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using narrowband on a RASA + OSC as a temporary solution?

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#1 DrGomer

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Posted 14 February 2020 - 03:47 PM

I'm at the "want it all, but no way I have the budget for it all" situation that most of us are probably in.
I really want a RASA because I really like to dig deep but also want to go mono.  ..and I have neither a mono cam, nor a RASA.

I also don't like the current affordable mono cams (amp glow and tiny pixels with the 183, and microlens reflections with the 1600). 

So as a stepping stone, waiting for funds to go mono and RASA, thought I can almost get there. 
So...gasp...use narrow band on a OSC with the RASA. Yes, not efficient in the slightest, but I think the efficiency with the RASA is close to a typical refractor with mono. And being that I already have the ASI533, I can still go full on light bucket with the OSC when I want till I can get a true mono cam.

Does the math roughly check out? 

Currently using an SV80A @~450mm f/5.6 with the FF/FR (f/7, 0.8x).
RASA throughput would be 5.6^2/ 2^2 = 7.8x faster photon collection. 
Lets just pretend the RASA at 400mm is about the same as my current 450mm refractor.
But the RASA has a central obstruction, so it is actually [(203/2)^2  - (93/2)^2 ]/(203/2)^2 * 7.8 = 6.2 x faster photon collection.

OSC only has (at worse) only 1/4 of the pixels dedicated to a specific filter color which is about the same factor as the RASA efficiency increase

Rasa is still faster at photon collecting but not but much, and that difference probably will be eaten by the double filter loss.

 

So my current refractor with a mono camera and narrowband filters will be similar to the RASA with with OSC and narrow band filters...with the bonus of potential higher spacial resolution should seeing allow.
Does that check out? Again, this would only be a temporary solution till I can also get a mono cam I'd be happy with to pair up with the RASA.



#2 bmhjr

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Posted 14 February 2020 - 03:57 PM

This spreadsheet will let you select your camera and filter combination, select your scope possibilities, and determine the calculated photons collected for comparison.  A good way to check your assumptions:

 

https://www.cloudyni...-5#entry9846108


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#3 bobzeq25

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Posted 14 February 2020 - 05:07 PM

I'm at the "want it all, but no way I have the budget for it all" situation that most of us are probably in.
I really want a RASA because I really like to dig deep but also want to go mono.  ..and I have neither a mono cam, nor a RASA.

I also don't like the current affordable mono cams (amp glow and tiny pixels with the 183, and microlens reflections with the 1600). 

So as a stepping stone, waiting for funds to go mono and RASA, thought I can almost get there. 
So...gasp...use narrow band on a OSC with the RASA. Yes, not efficient in the slightest, but I think the efficiency with the RASA is close to a typical refractor with mono. And being that I already have the ASI533, I can still go full on light bucket with the OSC when I want till I can get a true mono cam.

Does the math roughly check out? 

Currently using an SV80A @~450mm f/5.6 with the FF/FR (f/7, 0.8x).
RASA throughput would be 5.6^2/ 2^2 = 7.8x faster photon collection. 
Lets just pretend the RASA at 400mm is about the same as my current 450mm refractor.
But the RASA has a central obstruction, so it is actually [(203/2)^2  - (93/2)^2 ]/(203/2)^2 * 7.8 = 6.2 x faster photon collection.

OSC only has (at worse) only 1/4 of the pixels dedicated to a specific filter color which is about the same factor as the RASA efficiency increase

Rasa is still faster at photon collecting but not but much, and that difference probably will be eaten by the double filter loss.

 

So my current refractor with a mono camera and narrowband filters will be similar to the RASA with with OSC and narrow band filters...with the bonus of potential higher spacial resolution should seeing allow.
Does that check out? Again, this would only be a temporary solution till I can also get a mono cam I'd be happy with to pair up with the RASA.

It's a _lot_ more complicated than your analysis indicates.  The wavelengths of stuff count, as do the debayering equations.  OSC is not 4 times worse than mono + filters, although it is worse.

 

Assuming you acquire the RASA, better would be to use the RASA and OSC with either the Celestron LP filter, or (better yet) one of the new duoband filters, which are designed specifically for OSC cameras, and are a compromise between narrowband filters and filters like the Celestron LP filter.  That will be substantially better than the SV80A with a mono camera.  I have both setups (although my scope is the SV70T).

 

Here's the RASA, Celestron LP filter, OSC.  2.0 hours total imaging time in Bortle 7 skies.

 

https://www.astrobin.com/ky6juu

 

I'm not bothering with the duoband, simply because I have a suitable mono camera.

 

Note that I spent a lot of time, and some money, fixing tilt issues with the RASA.  That's the chief problem here, F2 is crazy sensitive to tilt.  1/1000 of an inch is too much.


Edited by bobzeq25, 14 February 2020 - 05:14 PM.

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#4 DrGomer

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Posted 14 February 2020 - 06:26 PM

Thanks for the info.  I have the duoband already since it came with the 533.  That's really impressive at 2hrs.  
As for tilt, if Baader doesn't release their tilt drawer solution soon, I might have to talk with my machinist to see what it costs to make one :p  Or I'll just use shims as a temporary solution (got plenty in the shop). 




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