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M3 with a Stellarvue SV80 and ZWO ASI183MM

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#1 james7ca

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Posted 27 February 2020 - 09:03 AM

Taken on the morning of February 26, 2020 using a Stellarvue SV80ST2 with 0.8X reducer (f/4.8) and a ZWO ASI183MM Pro camera (lowest read noise, ten minutes total integration time, 20 x 30s, for each RGB channel). Guided capture on an A-P Mach1GTO mount.

 

It was a balmy but somewhat windy morning (Santa Ana conditions) with M3 fairly high in the sky but under red/orange zone light pollution. I have about eight times as many subs to process and run through PixInsight's subframe selector (some from a previous night) so in the end I may end up with about one hour for each channel (after the "bad" subs are thrown out, the seeing conditions weren't very good and for this first pass I just selected the subs sequentially with no quality assessment).

 

Image capture with Sequence Generator Pro and guiding with PhD v2. Image processing with PixInsight and Photoshop CC2019.

 

Near the bottom center you can see -- faintly -- NGC 5263, a 14th magnitude spiral galaxy. The bright star toward the lower left is HR 5145 a 6th magnitude, orange-red giant that is nearly twenty-two times the diameter of our sun. The limiting magnitude on this reproduction seems to be somewhere between magnitude 16 and 17 (the faintest stars).

 

The reproduction scale is 3.8 arc seconds per pixel (capture scale was 1.29) and the median FWHM on the mono-converted, linear RGB master was 6.1 arc seconds (didn't I just say that the seeing were pretty bad?) with a median eccentricity of 0.3 (thus, big and round stars).

 

C&C Welcomed.

Attached Thumbnails

  • M3 with SV80 and ASI183MM (small).jpg

Edited by james7ca, 27 February 2020 - 09:46 AM.

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#2 Astroman007

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Posted 27 February 2020 - 09:47 AM

Great image!

 

Never ceases to amaze me, what one can see and the images one can obtain through these fine small apos.


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#3 james7ca

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Posted 27 February 2020 - 10:17 AM

Martin, thanks for the notice.

 

However, maybe not so "fine" of an APO given that with the Stellarvue 0.8X reducer (since discontinued) and under better seeing conditions the stars come out smaller but not very round. The reducer also adds a little chromatic aberration which I know isn't present on axis without the reducer. That said, the SV80ST2 is the most color-aberration-free refractor that I have, definitely better than my Tele Vue NP127is (although the latter still does pretty well in terms of chromatic aberration and on-axis the stars are mostly round).

 

Oddly enough, it's possible that when using the "best" subs (particularly from the previous night, not used in the above image) that I may get a less good looking result because the poor seeing rounded out the stars in this first image. So, it might be the case of either big and round stars or smaller but misshaped stars.


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#4 bobzeq25

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Posted 27 February 2020 - 11:19 AM

Excellent for the conditions.  When the FWHM of my subs is over 5 arc sec, I just close the observatory, and go do something else.  <smile>



#5 james7ca

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Posted 27 February 2020 - 11:34 AM

Excellent for the conditions.  When the FWHM of my subs is over 5 arc sec, I just close the observatory, and go do something else.  <smile>

I think if I use PI's subframe selector and cut pretty deeply into my set of images (from both nights) then I should be able to get the FWHM down to about 5 arc seconds. However, we had a pretty radical shift in the weather from Monday to Tuesday and on the first night the seeing was better but the humidity was nearly 100% and I actually had some problems keeping the dew from forming on the SV80 (we almost had fog and the ground was still wet from the rain over the weekend). Then on Tuesday we had a dry Santa Ana condition with moderate winds and the seeing was much worse, but the transparency was much better.

 

I have run subframe selector on some of the subs from the second night and several had FWHMs of over 8 arc seconds. One (or two) were even at 9 arc seconds which I think is a new record for me, at least when using a scope the size of the SV80. I need to go back and look at some of those outliers, since it may have been a guiding error or something beyond bad seeing (focus seemed to hold and there was no obvious trend of drift in the star sizes -- they were all pretty bad with a few real clunkers).



#6 namh

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Posted 27 February 2020 - 12:50 PM

The conditions here by the coast are really wild aren't they? Like you say, moist and clear and then dry and breezy. Rarely dry and clear! I think it is worth trying even in not so ideal conditions certainly if you automate.

 

JB



#7 james7ca

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Posted 27 February 2020 - 08:40 PM

The conditions here by the coast are really wild aren't they? Like you say, moist and clear and then dry and breezy. Rarely dry and clear! I think it is worth trying even in not so ideal conditions certainly if you automate.

 

JB

Actually, given the dreaded marine layer from the Pacific the only nights that you can be really sure will be clear are during a dry Santa Ana condition. So, we do get dry and clear nights and those probably make up a high percentage of our usable nights. I don't know the ratio, but it could even be half if you count only the nights that stay clear from dusk till dawn. Unfortunately, during a Santa Ana the seeing conditions are always very poor. So, it's either high humidity from the ocean with very likely overcast or clear and dry with poor seeing.



#8 namh

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Posted 27 February 2020 - 08:54 PM

As you say; dry, clear, and breezy equals poor seeing. I only wish I had more of the dry and clear nights, no wind. Often I am clear and at 80-90% humidity which you know can presents problems.

 

JB




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