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Tell me your best FWHM with the RASA 8" and a small pixel camera

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#1 andysea

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Posted 05 June 2020 - 10:12 AM

The title says it all. With my ASI 1600 mono and Rasa8 the best I get is around 3" (~1.5 pixels). Sometimes just a hair lower than that. 

What is the best you guys got if any of you is using an ASI 183 or the QHY equivalent?

I'm not sure if mono or color makes any difference with respect to FWHM.


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#2 bobzeq25

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Posted 05 June 2020 - 10:54 AM

This was 2.8 arc sec with a 183.  I've seen maybe 2.4.  But 3 is more usual.

 

https://www.cloudyni...-4#entry9938910

 

I have notably bad seeing.



#3 555aaa

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Posted 06 June 2020 - 10:15 AM

It does make a difference with color being a lot worse. If your fwhm is 1.5 pixels I would think that's about as good as it's going to get. I've gotten as good as 1.8 pixels on the RASA 11 so 1.5 sounds right on the 8.

#4 andysea

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Posted 06 June 2020 - 12:53 PM

So far It looks like ~2.8" is the best people get consistently. I was wondering if with smaller pixels I could get better FWHM.



#5 bobzeq25

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Posted 06 June 2020 - 03:08 PM

So far It looks like ~2.8" is the best people get consistently. I was wondering if with smaller pixels I could get better FWHM.

Depends on the quality of your skies.  In mine, image scales numerically below 1.0 buy me little in FWHM.  Maybe they would, once or twice a year.  <smile>

 

The 183 would probably improve your FWHM a bit over the 1600.  Image scale 1.2 versus 1.9.

 

Many factors go into FWHM.  It also depends on your mount.  You'd really like to autoguide with RMS error numerically less than your image scale.


Edited by bobzeq25, 06 June 2020 - 03:15 PM.


#6 andysea

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Posted 06 June 2020 - 03:27 PM

Yeah the mount is OK, I think. I still use the Mach1, I guess I could try with the 900 and see if I get any better tracking. RMS Is usually hovering around 0.4"~0.6" Total in PHD. However on nights of bad seeing it can be as high as 1" or more. I'm really tempted by the 183 paired with the RASA.


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#7 tomb18

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Posted 06 June 2020 - 06:36 PM

Get rid of that mach 1. I'll take it off your hands, no problem,,


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#8 freestar8n

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Posted 06 June 2020 - 07:08 PM

So far It looks like ~2.8" is the best people get consistently. I was wondering if with smaller pixels I could get better FWHM.

I think it is usually the case that the fwhm will decrease if you use smaller pixels - and it's particularly true if the stars are small on the scale of the pixels so the star spots only span a few pixels.  This goes counter to theoretical assumptions that the fwhm is well defined and just measured in the sky - but in practice other factors play a role and a finer view of the star will reveal a smaller size to it.

 

On the other hand - I don't think RASA or hyperstar are diffraction limited - whereas an sct at f/10 would be.  So I doubt you could use RASA for good detail on Jupiter - but I think typical fwhm values from RASA are probably limited by pixel size.

 

Would be interesting to try RASA or hyperstar with the new Samsung sensor - with 0.7um pixels:

 

https://www.samsung....ocell-slim-gh1/

 

They would only be 0.23" on RASA 11 f/2.2 - which is comparable to my ASI1600 with EdgeHD11 at f/10.  f/ratio is 10/2.2 = 4.5 times bigger, and pixels are 5.4 times bigger - so there is a net reduction of pixel size in arc-sec with the RASA and 0.7um pixels.

 

Not sure when or if such sensors will be used for astro.

 

Frank



#9 andysea

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Posted 06 June 2020 - 07:56 PM

I'm starting to think that my idea of switching to the 183 with the rasa8 may be a fool's errand altogether. I currently use the ASI1600 and it has much larger pixels, theoretically they should collect more photons than the 183 and even the higher QE of the 183 may not be able to offset the larger pixel size of the 1600.


Edited by andysea, 06 June 2020 - 07:56 PM.


#10 freestar8n

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Posted 06 June 2020 - 08:10 PM

I'm starting to think that my idea of switching to the 183 with the rasa8 may be a fool's errand altogether. I currently use the ASI1600 and it has much larger pixels, theoretically they should collect more photons than the 183 and even the higher QE of the 183 may not be able to offset the larger pixel size of the 1600.

As long as the QE is larger and there isn't much sensor noise, you will be collecting more photons with the 183 over its sensor area than in the same size area on the 1600 (i.e. only the central region of the 1600 since it is larger).

 

If you want more detail then you need to use smaller pixels - and smaller pixels probably will result in smaller stars, in arc-sec, and more detail.  But the smaller sensor size won't give you as big a field - so it depends on exactly what you want to image and if it will fit.

 

But with higher QE you will be collecting *more* photons over a given patch of the sensor.  If you find you don't need the detail provided by those small pixels you can always bin or smooth to find the right compromise.

 

I don't know that the 183 would have a dramatic improvement in detail, but 2.4 vs. 3.78 um pixels should reveal more of what the RASA can deliver.  And a smaller sensor does have the advantage that it is less sensitive to alignment errors.

 

Frank



#11 bobzeq25

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Posted 06 June 2020 - 10:46 PM

I'm starting to think that my idea of switching to the 183 with the rasa8 may be a fool's errand altogether. I currently use the ASI1600 and it has much larger pixels, theoretically they should collect more photons than the 183 and even the higher QE of the 183 may not be able to offset the larger pixel size of the 1600.

I like the 183 on the RASA, I think it's practically designed for the scope.

 

Smaller (numerically) image scales do have less signal to noise ratio.  But, the RASA's F2 speed is good compensation for that.  It feeds the small pixels well.

 

It won't be a revolution.  It is definitely not a fool's errand.   I've done many foolish things, getting the 183s (I have both) for the RASA was not one of them.  <smile>

 

I think you'd find it an improvement.  Whether or not the improvement would be "worth it" is simply a personal decision.

 

Below is the image I'm happiest with, so far.  2 panel mosaic of Andromeda.  It's my first image that withstands printing larger than 8X10.  Fullsize here.

 

https://www.astrobin.../full/025umc/0/

 

M31 mosaic V6.jpg


Edited by bobzeq25, 07 June 2020 - 08:10 AM.

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#12 andysea

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Posted 06 June 2020 - 10:51 PM

Nice M31 Bob!

Ok I am going to give it a try :) Crazy thing is I had the 183, I bought it right after it came out. Then I sold it, I don't even remember why. Looking back at the data from that camera it was actually very nice.  The RASA didn't exist back then, maybe if I had it I would have kept the 183. Fingers crossed that I will get some nice images.



#13 Jared

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Posted 07 June 2020 - 01:23 AM

The title says it all. With my ASI 1600 mono and Rasa8 the best I get is around 3" (~1.5 pixels). Sometimes just a hair lower than that. 

What is the best you guys got if any of you is using an ASI 183 or the QHY equivalent?

I'm not sure if mono or color makes any difference with respect to FWHM.

Of course, some of it will depend on how good or bad your skies are, but expecting a decent quality 8" aperture telescope to deliver 2" FWHM stars on nights of good seeing is not unrealistic.  That's the scope itself, of course.  In order to get that resolution, you'd need something with appropriately sized pixels.  In the case of the RASA, you'd need something like 2 micron pixels to get down to 2" of resolution.  Maybe even a bit smaller if using a OSC camera. No absolute number, of course, but that's about where you would need to be.  So the 183 will likely allow higher resolution than you can get right now with ASI 1600, but probably wouldn't get you down to what most most people can achieve on a night of average or better seeing with longer focal length telescopes.  Honestly, though, high resolution isn't the RASA's forté. It excels as a wide field scope.  I know it's galaxy season still, so many of us are trying to find ways to improve resolution, and the 183 would likely let you capture smaller details.  But it probably won't be enough to make your system seeing limited.  If that's what you are after, you'll need a telescope with a bit more focal length given the current crop of cameras.  If you are happy with the field of view the 183 will provide, it's legitimately an upgrade on your 1600.  But it's not going to turn your setup into the ultimate "galaxy" or "planetary nebula" system.



#14 andysea

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Posted 07 June 2020 - 02:13 AM

That's OK I have a 10" f7.3 RC telescope that I can use for galaxies and it can easily cover the IMX455 sensor.

I was just hoping to squeeze a bit more resolution out of the RASA and I was wondering if smaller pixels would be a realistic way to go. I think that currently the ASI183 is the smallest pixel camera that I can use with the RASA8 and the Starizona filter slider system.



#15 Jon Rista

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Posted 07 June 2020 - 02:46 AM

I'm starting to think that my idea of switching to the 183 with the rasa8 may be a fool's errand altogether. I currently use the ASI1600 and it has much larger pixels, theoretically they should collect more photons than the 183 and even the higher QE of the 183 may not be able to offset the larger pixel size of the 1600.

You will likely have more clipping issues with the ASI1600. IMO one of the biggest benefits of using the IMX183 on a RASA is the fact that it has tiny pixels, and those pixels have excellent capacity. In fact, the per-micron effective FWC is worse for the ASI1600...it is 1778e-/µm^2 for the ASI1600, and 2604e-/µm^2 for the ASI183. This would be for minimum gain...which you should probably be using with the RASA. Further, the ASI183 has smaller pixels, by a factor of ~2.5x. Accounting for the higher Q.E. (~1.4x higher with the IMX183), that means you should, technically, be able to expose with the IMX183 ~2.7x longer than with the ASI1600 before you start to clip. Or, alternatively, expose for the same amount of time...and clip less! :p 

 

IMO, clipping is the biggest issue using the ASI1600 with any super fast, large aperture scopes. There are far too many threads on these forums with people complaining about star clipping when using the ASI1600 on Hyperstar or RASAs. In general, I think this is a particular issue with these super fast scopes in general...star intensity grows much faster when you enlarge the aperture and reduce the focal length than extended object signal...so you clip faster and faster, while your background signal doesn't grow fast enough, and you end up with dynamic range problems.

 

The best cameras to pair with a RASA or Hyperstar are either cameras with huge pixels and massive FWCs, or cameras with tiny pixels and large FWCs (which, in fact...at minimum gain, the native FWC of the IMX183, when scaled out to the pixel area of a KAF-16803...is a MONSTROUS 210,000e-). So basically, FWC is of supreme importance with RASA/Hyperstar. The IMX183 gives you tons of FWC on a unit-area basis. The ASI1600 does not. 


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#16 andysea

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Posted 07 June 2020 - 03:06 AM

Thank you Jon.
With the 1600 I use 30s subs at gain 139 and I don’t seem to have too much clipping. For some targets, like m45, I do HDR and that takes care of the bright stars. Are you saying that I should use gain zero with the 183?

#17 Jon Rista

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Posted 07 June 2020 - 04:24 AM

Thank you Jon.
With the 1600 I use 30s subs at gain 139 and I don’t seem to have too much clipping. For some targets, like m45, I do HDR and that takes care of the bright stars. Are you saying that I should use gain zero with the 183?

I would use zero gain with either camera. With a RASA, you get so much signal so fast, you can benefit from having that full native FWC at your disposal. You could probably get 150 second subs with the ASI1600 at minimum gain. If this is OSC, then  you should be getting plenty of signal to bury the read noise, quantization error, everything at minimum gain. You should be able to get even longer subs with the ASI183 at minimum gain, since it has smaller pixels, an in unit-area terms around 830e- greater FWC. The higher Q.E. will counteract just how much longer a bit, but you might be able to get 240 second subs with an ASI183 at minimum gain?

 

Looking at some of your RASA images on Flickr, you don't seem to have as big a problem with clipping as a lot of people have had with the ASI1600. That said, you do seem to be clipping a decent number of stars. I think, at the very least, it might be an interesting experiment to see how things go at Gain 0, and even maybe Gain 76. You would have more FWC and more dynamic range at both gains. Well, both would be 12 stops, one would have twice as much FWC as unity and the other 4.9x as much. That increase in FWC is what is going to affect exposure length. So you could try 60 second subs at Gain 76 on the ASI1600. Twice the exposure length, but you have increased dynamic range as well, so you should actually clip fewer stars. Gain 0 is interesting, in that the DR is limited by the bit depth. So, it isn't like you just chop off the top of your signal and clip a bunch of stars...the loss in DR is distributed throughout the signal range. So you may find that you clip even fewer stars at Gain 0...but, I'd experiment and see. 


Edited by Jon Rista, 07 June 2020 - 04:30 AM.



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