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Borg 90FL f4 astrograph versus Stellarvue SV80ST-25 triplet at f/4.8

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#126 StevenBellavia

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Posted 04 September 2018 - 01:13 PM

This was in the f/6 configuration, using the 1.08X DG-multi-Flattener. 

 

Full resolution image here:

 

https://www.astrobin.com/363807/

I added  3 hours of color data, for a total of 7 hours, combined:

 

https://www.astrobin.com/363807/

Attached Thumbnails

  • NGC_7635_Aug-28_and_Sep-03_2018_Borg_90FL_f6_ZWO_ASI_183MM-Ha_36x300sec_and_183MC_120x90sec_2D_4CN.jpg

Edited by StevenBellavia, 04 September 2018 - 02:39 PM.

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#127 StevenBellavia

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Posted 04 September 2018 - 01:14 PM

Please, can you show me the pic of your setup exactly as you use it for imaging?

This is a photo of the Borg 90FL in its f/6 configuration, using the #7108, 1.08X DG Multi-flattener, along with a Starizona filter slider and a ZWO 5 position manual filter wheel, with a ZWO ASI 183 camera.

 

I hope to get a photo of the f/4 configuration as soon as I can find one or take one again.

 

I highly recommend the scope at f/6.  At f/4, it is a little more challenging to get good focus, color and star shapes.  The spacing has to be accurate, and even then, it is not as nice as the f/6.

Attached Thumbnails

  • Borg_90FL_f6_IMG_5163.JPG

Edited by StevenBellavia, 04 September 2018 - 09:07 PM.

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#128 25585

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Posted 08 September 2018 - 04:00 AM

If you re-purchased the same components you originally had, and the scope is better at F6, is there a possibility that somehow F5.6 lenses were fitted into a F4 by mistake in the factory?



#129 StevenBellavia

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Posted 19 September 2018 - 07:55 AM

If you re-purchased the same components you originally had, and the scope is better at F6, is there a possibility that somehow F5.6 lenses were fitted into a F4 by mistake in the factory?

The f/4 versus f/6 configuration happens downstream of the 2-element objective, via screw-in threads, so I don't think there is anything to "mix up"..

 

For f/4 you use this:

 

https://www.sciencec...s/html/7872.htm

 

For f/6, you use this:

 

https://www.sciencec...s/html/7108.htm

 

Both are downstream.

 

I think this scope can be used at f/4, if you are willing to have some chromatic aberration.  I think it is a similar trade-off to using an SCT at f/6.3, with a reducer, where you get more vignetting and internal reflections, but is a trade-off to imaging with much longer exposures in the slower, but better f/10 configuration.

 

Except for very high-end refractors (like Vixen or Takahashi), I don't think there are many f/4 refractors out there, so it is nice to be able to use the Borg this way. As I said in an earlier post, fixing color in post-processing is much easier than fixing blurry or distorted stars. I think the Borg has razor sharp focus, and shows more details than any refractor I have owned so far, which is why I went back to it.


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#130 StevenBellavia

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Posted 13 October 2018 - 10:20 AM

Last night was our first clear night in a very long time, and even then, it only lasted 2 hours before clouds came and rain a little later.

 

I had issues with my ZWO camera, switching gain and offset all on its own, and lost the first 90 minutes of data collection.

 

The following image is the last 35 minutes of data, calibrated and stacked  with only 7 frames. I also did not crop the image.  The darker regions on the borders are from dithering.

 

Maybe not the best elephant's trunk image,  but also not bad for only 35 minutes of data, with 7 frames. (no flat frames either)

 

I think it really shows what the Borg 90FL at f/4  can do using narrowband filters and a high-resolution (small pixel) camera (ZWO ASI183MM).

 

This is the full resolution un-cropped image (Note black borders are from dithering)

 

https://www.flickr.c...57699147279432/

Attached Thumbnails

  • IC1396_Ha_7x5min_G140_O20-uncropped_4-rotate-resize_4CN.JPG

Edited by StevenBellavia, 13 October 2018 - 02:26 PM.

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#131 StevenBellavia

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Posted 14 October 2018 - 04:28 PM

Last night was our first clear night in a very long time, and even then, it only lasted 2 hours before clouds came and rain a little later.

 

I had issues with my ZWO camera, switching gain and offset all on its own, and lost the first 90 minutes of data collection.

 

The following image is the last 35 minutes of data, calibrated and stacked  with only 7 frames. I also did not crop the image.  The darker regions on the borders are from dithering.

 

Maybe not the best elephant's trunk image,  but also not bad for only 35 minutes of data, with 7 frames. (no flat frames either)

 

I think it really shows what the Borg 90FL at f/4  can do using narrowband filters and a high-resolution (small pixel) camera (ZWO ASI183MM).

 

This is the full resolution un-cropped image (Note black borders are from dithering)

 

https://www.flickr.c...57699147279432/

This is with another 2 hours of data, adding OIII and a little more Ha data, and doing bi-color RGB synthesis.

 

The full resolution image and details are here:

 

https://www.astrobin.com/371763/

Attached Thumbnails

  • IC1396_Borg_90FL_f4_ZWO_ASI183MM_Ha-70min_OIII-85min_resized_4CN.JPG

Edited by StevenBellavia, 14 October 2018 - 04:29 PM.

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#132 EricCCD

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Posted 14 October 2018 - 05:12 PM

Looking good! Even the corner stars look pretty good with these versions!

 

Eric



#133 StevenBellavia

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Posted 15 October 2018 - 08:10 AM

Looking good! Even the corner stars look pretty good with these versions!

 

Eric

Thanks Eric!

Yes, these images only have very minor cropping to eliminate the overlap from dithering, and the stars were good to the corners, even un-cropped.

Spacing and focus are critical at f/4, but achievable. I have left the spacing as I indicated on a previous post, but re-posting again here for others with this same scope.

smile.gif

Steve

Attached Thumbnails

  • Borg_90FL_f4_Spacing_Starizona_+_ZWO.JPG


#134 sparrowhawk

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Posted 17 October 2018 - 03:12 PM

Hi Steve,

It’s been a while since I checked in on your thread. It appears the initial issues are resolved. I’m not sure I followed everything, could you summarize what you determined the original issue was?

Kind regards,

Shawn

#135 StevenBellavia

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Posted 18 October 2018 - 11:16 AM

Hi Steve,

It’s been a while since I checked in on your thread. It appears the initial issues are resolved. I’m not sure I followed everything, could you summarize what you determined the original issue was?

Kind regards,

Shawn

Hi Shawn,

 

A summary is an excellent idea!

 

Let me work on that, so it is concise and makes sense.

 

I'll get back to you and the thread shortly.

 

Steve



#136 old_enough

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Posted 18 October 2018 - 01:41 PM

I really like your images! I‘m a purely visual observer but your wider than usual framing is immersive.

All the best,
Sebastian
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#137 StevenBellavia

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Posted 18 October 2018 - 03:21 PM

My experience with a Borg 90FL f/4 astrograph, as of today's date:

 

The #66900 Borg 90FL f/4 astrograph set, has a 2-element objective with a fluorite front element, and a 4-element, 0.72X reducer-flattener downstream of the focuser, so it moves when focusing.

 

1. I was initially disappointed with the scope, as the bright stars had purple halos, when at "best-focus", using a modified DSLR and no filters.

 

2. It also produced purple halos with a stock DSLR, but not quite as bad as with the modified DSLR

 

3. You can reduce or eliminate the purple halos with a very slight de-focusing towards intra-focus (sensor further in).  But that was not acceptable to me. I like ultra-sharp focus.

 

Many of my test images are shown here:

 

https://www.flickr.c...th/45408700801/

 

4. I am not certain of this, but I believe at best focus, the Borg 90FL f/4 is sharper than the Stellarvue SV80ST-25. On the one night that I swapped them back and forth, within minutes of each other, most stars in the Borg images had a slightly lower FWHM.  Both are excellent scopes, with pros and cons for each.

 

5. When using stock 100nm bandwidth R-G-B filters, the blue will cause bloated stars, even at best-focus, and the red will too, but not nearly as bad. If a more aggressive luminance filter is placed in-line with the R-G-B, such as the IDAS UIBAR-III, the stars will then be similar, if not identical in size.  What I find to be the best resolution of this issue is to use a Baader semi-apo filter placed in-line. This is also true for monochrome, where the semi-apo filter becomes the luminance filter, as well as for OSC cameras. This 3-image set clearly shows the benefit of using an aggressive luminance filter (425 to 665nm) and the semi-apo filter:

 

https://www.flickr.c...157693331796960

 

And this set and animated GIF shows the benefit of using the semi-apo filter with the blue (It also shows some red bloat, which I did not try with the IDAS or semi-APO filter, but should have):

 

https://www.flickr.c...157670886053658

 

6. When used for narrowband, the Borg performs extremely well. This is where having an f/4 astrograph really pays off. I am using a small pixel (2.4 micron) pixel camera, to get 1.4 arc-sec/pixel sampling, so speed is critical. I can barely swamp the read noise by 2X in 300 second sub-frames at f/4.  The f/4.8 (my Stellarvue) or f/4.9 (my William Optics Petzval),  would need 430 to 450 seconds, and also the same ratio of total integration time, that is 1.4X to 1.5X longer, to get the same SNR. That is a significant difference.

 

Summary:

 

In the end, I chose the Borg over the Stellarvue SV80ST-25 for imaging, primarily because the Borg is about 1/2 the weight as the Stellarvue, making it much easier to auto-guide. I also prefer the clearly marked, large helical focuser where each index mark is approximately 120 microns, and you can adjust to locations in-between index marks, and is very repeatable. That is, if you return to the same location, you will have the same focus (if everything is at the same temperature).

 

The Borg 90FL F/4 astrograph has noticeable chromatic aberration, especially towards blue. But I believe this is solved by using appropriate filters.  None of the bloating or halo issues are present in narrowband HA and OIII (I have not tried SII or H-beta, but since close to Ha and OIII, I would not expect any issues).

 

In my opinion, based on many tests and currently over 50 hours of imaging with this scope, that the overall benefit of having a fast, ultra-light, well-built scope with sharp focus, flat field, and an excellent focuser, outweighs the other shortcomings.

 

If you want to see calibrated, stacked and post-processed images taken so far, go to my astrobin site and sort by gear, and choose Borg 90FL.

 

Steve

 

Disclaimer: I am not an expert on optics or digital photography.  I am a self-taught amateur, with only a few years of astrophotography experience.

 

Attached Thumbnails

  • Borg_1_4CN.jpg

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#138 james7ca

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Posted 19 October 2018 - 07:55 PM

Steve,  thanks for this report.

 

I guess it doesn't surprise me that you're seeing chromatic aberration with a fast, two-element, fluorite scope (see below). However, you'd think that they could improve that with the 0.72X reducer (4 elements), but maybe it is working hard just to give the reduction and flat field (with little regard for improving the color correction).

 

I tried a Tak FS60CB which also has a two-element fluorite objective and I was surprised at the amount of chromatic aberration that that scope produced (even at f/5.9). However, most reports suggest that it performs better when paired with the optional reducer.

 

Frankly, your experience with the Borg and my experience with the FS60CB (and a Stellarvue SV50ED) has led me to conclude that all of these relatively fast two-element designs that use either fluorite or ED glass are something that should be termed visual APOs, as they really don't provide the type of color correction that is needed for straight RGB astrophotography. You either need to work in the narrow band or add some restrictive filtering if you want to avoid the chromatic aberration.

 

However, my Stellarvue SV80ST2 (three-element FPL53) gives perfect color correction on axis, but can't cover a full APS-C field even when paired with Stellarvue's reducer (image quality issues near the corners and edges).

 

I also have a Tele Vue NP127is (four elements, with -- apparently -- two ED components) that does reasonably well in RGB, but it has about 100 microns of focus shift between the red and blue ends of the spectrum so it too is not completely color free (for photography, I don't think you'd ever see much visually). It does, however, produce acceptable images even with a one-shot-color camera so it's definitely better than either the FS60CB or the SV50ED.


Edited by james7ca, 20 October 2018 - 06:41 AM.

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#139 Fomalhaut

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Posted 20 October 2018 - 04:07 AM

 

I tried a Tak FS60CB which also has a two-element fluorite objective and I was surprised at the amount of chromatic aberration that that scope produced (even at f/5.9). However, most reports suggest that it performs better when paired with the optional reducer.

 

 

 

I have the FS-60CB with its dedicated "Reducer C 0.72x" and confirm to consider it a very nice small astrograph over the full APS-C field of my modded Canon 700d.

 

Chris


Edited by Fomalhaut, 20 October 2018 - 04:09 AM.


#140 StevenBellavia

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Posted 21 October 2018 - 08:17 PM

Steve,  thanks for this report.

 

I guess it doesn't surprise me that you're seeing chromatic aberration with a fast, two-element, fluorite scope (see below). However, you'd think that they could improve that with the 0.72X reducer (4 elements), but maybe it is working hard just to give the reduction and flat field (with little regard for improving the color correction).

 

I tried a Tak FS60CB which also has a two-element fluorite objective and I was surprised at the amount of chromatic aberration that that scope produced (even at f/5.9). However, most reports suggest that it performs better when paired with the optional reducer.

 

Frankly, your experience with the Borg and my experience with the FS60CB (and a Stellarvue SV50ED) has led me to conclude that all of these relatively fast two-element designs that use either fluorite or ED glass are something that should be termed visual APOs, as they really don't provide the type of color correction that is needed for straight RGB astrophotography. You either need to work in the narrow band or add some restrictive filtering if you want to avoid the chromatic aberration.

 

However, my Stellarvue SV80ST2 (three-element FPL53) gives perfect color correction on axis, but can't cover a full APS-C field even when paired with Stellarvue's reducer (image quality issues near the corners and edges).

 

I also have a Tele Vue NP127is (four elements, with -- apparently -- two ED components) that does reasonably well in RGB, but it has about 100 microns of focus shift between the red and blue ends of the spectrum so it too is not completely color free (for photography, I don't think you'd ever see much visually). It does, however, produce acceptable images even with a one-shot-color camera so it's definitely better than either the FS60CB or the SV50ED.

Hi James,

(funny, I just sent the link to your thread regarding the 200mm Kenko lens and ASI178 camera to a friend, before I saw that you replied to this).

 

I agree 100%

 

The Televue NP127is is a Petzval design, which I think is the best thing out there today for imaging refractors.

 

I just picked up a William Optics 71 Star-II, also a Petzval, and there is essentially no CA and it has a large flat field too. It is f/4.9.

And I also like that the correcting elements are ahead of the focuser, so you don't have to worry about precise spacing. If you are at-focus, then you are at the correct spacing, automatically.

 

I wonder if I had gotten that first, if I would have even gotten the Borg?  But now I feel I cannot part with either, which is sort of a problem for me, as I am poor at decision-making. Now every time it is clear, I have to decide on color or narrow-band, William-Optics or Borg and if Borg, f/4 or f/6 (I also have the 1.08X DG-Multi-flattener.  There is virtually no CA at f/6).

 

I also wonder if Borg should have just made a reducer-corrector to get it to around f/5 and skipped the f/4 and f/6 configurations?

 

I only dabble in optics deign as a hobby, and play around with OSLO and Zemax occasionally, and have learned that when you change lens material, radii and spacing, to get one parameter to go in a favorable direction (like field curvature), it is nearly impossible to get another parameter to not go in a bad direction (like CA, or spot size).  It's like pushing your finger into a blob of jello - one part goes in and another pops out.  Maybe they could have paid a little more attention to the shorter (blue) wavelength in their reducer-corrector design? I don't know. But I do know it is all easier said than done.

 

The Borg 90 at f/4 is perfect for narrowband. So now I can't give it up. If I do need to add color, I may do it with the William-Optics 71, which at f/4.9 will be at the same image scale and sampling, but just a little slower, which is fine for color.  And I could bin if I had to. Or just use the semi-apo filter on the Borg at f/4, which works great, and whatever light loss there is due to the narrower pass-bands of the filter, (about 30% loss in blue, some green, most of yellow, and some red too) is perhaps compensated for by the faster optics?

 

Steve

Attached Thumbnails

  • baader_semi_apo_spectrum.png

Edited by StevenBellavia, 21 October 2018 - 08:34 PM.


#141 james7ca

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Posted 22 October 2018 - 09:24 AM

I waiting to see what happens with the recently pre-announced William Optics 51mm quad (suggested to be a Petzval, 250mm F4.9).

 

Beyond that I'm considering the Borg 55FL with its reducer that produces a 200mm focal length f/3.6. However, given your recent experience with the Borg 90FL I'm not so sure it would be a good choice for RGB imaging.

 

The Kenko does fairly well in the narrow band, but it can't really handle RGB and thus I don't need another short focus scope that puts me in that same situation.

 

The IMX178 combined with a very short focus lens actually makes a decent wide-field solution. Then, if I tried my IMX183 I'd have an even wide setup, assuming that I can find a lens that can handle its 2.4 micron pixels.




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