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Small aperture challenge: Classic doubles with 51 mm of perfect aperture

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#1 db2005

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Posted 09 August 2020 - 01:40 AM

Hi all,

 

After observing Jupiter and Saturn with my FC-100 yesterday night, and before finalizing the observing session, I went on a small tour among some of my favourite double stars. To spice things up a little, I reduced the aperture on the FC-100 to around 51 mm to hopefully challenge my eyesight and my scope. As things turned out I was quite impressed by how many classic doubles are actually possible to see with very modest aperture and 133x magnification. Here are the highlights of the observing session:

  • Epsilon Lyrae (the "double-double"): First pair easily split, second pair barely split in moments of clear seeing.
  • Epsilon Bootis (Izar): The B-component was barely resolved just inside the diffraction ring of the primary. In moments of steady seeing the B component was very clearly evident with its bluish color separating it from the primary component's first diffraction ring.
  • Iota Cassiopeiae: This classic triple was very easily resolved and A, B, C components identified.
  • Polaris: This visual double is a challenge to small apertures not because of the separation (which is wide) but because the secondary is much fainter (magnitude 9) than the primary (magnitude 2.1). Observing this pair requires good optics and good seeing and used to be considered a good test for a 3" aperture. But with 51 mm of perfect aperture Polaris was easy.
  • Mizar: This classic, bright and widely separated double was of course easy.

I am impressed by how much one can actally see with modest, high quality aperture. Amateurs of long-gone days fortunate enough to own a high-quality 50 or 60 mm achromat would indeed have been able to see many classic double stars. Of course, the observer's level of experience plays a role as well. In my experience, once you have seen some targets clearly in a 100 mm scope they seem to become much easier to see in significantly smaller apertures too. Ironically, this also means that experienced observers - while perhaps wanting more aperture - actually need less aperture to see the same things.

 

CS,
Daniel

 


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#2 Astrojensen

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Posted 09 August 2020 - 01:52 AM

I observed for more than a year with a Zeiss 50/870mm objective from a naval rangefinder and have seen all the pairs you mention, plus many, many more. The optical quality of that lens is truly stunning and it will do insane things. If you want a real challenge for a 50mm, try Theta Aurigae. 

 

 

Clear skies!
Thomas, Denmark


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#3 Lappe Lad

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Posted 09 August 2020 - 10:42 AM

As an observer who considers his 63mm refractor a “portable observatory” I naturally gravitate towards threads like this. They inspire me to stay happy with what I’ve got and keep pushing my observational skills instead of pondering getting a bigger scope. I know some people might say “Dude, you have no idea what you’re missing.” Well…

Not only is it rewarding to make observations with instruments that others might consider wholly inadequate, but the image quality often holds up beautifully under difficult atmospheric conditions.

 

Robert


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#4 Astrojensen

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Posted 09 August 2020 - 11:48 AM

As an observer who considers his 63mm refractor a “portable observatory” I naturally gravitate towards threads like this. They inspire me to stay happy with what I’ve got and keep pushing my observational skills instead of pondering getting a bigger scope. I know some people might say “Dude, you have no idea what you’re missing.” Well…

Not only is it rewarding to make observations with instruments that others might consider wholly inadequate, but the image quality often holds up beautifully under difficult atmospheric conditions.

 

Robert

Observing with a small telescope can be intensely rewarding, especially when you bag something that is usually considered a challenge in larger apertures. 

 

I've observed with 60mm scopes for almost three decades and still find new things to see. Some challenging, some not. 

 

 

Clear skies!
Thomas, Denmark 


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#5 Rutilus

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Posted 09 August 2020 - 01:04 PM

I do the stopping down business all the time, however my scopes are modern Chinese achromats. 

When stopping down to 50mm, my 120mm f/8.3 and 150mm f/8 are over Conrady levels and give excellent

views for 50mm aperture.

One favorite of mine is Delta Cygni. I can see the companion star in both scopes.  


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#6 db2005

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Posted 09 August 2020 - 01:06 PM

Observing with a small telescope can be intensely rewarding, especially when you bag something that is usually considered a challenge in larger apertures. 

 

I've observed with 60mm scopes for almost three decades and still find new things to see. Some challenging, some not. 

 

 

Clear skies!
Thomas, Denmark 

I have found that one very nice thing about "small aperture observing" with a stopped-down scope is that if I'm not completely sure what I saw, I can always quickly revert to the scope's full aperture and verify a split double star's location. This makes for some very quick and easy comparisons. Also, observing at close to the limits of the laws of physics provides valuable chances to improve my observing skills, something that might be useful when taking on targets that are challenging even when using the scope's full aperture.


Edited by db2005, 09 August 2020 - 01:08 PM.

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#7 Rutilus

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Posted 09 August 2020 - 01:18 PM

Also, if the companion star sits in the diffraction ring, then stopping down will move the companion into the

dark interspace, making it sometimes easier to see with a smaller aperture scope.


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#8 db2005

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Posted 09 August 2020 - 01:23 PM

Also, if the companion star sits in the diffraction ring, then stopping down will move the companion into the

dark interspace, making it sometimes easier to see with a smaller aperture scope.

That's an interesting idea - exploiting a limitation imposed by the laws of physics to actually enhance the performance of the instrument. Clever! waytogo.gif



#9 Nucleophile

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Posted 17 August 2020 - 09:47 PM

I'll be taking an SV60 (former life as a finderscope on the 15") on a tripod to dark skies for a few weeks starting tomorrow--this is a good list of objects to examine with that little refractor.  I'll also take along the CDSA (2nd Ed).  Probably next time I will take the 8 inch if there is room in the van not taken up by tools of every sort.  The forecast is for clear skies ad infinitum with rain coming next time Neowise swings on by.

 

 I'll attempt to assemble a report for submission once my internet come online...


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