27 replies to this topic

[APshooter]

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:

 

 

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[APshooter]

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

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[APshooter]

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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[APshooter]

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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[tjones84]

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 spin  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! 

 

Cheers, and thanks again.

 

Trevor

[APshooter]

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.

[tjones84]

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

[lakeorion]

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.

[APshooter]

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. 

[APshooter]

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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[Rayastro]

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

[APshooter]

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.

[viniguez487]

NIGHTFLATS^TM!!! 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!

[lucutes]

Hoping I can piggyback on this forum,

I am having problems possible with flat frames on my RASA 8 and Nikon Z50. I was wondering if some expert on here had an idea what is going on. Here is a quick post calibration stretch of the integrated frame.

 

Note, this was taken in my Bortle 6.5 backyard but the night in question didn't have any localized light sources (such as lights on around the houses next to me). Flats were taken with a flat panel and might have been underexposed. 

 

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[sharkmelley]

Hoping I can piggyback on this forum,

I am having problems possible with flat frames on my RASA 8 and Nikon Z50. I was wondering if some expert on here had an idea what is going on. Here is a quick post calibration stretch of the integrated frame.

 

Note, this was taken in my Bortle 6.5 backyard but the night in question didn't have any localized light sources (such as lights on around the houses next to me). Flats were taken with a flat panel and might have been underexposed. 

This is the same coloured concentric banding issue I came across when testing the Nikon Z6, caused by Nikon's undocumented internal processing of the raw data.  Lots of info in this thread:  https://www.cloudyni...ting/?p=9610122

 

Your image is a very clear example of the problem and it's the first I've seen on the Z50.  The only workaround is to ensure both your lights and your flats are well-exposed.

 

Mark

Edited by sharkmelley, 01 May 2020 - 02:50 AM.

[John Miele]

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!

Where did you buy the acrylic sheet? Do they sell it at art stores or did you have to go online?

 

Thanks

[lucutes]

This is the same coloured concentric banding issue I came across when testing the Nikon Z6, caused by Nikon's undocumented internal processing of the raw data.  Lots of info in this thread:  https://www.cloudyni...ting/?p=9610122

 

Your image is a very clear example of the problem and it's the first I've seen on the Z50.  The only workaround is to ensure both your lights and your flats are well-exposed.

 

Mark

Thank you Mark for directing me to that thread. I thought it might have had something to do with the RASA.

[APshooter]

Where did you buy the acrylic sheet? Do they sell it at art stores or did you have to go online?

 

I bought it at Menards, a big box hardware store.  The acrylic sheet is used in photo frames.  After testing I found bright stars showed up through the acrylic and made bright spots on the flats, so I added parchment to the back of the acrylic.

 

Since that day I've since gone back to my Spike-a-flat panel.  I cut a piece of white posterboard the same size as the panel and taped it to the front of the unit.  This dims down the light coming from the panel enough so that I can get reasonable exposure times again at F2.

[arrowspace90]

I keep trying flats with my RASA 8 while being told that my flats are no good.

 

Ok fine.  Went from overexposed sky flats at 10 seconds to "night flats" with a white tee and an LED trace screen at low setting.

I am currently taking them down around .05 seconds exposure.  Last night at .07 seconds.

 

I noticed a black dot on 2 stars of different colors.  Is that a bad flat artifact?  I try to get the histogram to where the high point of the light curve is just left of center, but it's again difficult to get it right.

 

If flats are this important, there needs to be a more precise way to get them right.

[RogeZ]

Arrow:

You should not be taking flats that are that fast. Lower the gain in the camera and try to be no faster than 1 second.

[Rich139]

Careful - gain should match your lights!

[arrowspace90]

Last night I guessed at .2 seconds as that seem to produce a even dark grey shade

[arrowspace90]

Please, if you think you are getting great flats, tell me if this looks too dark, too light, too whatever.  This is a white tee over the RASA dew shield.  I took 30 of these using a large LED tracing pad.  Each exposure was .2 seconds and the histogram was just to the left of the middle.

 

I don't compete with anyone and I am not out to win an APOD.  I just want to get better at this hobby even though I lack a PhD in optics.

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[RogeZ]

Flat gain does not have to match Light gain.

Again, try to use images of 1s or more or you get all sorts of FPN. Also, make sure you calibrate your flats.

[APshooter]

Back when I was using night flats, it took 15 seconds to reach 7500ADU, which isn't all that much.  I found I had to put a sheet of poster board over my spike-a panel to get exposures down to 1 second, and even then the panel is set to something like '60' out of 1000, which is almost turned off.  Once second with my light panel now brings me to an ADU of 24000.

Edited by APshooter, 17 July 2020 - 08:45 PM.