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#1 mistateo

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Posted 26 May 2019 - 02:03 AM

First break in the clouds for the month of May last night. Snuck out and got 2 hours on M92, of which I kept the best 90 minutes to stack this image. Very minor clipping in the core as I was shooting at unity gain and needed 3 minute exposures (wasn't willing to change any params and reshoot darks). I think it came out pretty well.
 

Higher res version here: https://flic.kr/p/2g2LGhU

 

RGB-Proc-DS.jpg
 

Scope: Orion Eon 120mm @765mm FL
Camera: QHY247C
Integration: 30 x 180s
Stacked in DSS, processed in PixInsight.


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#2 elmiko

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Posted 26 May 2019 - 02:11 AM

Beautiful Star cluster! You did a great job on the processing.

Mike


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#3 Mert

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Posted 26 May 2019 - 03:24 AM

That came out beautiful, very sharp focus, nice starcolors

and many small background galaxies in there!

Well done.

Mert


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#4 james7ca

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Posted 26 May 2019 - 04:08 AM

That looks nice. What flattener/reducer did you use?


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#5 mistateo

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Posted 26 May 2019 - 06:23 AM

That looks nice. What flattener/reducer did you use?

The Skywatcher .85x


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#6 james7ca

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Posted 26 May 2019 - 10:45 PM

The Skywatcher .85x

Thanks. I've looked at the full-sized image and the star shape is very good even in the corners which I think is pretty amazing for an APS-C format. The pixel count seems to be slightly below what would be for the entire QHY247C format (6024 x 4024, your image is 5760 x 3848), so did you crop or resize the image?

 

All that said, I do see a very mild purplish bias around some of the brighter stars and on close inspection there seems to be a somewhat "hard" transition between some of the star cores and their outer color halos (which I think could be addressed in processing). However, the image quality is very good and amazingly so for a reasonably priced doublet refractor. What's particularly impressive is the flatness of the field. Did you have to make any adjustments in the spacing or tilt when using the Skywatcher reducer?



#7 mistateo

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Posted 27 May 2019 - 01:13 AM

Thanks. I've looked at the full-sized image and the star shape is very good even in the corners which I think is pretty amazing for an APS-C format. The pixel count seems to be slightly below what would be for the entire QHY247C format (6024 x 4024, your image is 5760 x 3848), so did you crop or resize the image?

 

All that said, I do see a very mild purplish bias around some of the brighter stars and on close inspection there seems to be a somewhat "hard" transition between some of the star cores and their outer color halos (which I think could be addressed in processing). However, the image quality is very good and amazingly so for a reasonably priced doublet refractor. What's particularly impressive is the flatness of the field. Did you have to make any adjustments in the spacing or tilt when using the Skywatcher reducer?

The 6024 x 4024 includes a 24 pixel "overscan" area on each axis, so the usable frame is 6000 x 4000 (if you don't dither, which I did).  So it is cropped, but the extra little bit of area won't change the star shape.  I have been working with this scope for 2 years now, it is a good sample of the 120mm f/7.5 doublet.  I have a moonlite focuser and have adjusted the collimation of the drawtube so that a (well collimated) laser hit the objective dead center.  I also am using all threaded connections (including a moonlite adapter from drawtube thread to reducer thread), so tilt is not really an issue with my setup.  I have the back focus set at 55.5 mm (I am using a 2mm thick uv/ir cut filter).

 

The minor purple fringing is just the nature of the beast with the brighter stars and a doublet when using a OSC camera.  When using my ASI1600 with LRGB, I see no fringing at all, given relatively similar FWHM values between channels.  The hard transitions in the stars were likely a result of my curves adjustments.  The centers of all reasonably bright stars were clipped initially and there was VERY SLIGHT clipping in the core of the cluster due to my exposure.  So I tried to smooth it over manually with curves adjustments (tried HDR transformation in with various layer amounts, but they just made the cluster look dead, no glow and I didn't like it).

I have 3 imaging scopes and this one gets the most use, it was a great deal.  It will be a difficult one to upgrade from.  I think a Tak TSA120 would be a worthy step up, but that can wait until I replace my Atlas Pro and EQ6-R with better mounts.



#8 james7ca

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Posted 27 May 2019 - 01:38 AM

Okay, thanks for the additional details. Like I said, your APS-C coverage looks very good (better than any scope that I've ever used, including my Tele Vue NP127is).

 

You might be able to smooth out the cores of the larger stars by doing a selective mask with a slight blurring of the star's center. If done carefully that can even restore some color to the center of the star.



#9 mistateo

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Posted 27 May 2019 - 01:03 PM

Okay, thanks for the additional details. Like I said, your APS-C coverage looks very good (better than any scope that I've ever used, including my Tele Vue NP127is).

 

You might be able to smooth out the cores of the larger stars by doing a selective mask with a slight blurring of the star's center. If done carefully that can even restore some color to the center of the star.

That is concerning to hear.  I thought the NP127is was corrected for full frame cameras, but I just read on the OPT site it is corrected for 30mm.  The IMX193 is 28mm diagonal, which is pretty standard size for APS-C I think.  I know Ross (RGSalinger) has the same scope (NP127is), but last time I saw it, he had an ASI1600 on it, so not sure if he has used an APS-C sized sensor with it or not.  It can't be a spacing issue being the petzval design has the flattener already built in right?



#10 james7ca

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Posted 27 May 2019 - 09:29 PM

As for Tele Vue's NP127is, it's certainly not a bad design but I think the original formulation was done for visual work and for that reason it kind of struggles with astrophotography (even though it has been updated with the so-called imaging systems tweaks). It's also a VERY old design and I don't think it can really be compared to more modern, triplet APOs that are using FPL-53 glass.

 

In terms of field coverage, yes, the prime focus doesn't need any kind of flattener or special spacing requirement unless you want to go full-frame and for that Tele Vue sells their Large Field Corrector (LCL-1069). However, I purchased that with some hope that it might improve the field coverage even on an APS-C format and I found that it made little to no difference (on that smaller format, but I don't have a full-frame camera to verify that it helps there, although I'm sure it does).

 

That said, I suspect that the NP127is (and the smaller, but similar, NP101is) would work very well as a fast astrograph if paired with a large format CCD camera that had very large pixels. In fact, that's probably how Tele Vue expects it to be used (given the promotion of their NP127fli that is often paired with the FLI ProLine 16803 camera that has 9 micron pixels). However, with modern, smaller-pixel CMOS cameras I think the Nagler-Petzval design is beginning to show its limitations (as is true with many scopes).

 

For sample field coverage on APS-C you can check here:

 

  https://www.cloudyni...y/#entry6627920

 

  https://www.cloudyni...s/#entry7301481

 

Okay, to show the better side and the kind of quality that can be had over a limited field with the NP127is you can look at the following threads:

 

  https://www.cloudyni...6/#entry8059803

 

  https://www.cloudyni...dpost&p=7976663

 

  https://www.cloudyni...dpost&p=7137761

 

  https://www.cloudyni...s/#entry7893318

 

I should also add that for visual work the NP127is offers very good performance (perhaps nearly flawless).


Edited by james7ca, 27 May 2019 - 09:58 PM.


#11 mistateo

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Posted 28 May 2019 - 12:56 PM

Interesting threads.  I want to try that tool on all of my scopes with the QHY247 to see what values I come up with.  Have you been able to find anyone else with the NP127|101 that has done the same test on their scopes with an APS-C or larger sized sensor?



#12 james7ca

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Posted 28 May 2019 - 06:36 PM

Measurements, yes, that's one caveat I have to add when comparing your image to what I've seen on my NP127is. Although your stars look well-formed, round, and relatively small without something like a FWHM measurement it's kind of hard to make an absolute judgement on image quality. That being, small and round stars are difficult (my criteria for the NP127is) while just being round may not tell you much. On nights with poor seeing I can get very round stars but they are also significantly larger than when I have a night with good seeing (and focus). The differences can be very dramatic, going from a FWHM of five arc seconds on a "bad" night to something under two arc seconds on a good night (or even approaching one arc second in H-alpha).

 

That said, your stars look very good for roundness and the center of the globular cluster looks well resolved, suggesting pretty small and tight stars. But, I'm looking at a fully processed image which can also introduce its own problems when evaluating image quality. So, if you do take some FWHM and eccentricity measurements remember that those have to be taken on the linear original or master integrations before any form of processing (except calibration and possibly stacking). That is, no histogram adjustments and certainly nothing like sharpening or deconvolution.

 

In the end, however, it's really how the images appear to your own eyes. Meaning if it looks fine to you then things like FWHM and eccentricity really don't matter much.

 

In terms of other samples for the NP127is, I haven't been able to find much. Most images you see are fully processed and that can change the image quality in both good and bad ways. Also, it isn't that common to find image samples that are presented at full, original scale and then there is the issue of the pixel size of the camera and the linear size of the sensor. The thread on image quality that I started several years ago shows the results from at least one other NP127 (the older non-IS version), but in that case the owner was using a fairly small sensor.

 

The only other thread on Nagler-Petzval image quality that I know of here on CN ended when the owner returned his scope with a partial credit (from the vendor) after first allowing Tele Vue to check and adjust the scope back at the factory. He wasn't happy with the full-field coverage given by the NP101is and finally purchased a Takahashi as a replacement (given a 60% credit on the NP101is). Here is that thread:

 

  https://www.cloudyni...s/#entry6899559

 

All this said, I think I've produced some very good images with my NP127is. In fact, if you search here on CN you'll probably find that I've been the principal contributor of images that have been taken with that scope (perhaps even if you count everything from both the NP127 and the NP101). Of course, samples vary, so I can't really say whether my experience represents the best or worst of what Tele Vue has to offer. I doubt, however, that it is either the best or worst.


Edited by james7ca, 28 May 2019 - 09:08 PM.



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