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NV: Tips for Viewing Globular Clusters

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#1 chemisted

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Posted 29 September 2017 - 11:31 AM

Fifteen years of observing these marvels has taught me a few things that I would like to share.  First, and foremost, if your instrument of choice is a refractor or cassegrain reflector NEVER use a mirror diagonal with dielectric coatings.  They have no reflectance beyond visual red and you will loose the majority of the photons from the cluster's red giants. Recently I have even seen RCs for sale with this kind of coating on its mirrors.  Avoid these if you want to use NV devices on globs.

The remaining items are in no particular order of importance.  I try to operate at a power sufficient to reach the maximum resolution of my objective. This is 13X its diameter in inches.  For my 10" that is 130X (f/13).  I will do less if I want a larger FOV.  I have found it is a good thing not to be in a hurry.  Many times clusters sharpen up a lot with concentrated attention.  The faintest stars are tiny pinpoints that sometime evade casual inspection.  As far as sky conditions go, the brightest globulars are wonderful under just about any sky but the fainter ones really benefit from good transparency.  As always, higher in the sky is better.  Last of all, I have never found the need to use a filter of any sort.

 


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#2 Eddgie

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Posted 01 October 2017 - 09:20 AM

More and more, for stellar targets even from my light polluted home, I am running unfiltered and on Globulars, I agree that Unfiltered has I think given me the best view of this particular class of target.  (If not the best, then at least no worse than 610nm and I think better than 650nm long pass).



#3 Jeff Morgan

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Posted 01 October 2017 - 09:46 AM

Interesting find. Although currently not in use, I do have an Astro-Physics MaxBrite dielectric diagonal so I thought I would check out the specs on it.

 

Unfortunately, they say how well it does in the visual range (99% from 4000a to 7000a) but not how fast performance falls off outside that range.

 

Since the NV technology is sensitive down to 900nm, I think this would be a very real concern.

 

An A/B comparison with silvered mirror or prism diagonals could be ... illuminating smirk.gif


Edited by Jeff Morgan, 01 October 2017 - 09:46 AM.


#4 chemisted

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Posted 11 January 2018 - 12:33 PM

Interesting find. Although currently not in use, I do have an Astro-Physics MaxBrite dielectric diagonal so I thought I would check out the specs on it.

 

Unfortunately, they say how well it does in the visual range (99% from 4000a to 7000a) but not how fast performance falls off outside that range.

 

Since the NV technology is sensitive down to 900nm, I think this would be a very real concern.

 

An A/B comparison with silvered mirror or prism diagonals could be ... illuminating smirk.gif

Baader has relatively new hardened silver diagonals that reflect beyond 1 micron and have even been reported to enhance visual red LINK.  I have just ordered one and hope to be able to report on it in the not-too-distant future.


Edited by chemisted, 11 January 2018 - 12:34 PM.


#5 11769

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Posted 11 January 2018 - 04:55 PM

Very valid concern and without sweeping a mirror on a spectrometer, it's good to be keep this limitation in mind. That being said, 400-700nm or 400-750nm is a common nominal reflectance range specified for broadband dielectric filters. Not uncommon for a mirror to still exhibit adequate (>98%) reflectivity out to 850nm or even a bit beyond.

 

For the curious, a quick and dirty qualitative test would be to shine an IR illuminator through the mirror coating. Dielectric mirrors also act like dielectric filters and what doesn't get reflected passes through. If the IR illuminator (~850nm typically) is heavily attenuated passing thru the coating then the mirror is probably an adequate candidate for use with NV devices.


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#6 Jeff Morgan

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Posted 11 January 2018 - 05:32 PM

Baader has relatively new hardened silver diagonals that reflect beyond 1 micron and have even been reported to enhance visual red LINK.  I have just ordered one and hope to be able to report on it in the not-too-distant future.

 

 

Outstanding! Look forward to hearing your report.



#7 PEterW

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Posted 12 January 2018 - 08:31 AM

I found not brilliant improvement on galaxies and a “ruined view” (lacked that sparkly, dusted puff ball look) when I looked with a big Obsession... which has dielectric enhanced secondary..... hmmm, may have hindered the comparison. Need to find another big dob used for a replay.

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#8 chemisted

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Posted 28 January 2018 - 08:23 AM

My diagonal comparison has turned out interesting.  The test subject was NGC 2158 adjacent to M35.  Although not a globular, this is an ancient cluster with highly evolved stars that make it a good surrogate. The telescope was my 140 mm refractor and I first used a wide field view of about 1.5 degrees with my NVD Micro.  The new Baader BBHS (silver) diagonal showed the little cluster as an inviting bright glow that immediately drew the eye to it.  For comparison I used two TeleVue diagonals.  The aluminum also gave an inviting glow but, with the dielectric EverBrite, only a barely detectable mist was visible.  Going to higher power with a 2.5X PowerMate (34' FOV) gave the following results.  The Everbrite showed 7 stars where the cluster was but nothing else easily.  The aluminum diagonal showed many cluster stars that were not visible with the Everbrite including quite a few double stars.  I picked one of these to follow as I changed to the BBHS.  The silver diagonal showed it to be triple, not double!  There were quite a number of additional stars in the cluster as well.  So, in a cluster with evolved reddish stars,  the silver coating is showing me more than standard aluminum and both of these are vastly superior to dielectric.  The young cluster M35 was the same with all three diagonals using the wide field view.


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#9 Jeff Morgan

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Posted 28 January 2018 - 10:31 PM

Dang. There goes another $500.

 

wink.gif

 

Nice report though, look forward to hearing more.



#10 moshen

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Posted 29 January 2018 - 06:08 PM

Oh boy..

 

Looks like I might be in for one of these diagonals too. So $$ though. bawling.gif



#11 chemisted

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Posted 30 January 2018 - 09:04 AM

Oh boy..

 

Looks like I might be in for one of these diagonals too. So $$ though. bawling.gif

I purchased the T-2 version which is quite a bit less than the 2 inch.  I already had a variety of nosepieces and I wanted the shorter path length this smaller diagonal affords.  The clear aperture is about 33 mm and if this is good enough for your intended purposes, as it was for mine, it is a good way to save some bucks.  I got the 1.25" helical eyepiece holder to give me some flexibility and fine focus potential.  The diagonal does not have an eyepiece stop so you have to be careful to not damage the mirror.  For example, I screwed the helical element out before putting in the 2.5X PowerMate just to be sure it did not contact the mirror.



#12 GeezerGazer

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Posted 01 February 2018 - 01:38 AM

Interesting about dielectric coated diagonals.  The Alpine Astro website (USA distributors of Baader products) indicates that the BBHS, Baader Broadband Hardcoated Silver diagonal, uses dielectric coatings OVER the silver application to seal it.  These clear dielectric coatings must not impede reflectivity of the longer IR wavelengths.  

 

I used a BBHS diagonal for about a year but found no advantage through my 140 or 100mm refractors when compared to my Baader Zeiss prism diagonal... but my comparisons were at visual wavelengths just as I started using NV.  It would be interesting to test the silver coated and prism diagonals to see if there is a difference between their presentation of the IR wavelengths.  I cannot compare them because I sold the BBHS.  

 

When I have clear weather again, if EVER, I'll compare my prism diagonal to the straight-through image (no diagonal) with my Mod 3C.  That should reveal if the prism or its coatings are eroding transmission of the IR wavelengths.  



#13 chemisted

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Posted 01 February 2018 - 06:09 AM

Interesting about dielectric coated diagonals.  The Alpine Astro website (USA distributors of Baader products) indicates that the BBHS, Baader Broadband Hardcoated Silver diagonal, uses dielectric coatings OVER the silver application to seal it.  These clear dielectric coatings must not impede reflectivity of the longer IR wavelengths.  

 

I used a BBHS diagonal for about a year but found no advantage through my 140 or 100mm refractors when compared to my Baader Zeiss prism diagonal... but my comparisons were at visual wavelengths just as I started using NV.  It would be interesting to test the silver coated and prism diagonals to see if there is a difference between their presentation of the IR wavelengths.  I cannot compare them because I sold the BBHS.  

 

When I have clear weather again, if EVER, I'll compare my prism diagonal to the straight-through image (no diagonal) with my Mod 3C.  That should reveal if the prism or its coatings are eroding transmission of the IR wavelengths.  

Yes, the overcoats are absolutely clear to the near infra-red.  The Baader  home site indicates the BBHS diagonals go out to 2 microns so if they hold up with time they should be excellent for NV work on old clusters.

 

I haven't investigated prism diagonals since I am working at f/5 for my wide-field view.  It will be good to hear how your experiment goes.  



#14 moshen

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Posted 03 February 2018 - 06:09 AM

I did some viewing of NGS2158 tonight with my Mod3 and Televue Everbrite and just straight through a refractor without diagonal. I also used a 650nm bandpass for LP. I seem to notice what chemisted mentioned. Straight through I see more stars and a slight mistyness is there that is not visible with the dielectric.

 

It was tough to compare because straight through viewing had me on the ground in an awkward position but I think there's enough personal evidence for me that switching off a dielectric may be worthwhile when using an image intensifier.

 

I think I'll pick up the Baader T2 bbhs diagonal.



#15 chemisted

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Posted 19 May 2018 - 08:58 AM

Diagonal Comparison - Part 2:

 

Using the 140 mm refractor with the 2.5X Powermate I recently spent some time on probably my most studied globular.  M3 with the TeleVue EverBrite shows so many stars that you think you are getting a good view.  However, I have favorite double stars in this cluster that were not showing well at all.  Changing to the Baader silver made these doubles easy.  In particular, there is a bright double that had an adjacent star that came and went with the TeleVue whereas the Baader showed it easily as well as its fainter companion.  So it is really a double double that was severely compromised by the dielectric coatings.  Overall, with the EverBrite, the cluster looked dull compared to the view with the silver diagonal.  It reminded me of using the Micro with an H-alpha filter on a camera lens and then stopping the lens down one click while looking at a nebula.  Details were still there but I immediately opened the lens up to take the shades off!


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#16 pwang99

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Posted 20 May 2018 - 08:07 AM

Wow, had never considered this.  I have an AP Maxbright as well as a Baader click-lock.  I tend to use the latter for doing NV.  It has never occurred to me that dielectric coatings could limit the range.  The AP page says "Reflectivity is above 99% over the entire 4000 to 7000 Å photo-visual range", which doesn't bode well...

 

Will need to do a comparison next time I'm out with the refractor.




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