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Rasa 8-An Experiment with 'Night Flats'

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#1 APshooter

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Posted 28 March 2019 - 09:10 PM

The last 8 days' weather has been unusually clear for us in the Midwest, a golden opportunity to try out my new Rasa 8.  As I get more familiar with the 1600MM camera, I'm finding a slightly lower gain helps in my Bortle Class 5 skies.  I reshot Markarian's chain the other night at gain 76, but was disappointed in the way the flats seemed to be correcting the image.  The centers of the stars had small black dots just off center in each of the stars and there was a general unevenness to the image, as if the flats weren't doing the job.  I wasn't sure if this was my processing in this image or something to do with the flats themselves.

 

Normally, I'd set my Spike-A panel atop the dew shield to take my flats.  At F2, the Spike-A panel has to be set very low in brightness in order to get something like 28K ADU.  So low, in fact, that out of 1026 brightness levels, I have the panel turned down to 1 with an exposure time of just .04 seconds.  Since the images with flats at this setting were giving me issues, I thought I'd try a different approach.  I had read in a recent Jon Rista post that he's using sky flats again.  Something about collimated light that seemed to be better than using a flat panel.  

 

I wondered if I couldn't do the same thing, but use the night sky itself as the light source.  Since my Spike-A panel was so dim as to be almost black anyway, I figured something like parchment paper over the top of the dew shield might provide a sufficiently diffuse surface to act as a panel, lit by light pollution, if you will.  The other night I made up a test flat panel, made of cardboard and tape.  A cardboard ring holds the slips over the dew shield, while a flat ring of cardboard on top forms a drum head, sort of like a tambourine.  I set this atop the dew shield and started taking pictures, monitoring the ADU in TheSkyX.  At 15 seconds I was getting 7800 ADU.  I had spoken to andysea about this subject and he said that that was the number he was shooting for in his flats.  I then took two sets of flats; one the normal way, with the Spike-A, the other with Nightflats, as I call them. 

 

I process the night flats and in Pixinsight using manual calibration, and got this.  The Nightflat is on top, the Spike-A flat on the bottom:

 

 

Attached Thumbnails

  • Night flat integration cn.png
  • flat integration cn.png

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#2 APshooter

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Posted 28 March 2019 - 09:13 PM

Here's a shot of the makeshift 'flat panel' for nightflats:

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  • 20190328_220418.jpg

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#3 APshooter

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Posted 28 March 2019 - 09:19 PM

And here's the new Markarian's Chain image calibrated with 15 second 'nightflats'.  I processed the bulk of this in APP, using DBE in Pixinsight to clean up the last of of a faint gradient in the image.  The gradient was very minor, but was noticeable if you looked for it.  At any rate, I was quite pleased with the way the image turned out.  I'm putting this information out there for anyone else who might have issues shooting flats with a Rasa.  This in no way should cast a bad light on the Spike-A panel itself.  It performs excellently with my slower scopes like my F6 refractor and F10 SCT.  

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  • Markarian_s_Chain_APP_stretch_DBE cn.jpg

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#4 APshooter

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Posted 28 March 2019 - 09:30 PM

Here's a side by side example of what I was seeing in the .04 second exposure flats vs 15 seconds.

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  • fLATS COMPARISON.jpg

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#5 tjones84

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Posted 29 March 2019 - 11:16 AM

This is incredibly helpful information! Thank you so much for sharing this experiment and your results.

 

I will try taking some "nightflats" the next time I take the RASA out for a spinsmile.gif  Do you mind if I share your experiment in my article about the 8" RASA

 

(Celestron 8" RASA F/2)

 

I'm a Bortle Class 6 - so for once, my light polluted skies will contribute to my astrophotography in a positive way! lol.gif

 

Cheers, and thanks again.

 

Trevor


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#6 APshooter

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Posted 29 March 2019 - 01:02 PM

Thanks for the kind words, Trevor.  Yes, by all means share the information!  I'd be curious to hear if you could reproduce the same results by taking your nightflats with your own rig.


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#7 tjones84

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Posted 29 March 2019 - 02:13 PM

Thanks for the kind words, Trevor.  Yes, by all means share the information!  I'd be curious to hear if you could reproduce the same results by taking your nightflats with your own rig.

I will be sure to report back on this! Thanks a million



#8 lakeorion

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Posted 29 March 2019 - 02:53 PM

My scope isn't nearly as fast but I also had flat problems with short exposures.  I ended up putting neutral density filters intended for a portrait flash unit in front of my Gerd Neumann EL panel to dim it down and going with 5 second flats.  EL panels display flicker with short exposures.

 

I also thought of attempting complimentary colors of Saran Wrap as a 'dimmer' but never followed through with the concept.  Around Christmas it's easy to find red and green...


Edited by lakeorion, 29 March 2019 - 02:53 PM.


#9 APshooter

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Posted 29 March 2019 - 03:25 PM

lakeorion, good call with the ND filters.  I almost went with several layers of parchment paper or light posterboard over my flat panel to do something similar.  I also had read something about even biases having issues (readout?) when taking exposures less than 1 second and to instead take dark flats. 



#10 APshooter

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Posted 08 April 2019 - 12:27 PM

I redid the cap and replaced the parchment with .050 non glare acrylic.  It's used as a replacement for photo frame glass.  It holds up much better than the parchment paper which tended to wrinkle when it absorbed moisture from the air.  I painted the foamcore as well, the gorilla tape thing wasn't doing it for me!

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  • cap cloudy nights.jpg

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#11 Rayastro

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Posted 02 June 2019 - 09:29 AM

AP Shooter, I do have the similar problems. Please check my flats video for RASA 11". I am trying both 0 second exposures and 10k ADU. One of them usually work. I still need to use DBE to correct any remaining. Hope this helps.

https://youtu.be/fRdUlWsSOuE

#12 APshooter

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Posted 02 June 2019 - 01:57 PM

Thanks, Rayastro! By chance I watched your video today while searching for new Rasa videos. I've been getting better results since that first outing. You're right, I'm using DBE for final touchups after processing in APP.

#13 viniguez487

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Posted 07 August 2019 - 12:17 AM

NIGHTFLATSTM!!! I love it. I was thinking of building the exact same thing you made and hooking it to some battery powered LED lights with a dim switch. I am a traveling astrophotographer so I need something portable for flats I can use at night. Gonna try this out on my RASA 11 next time I’m out. It’s also amazing to see two of my favorite Astro-Youtubers in the same thread! Shout out to AstroBackyard and Ray’s Astrophotography!




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