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RASA Exposure Length

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#1 MountainAir

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Posted 31 October 2020 - 01:33 PM

I'm having difficulties with a RASA 11 V2: even smaller stars seem to be clipping.

 

I'm new to the RASA, and I'm still trying to get a feel for her.  I started by using the fantastic Celestron RASA LPR filter and an ASI294MC-Pro.  The filter was not very necessary at my dark-sky site (Bortle 4), and it physically interferes with the flatness of my image train when using the Baader UFC system, so I removed it.  I also switched to the ASI183MC-Pro.  I feel like both these changes were a step in the wrong direction:  I'm letting in more light (30s exposures vs 3m), and the full-well depth of the ASI183MC-Pro is only 15k.

 

After astronomical darkness settles, I run through the SharpCap Smart Histogram feature to estimate the subframe exposure length and gain for a 5-hour set.  I typically get around Gain 120 and 31 seconds, which I round to my standard settings of Gain 139 and 30 seconds (I know, that's not linear for both settings but G139_30s is what's in my dark library).

 

That has been yielding frames where many stars are clipping (as seen in PixInsight after stacking).  See attached.

 

Test.jpg

 

Ordinarily I would reduce exposure length and/or gain... but something strange, I seem to loose more of the nebulosity than I save in terms of star clipping.  I need to play more with this.

 

I'm wondering what the best course of action is.

  • I could switch back to the ASI294MC-Pro, but it's under sampled with this OTA.
  • I could find a new camera, but Santa isn't likely to bring me an ASI6200MC-Pro, and I like my kidneys.
  • I could figure out how to get the Celestron RASA 11 LPR to work with the Baader UFC (or get an image train custom-engineered for OSC use).
  • I could buy a filter for the Baader UFC filter drawer, but didn't want the expense of a 50mm square LPR filter when I already have a great one with the RASA.
  • And as I said above, I could figure out what's up with shorter exposure times even with more subs.

To make matters worse, I'm planning on heading to two Bortle 1 zones over the next two months.  I need to have this worked out by then.

 

Anyone have any "handling" insight for the RASA?



#2 Faris

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Posted 31 October 2020 - 02:34 PM

Hi

 

Can you share your raw data for inspection?

 

I use the hyperstar system on the 8SE. That is around f2 so I have some experience with fast optical systems. It is no a RASA but if you share your raw data it can help diagnose the problem. 



#3 bobzeq25

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Posted 31 October 2020 - 02:35 PM

I'm having difficulties with a RASA 11 V2: even smaller stars seem to be clipping.

 

I'm new to the RASA, and I'm still trying to get a feel for her.  I started by using the fantastic Celestron RASA LPR filter and an ASI294MC-Pro.  The filter was not very necessary at my dark-sky site (Bortle 4), and it physically interferes with the flatness of my image train when using the Baader UFC system, so I removed it.  I also switched to the ASI183MC-Pro.  I feel like both these changes were a step in the wrong direction:  I'm letting in more light (30s exposures vs 3m), and the full-well depth of the ASI183MC-Pro is only 15k.

 

After astronomical darkness settles, I run through the SharpCap Smart Histogram feature to estimate the subframe exposure length and gain for a 5-hour set.  I typically get around Gain 120 and 31 seconds, which I round to my standard settings of Gain 139 and 30 seconds (I know, that's not linear for both settings but G139_30s is what's in my dark library).

 

That has been yielding frames where many stars are clipping (as seen in PixInsight after stacking).  See attached.

 

attachicon.gifTest.jpg

 

Ordinarily I would reduce exposure length and/or gain... but something strange, I seem to loose more of the nebulosity than I save in terms of star clipping.  I need to play more with this.

 

I'm wondering what the best course of action is.

  • I could switch back to the ASI294MC-Pro, but it's under sampled with this OTA.
  • I could find a new camera, but Santa isn't likely to bring me an ASI6200MC-Pro, and I like my kidneys.
  • I could figure out how to get the Celestron RASA 11 LPR to work with the Baader UFC (or get an image train custom-engineered for OSC use).
  • I could buy a filter for the Baader UFC filter drawer, but didn't want the expense of a 50mm square LPR filter when I already have a great one with the RASA.
  • And as I said above, I could figure out what's up with shorter exposure times even with more subs.

To make matters worse, I'm planning on heading to two Bortle 1 zones over the next two months.  I need to have this worked out by then.

 

Anyone have any "handling" insight for the RASA?

Yep.  _Considerable_ experience.  Bortle 7, which only means my subexposures are shorter than yours.  The general advice still applies.  The key piece of advice is last.

 

Use the 183.  It could have been made for this scope, the tiny pixels have a useful image scale, and the F2 speed feeds them well.

 

Forget "light pollution" filters EXCEPT on emission nebulae, where they, by far, work best.

 

Most importantly, shoot lots of short low gain subs.  Subexposure should be small, and dim detail recovered by longer total imaging time.

 

This thread can get you "in the ballpark" and this advice should _absolutely_ be listened to, as it's _exactly_ right.

 

https://www.cloudyni...olour-versions/

 

"Don't go by the visual appearance of a sub – short broadband subs with the 183 may look very thin, but when stacked, the final result will be fine"

 

Example.  The numbers will be different for you, but this shows the principle.  (Remember, Bortle 7)  650X10" at Gain 0.  Better version than the crummy CN jpg here.

 

https://www.astrobin.com/t5173s/

 

Needless to say, one sub looks poor.  650 of them looks something like this.  <grin>  Note that 10" at gain 0 was NOT underexposed, skyfog was a bit over 10X read noise squared.  The subexposure is reasonably consistent with the tables.

 

Pleadies 2019 V3 small.jpg


Edited by bobzeq25, 31 October 2020 - 02:45 PM.

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#4 MountainAir

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Posted 31 October 2020 - 03:10 PM

This thread can get you "in the ballpark" and this advice should _absolutely_ be listened to, as it's _exactly_ right.

 

https://www.cloudyni...olour-versions/

 

Thanks for the feedback -- this is very useful, though I have read that thread before.  I actually put all my cameras through Sensor Analysis, then run them through a couple different spreadsheets to produce very similar tables of gain vs exposure vs sky brightness.  Eventually I learned that SharpCap's smart histogram feature does this nice and quickly.

 

In reading through that, though, I think I stumbled across my issue:  I was used to the ASI294MC-Pro, which has HCG mode where read noise falls to the floor when you hit gain 120.  That's why I stuck to gain 139, but the ASI183MC-Pro has no HCG mode.  I should probably be shooting with gain 0 and 30s exposures  in my 21.4 skies.  The reduced dynamic range at G139 may also be my issue with the 183.  I will try this out!


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#5 Topographic

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Posted 31 October 2020 - 03:17 PM

Fully concur with Bob. The 183 is a great RASA camera, so good I am buying a mono version to supplement my OSC version to do Mono L/Ha /OSC/. 

 

Low gain gives better dynamic range,  I tend to do most of mine at 53 gain. 40 " seconds or thereabouts in Bortle 3.


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