A 4-Inch (102mm) Refractor Can Be An Excellent Deep-Sky Telescope: Finally Sirius B, Barnard’s Galaxy, and Much More….

All Observer’s Challenge Reports to-date:


      Walter Scott Houston, most often used a 4-inch Clark refractor for his observations of deep-sky objects while writing the monthly “Deep-Sky Wonder’s” column in “Sky & Telescope Magazine” for 46 years until his death in December, 1993.  

     When Sue French picked up DSW’s, she seemed to favor the use of a 105mm refractor for many observations over her twenty years writing the column.  

      I purchased my 102mm f/9.8 refractor in 1997, and have enjoyed using this scope over the years.  It provides for a nice velvety black background with excellent contrast of faint deep-sky objects within its reach.  

      On nights of good seeing, it will easily exceed Dawes Limit:  https://www.astronomics.com/info-library/astronomical-terms/dawes-limit/  

      Double and multiple stars have always been of interest to me, and I’ve observed hundreds of close double and multiple stars over the years with this telescope.  


The value of a correct image diagonal:

I’m now using a correct image 90º diagonal.  The view through a standard 90º diagonal, whether using a refractor, Schmidt-Cassegrain or Maksutov-Cassegrain makes it impossible (at least for me) to sketch a deep-sky object, scientifically correct. I like to make my sketches as they are truly oriented in the sky, with north at the top and west to the right on my sketch card.    



An example of my pencil sketches as following:  Iota Cassiopeia…one of my favorite triple stars.   

Iota Cas Roger inv




Rogers M-081 Inverted

Rogers NGC-2300 Inverted

Scanned Image 160920001

Roger IC 1805

M13 And The Elusive Propeller

Seeing Sirius B: 

“Finally….after more than 40 years, I was able to see Sirius B, better known as the “Pup.”  Date of observation:  March 14th 2012 

Telescope:  4-inch (102mm) f/9.8 achromatic refractor.  Conditions:  Excellent seeing and transparency           

When Sirius B was at a relatively wide separation during the 70’s,  I was using a spherical mirror, 4 1/4-inch f/10 reflector, not exactly the preferred telescope for accomplishing this feat.  I had no idea just how difficult seeing “the pup” was/would be to see, but 40 years later (2012) I would find out.  

March 14th 2012, and using a 102mm refractor:

I made preparations in my attempt to observe Sirius.  After letting the telescope cool down for at least an hour and making a few visual test…the seeing appeared excellent!   

Afteadjusted my astro-chair and removing the diagonal, I started with a magnification of 83x, but to no avail.  I increased the magnification to 232x, and keeping my eye perfectly still.  

I looked very carefully for more than an hour, when a very faint star appeared so very close to Sirius.   

Knowing the position-angle really helped.  However, I could not hold the companion, or Sirius B constantly.  It was extremely difficult.  But after 40 years, I had finally seen the companion to Sirius.  

Seeing the companion with another 4-inch refractor:   A Takahashi FC-100H, f/8 fluorite doublet during the winter of 2016:

Another sighting of the companion of Sirius using a 4-inch Takahashi FC-100H, f/8 doublet fluorite.  Sirius B was, actually “very easy” with this scope.  It was much easier with this “premium” telescope as compared to the view through my 102mm f/10 achromat…as one would expect.  

A group of six amateurs took the test using the Takahashi refractor, and what did they see?   

Using the face of a clock, and all without sharing their perceived position of the companion until everyone had the opportunity to observe.  When the last person completed their observation, all correctly identified the position of Sirius B.  

Some were very skilled observers, but a couple or more were not, however, all could “fairly easily” locate and see the companion.   

The value of a small refractor telescope:              

I have always been interested in observing with a small good quality refractor.  A refractor can present pin-point star images and a velvety black background which can improve the contrast of most deep-sky objects.  The superior contrast of a refractor can often times allow views of deep-sky objects to rival or even exceed larger telescopes of more complex designs.  

NGC 6822 (Barnard’s galaxy) in Sagittarius:

It should also be noted that a very low surface brightness object, can often times be better observed at low magnification with a small refractor.  A good example of this would be NGC 6822 (Barnard’s Galaxy) in Sagittarius. 

“A weak glow but definite glow in 6 cm, where it appears elongated N-S and shows a very slight central concentration.  In 25 cm motion of the field helps in showing the low surface brightness galaxy, but it is difficult and ill-defined at best.”  Observing Handbook and Catalog Of Deep-Sky Objects by Christian B. Luginbuhl and Brian A. Skiff.  

Finally, after more than 20 years using my 10-inch reflector) I’d never been successful.  In September 2014, I was able to sketch and make notes of this elusive galaxy, NGC 6822, using the 102mm refractor.  However, much of my problem was/had been due to light glow from a pesky unshielded street light in close proximity to my backyard.  A dark sky is critical for locating and observing this faint and low surface galaxy.  

And so, with NGC 6822, another difficult object was checked off my list. 

The following sketch was made using the refractor with nothing more than a No. 2 pencil, a blank 5 x 8 notecard, with the colors inverted using a scanner.

Rogers NGC-6822

The following image by James Dire of Hawaii using a 190 mm Maksutov-Cassegrain telescope.



Seeing the companion to Polaris using an effective aperture of 60mm’s:   

Polaris has a magnitude of 2.1 and the secondary or companion at a much fainter 9.0 magnitude with a wide separation of 19 arc seconds.  The extreme difference in magnitudes can make this double more difficult to separate than you might think, especially if seeing is less than good. 

April 19th 2018:  102mm f/9.8 achromatic refractor with an effective aperture of 60mm’s.  

Seeing excellent: 12.5mm eyepiece plus a 2.8x Barlow for a magnification of 80x.  The companion was clearly visible as a tiny bluish dot.  



What is the smallest aperture that will allow me to see the companion to Polaris?  The following photo shows an effective aperture of 45 mm’s.  

However, as of current, I’ve been unable to see the companion with this aperture (45mm’s).  



Maintenance of a refractor: 

I have always kept the lens capped on the 102mm refactor, when not in use, the OTA is always stored in a padded case.  In the many years that I have owned this telescope, the objective lens has never required cleaning.  

The objective lens is also permanently collimated, without any adjusters, and was in perfect collimation when I received it.  There has been no change since purchasing the scope in 1997, and with plenty of use.   

For most part, a refractor is much more durable and able to suffer less from mishandling and rough treatment than reflector’s and catadioptric telescope’s.   

The modern era of refractor telescope’s arrive:

However, with the advent of exotic glass, better anti-reflection coatings and short focal lengths… like the slide-rule, the day of the 4-inch and larger f/15 refractor was over.     Roger Ivester

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