadBlockHead
    So I suggest you attempt some night sky flats...
    You should confirm these work... and work backwards from there.
    -the Blockhead
  • Edoardo Zanoni
    Yeah, I'm doing just that. I've tried before to do some synthetic flats, but there were not enough dithering to create it and it never got to me to take different part of the sky as images.

    My sky is not that good today, is that a problem? I can see stars in the images but there are clearly clouds.

    Regards,
    Edoardo
  • BlockHeadBlockHead
    Night sky flats require no clouds. 
    -the Blockhead
  • Edoardo Zanoni
    Ok, thank you! I will wait for a better day then! For the moment I'm subscribing to the Horizons Membership so I can check some more.

    I see in the video that the NSF were taken at 300s, it's 300 second the minimum amount?

    Regards,
    Edoardo
  • BlockHeadBlockHead
    No... but you need to take enough to get a good sky brightness and also so when you combine the images all of the stars will be rejected. So take like 50 or more frames.
    If your sky is very bright... even 120 seconds would be fine.
    -the Blockhead
  • Edoardo Zanoni
    Hi Adam,
    I've taken the NSF, but I don't understand how should I integrate the frames coming from an OSC camera.

    If I try to integrate the calibrated NSFs (without debayering) I get a flat that doesn't seems quite right - and the resulting integration of the lights it's wrong.
    If I integrate the debayered  NSFs the resulting flat looks good, tho, but I don't think it can be used as a calibration frame.

    What I'm doing wrong? If I integrate the debayered  NSFs the resulting flat looks good, tho.

    Regards,
    Edoardo
  • BlockHeadBlockHead
    Can you tell me more about the NSFs? Did you load them into the Flats panel?
    Did you calibrate them with the appropriate dark frame?
    Did you tell WBPP they are CFA?
    If yes, what leads you to believe "it doesn't seem quite right."

    Screenshots and/or more information I think will help.

    -the Blockhead

  • Edoardo Zanoni
    The NSFs I've taken are a mosaic of a little zone of the sky, with 120s exposures and the same temp, gain and offset as the lights.
    After taking them I've created the master flat as per your video:
    • 50 x 120s flat frames -> loaded in WBPP as light frames
    • 50 x 120s dark frames -> loaded in WBPP as dark frames
    • Enabled only the integration step
    The final result is a debayered flat frame  that looks good for a debayered flat.
    Here comes my first doubt: can I use a debayered flat master to calibrate? If I try to use it as it is, then WBPP will tell me that the CFA pattern is not right.
    I've tried to create the master flat by integrating the calibrated NSFs without debayering, but that means that the resulting image does not preserve the bayer information that get lost and the resulting flat does not work correctly.

    The only thing I can think of is to create the flat separated in RGB, debayer all the images, separate them and then run WBPP as if it was an image with separated RGB channels.

    I'm doing just that to try by myself and report back with some screenshots ora data if you prefer.

    Regards,
    Edoardo
  • BlockHeadBlockHead
    I didn't show using WBPP in my video because the data was monochrome. I just did the integration my self.
    I
    For OSC... you should load the NSFs into your FLATs panel. Use the FLATs button (or the Custom button) to do this.  You would match this flats with your 120sec dark frames (not bias). WBPP will create a master flat from your NSFs that is not debayered. This should take care of your issue.

    Please try this (show me screen shots of what I suggest above.

    -the Blockhead

  • Edoardo Zanoni
    You're right, I'm just plain stupid.
    I've used the NSFs as you said, as flats images, and the process went on smoothly.

    The result shows an image that's a lot more in line with a correctly flattened master light.

    In the attached files you can find, as you requested, the screenshots of WBPP, the resulting image and an example of an image done with a flat made out of th usual way of doing flats.

    Now I need to understand what's causing my flats to not be working.
    • I've already removed all the reflections I can think of inside the OTA
    • closed every possible path a light can enter (using a flashlight, I cannot see leaks when the OTA is closed)
    • used different systems to make the flats: sky flats with a t-shirts, t-shirt against an illuminated wall, flat field generator, flat field generator with a t-shirt, etc
    • used every ADU, from 20k to 40k.
    I don't know what should I look for any more.

    Regards,
    Edoardo
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  • BlockHeadBlockHead
    Comments...

    • I've already removed all the reflections I can think of inside the OTA
    • Probably not... 
    • closed every possible path a light can enter (using a flashlight, I cannot see leaks when the OTA is closed)
    • I use a Tak 180 Epsilon. I think you would agree it is a similar design. I made a tube of black flocking paper that fits into the hole where light is directed from the secondary flat to the camera. The tube fits over the corrector lens that is in front of the camera. The tube does not extend so far as to jut into the light path of the rays coming from the outside to the primary. This blocks light that hits the entrance of the hole where the secondary is sending light. Some light can get there from off-axis rays of any flat field light you send into your tube. This black flocked tube prevents this. 
    • used different systems to make the flats: sky flats with a t-shirts, t-shirt against an illuminated wall, flat field generator, flat field generator with a t-shirt, etc
    • If my theory is correct... none of this would make a different. Do not use t-shirt method. 
    • used every ADU, from 20k to 40k.
    • Adjusting the light level has NO IMPACT. I just proved this to you. NSFs are very very low light level. People really get this wrong..the light level only controls the amount of noise a Flat adds to its calibration result.  People are so wrong about this..but no one listens to me. lol 
    -the BLockhead
  • Edoardo Zanoni
    edited January 2024
    • Probably not...
      • You're right, the problem with the flat indicate just that
    • I use a Tak 180 Epsilon. I think you would agree it is a similar design. I made a tube of black flocking paper that fits into the hole where light is directed from the secondary flat to the camera. The tube fits over the corrector lens that is in front of the camera. The tube does not extend so far as to jut into the light path of the rays coming from the outside to the primary. This blocks light that hits the entrance of the hole where the secondary is sending light. Some light can get there from off-axis rays of any flat field light you send into your tube. This black flocked tube prevents this.
      • You mean something like this? In your case it is a permanent installation or it can be removed? What If the CC is already almost out of the focuser, do I need to block a little the path through the primary?
        image
    • If my theory is correct... none of this would make a different. Do not use t-shirt method.
      • I've bought a flat field (albeit cheap) to remove the need of the t-shirt method; I hope that the addition of the floking tube will remove this problem.
    • Adjusting the light level has NO IMPACT. I just proved this to you. NSFs are very very low light level. People really get this wrong..the light level only controls the amount of noise a Flat adds to its calibration result.  People are so wrong about this..but no one listens to me. lol
      • I think that if people knew about the NSFs would be more prone to understand this fact. Sadly the internete is full of information that get repeated at nausem but it's not correct. Thank you for showing me how real flats works!
    This evening (I'm based in Italy) I will try to create the CC Shield and report back!

    thank you Adam.

    Regards,
    Edoardo
  • BlockHeadBlockHead
    You have the correct idea. The version I made literally slipped on over the corrector lens and was held on by friction. It did jut out very slightly into the incoming light. But it is very small. (In my system there is a bit of space between the primary and tube...the gap is somewhat a safe zone). 

    I can't promise the flocking tube will help..but it is a very good experiment I do not think you have tried.
    -the Blockhead
  • Edoardo Zanoni
    edited January 2024
    Hi Adam,
    finally had the time and the sky to  try to add the "extension" to the CC.
    I've disassembled again the OTA to work on the focuser. I've created 2 "extensions", one attached directly to the focuser, but that one looked too broad and not covering enough so I've created another extension to wrap directly on the CC. Sadly the extensions are only 20 (the CC one) to 30mm (the focuser one) thick and I think they're not covering enough.
    I've taken some new lights but sadly the problem is still there.This time the dithering process worked correctly and I will try to use the dithered images to create a NSF.
    Tonight, if skies remain clear, I will try to remove the CC completely and see if that's the problem.

    Edit: I'm going to buy a dew shield to both prevent more stray lights (I live in a flat with a lot of illumination around) and try to move the flat field away from the focuser to reduce possible stray light rays reaching the CC 

    Cheers,
    Edoardo 
  • Edoardo Zanoni
    I'm back to report in case someone else have the same problem.
    I've tried to take the flats using the dew shield without success. Nothing I'm doing is resolving the problem and I think that the problem is the combination between my OTA, the focuser and the CC.
    1. The OTA have a low profile between it's end and the focuser hole
    2. The focuser has an high profile and require a long CC 
    3. The CC has a low profile between the outer lens and the end of the CC
    The result is a CC that catch stray light coming from light sources that are too near to it. Cannot prove this unless I make a flocking tube to cover the CC that is long enough to block stray light, but this will result in a blocked path to the main mirror.

    I had bad weather up untill now, so I could not test without the CC at all.

    Regards,
    Edoardo
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