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My Fully Optimized Globular Cluster Observing System

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#1 chemisted


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Posted 14 September 2019 - 09:50 AM

For some time I have been making changes to get the very best out of my OGS RC-10 specifically for the viewing of globular star clusters.  In 2017 I snagged an excellent NVD Micro with great response in the far red where the giant stars of these objects shine brightly.  In 2018 I added the Baader BBHS diagonal that also has remarkable reflectance at these wavelengths.  This year I was rounding out my Sky 90 system with the purchase of Takahashi's Extender-Q 1.6 X lens system.  It occurred to me that this optic is just what I was looking for to get the RC-10 to 140X and a 17' FOV.  I have tried a variety of approaches to achieve these results over the years.  J. B. Sidgwick states that to achieve full resolution of a telescopic system you need 13 times the objective diameter in inches and this arrangement gets me there.  Under these conditions my system is operating at f/15 and the Micro handles this effortlessly.


During the last new moon I had a stretch of uncommonly good evenings so I was able to try it out.  The results way exceeded my expectations.  Major clusters like M14 and M92 present themselves perfectly in this FOV and have an incredible depth to them as exceptionally faint stars are clearly displayed and separated from their neighbors.  Faint clusters similarly are now routine objects that show detail and extent that I have come to expect from their brighter brethren.  Perhaps the best example of all this is NGC 6749.  With my old equipment I described this cluster as a 4' glow with about half a dozen stars resolved.  With the Micro alone I was able to increase this to a dozen.  My last outing, which had a really wonderful clear and stable sky, showed so many stars that I quit counting at three dozen!


Recently I posted a comment on my observations of NGC 6229 with this new system over on the deep sky observing site.  It similarly describes more stars and better resolution than I have ever obtained before.


My job now is to re-observe the 100 clusters I have seen before in this new light.

Edited by chemisted, 14 September 2019 - 09:54 AM.

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#2 bobhen



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Posted 14 September 2019 - 10:33 AM

Nebula with NV gets all the press but open and globular clusters are fantastic as well. What were previously smudges now can be resolved. And in many cases, these objects now display unique characteristics.


With globulars, moderate aperture and increased image scale (I happen to use a Mewlon 210) can really deliver the goods.



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#3 jdbastro


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Posted 15 September 2019 - 01:04 AM

I use a Mewlon 250 at either f12 or f9.2 and globulars with NV are simply the bomb with this system.

M13, Mew250, Photonis 4G, 10sec 50iso, SONY ILCE-7M3


M13 Glob Her Mew250 F12 10sec 50iso
M3 GlobCluster Mew250f12 10sec 50iso


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#4 Jeff Morgan

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Posted 15 September 2019 - 11:44 AM

In my 16" f/7 Newtonian globulars are fantastic, even the fainter ones show many dozens of resolved stars.


The base magnification is about 103x and I can almost always profitably use a 1.5x barlow. I have a 2.4x barlow but atmospheric seeing is usually the limiting factor.

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#5 Organic Astrochemist

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Posted 15 September 2019 - 01:31 PM

Excellent work.

Please use these great setups on distant redshifted galaxies and tell us what you can see.

There is a large B-V color difference between cluster stars on the red giant branch and on the main sequence. It’s a shame how hard it is to capture that.

#6 chemisted


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Posted 13 October 2019 - 10:41 AM

GeezerGazer's very nice image of M22 has induced me to add this addendum to my post.  For a cluster this large a FOV of 40' works well as the image shows.  To achieve that with my equipment I attach an Astro-Physics CCDT67 reducer to the telescope side of my Baader T-2 silver diagonal (no nosepiece) and the NVD Micro directly to the diagonal with the shortest possible T-2 to C mount adapter.  This reducer also removes field curvature and I get pinpoint stars throughout with the setup coming to focus just before running out of drawtube.  


BTW, I can achieve an identical view by using my 40mm Celestron Axiom eyepiece afocally. So it really just depends on which approach I want to take on a particular night.

Edited by chemisted, 13 October 2019 - 12:11 PM.

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