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Meade Polaris 90: selection of oculars


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#1 Andrey

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Posted 24 July 2018 - 02:43 AM

Greetings, dear Meade users!
 
  I want to achieve maximum magnification on my Meade Polaris 90, but without using Barlow lens.
  As far as I understood theory for this model:
  • Clear Aperture (mm) 90
  • Clear Aperture (") 3,5
  • Focal Length f/10 (mm) 900
  • Focal Ratio 10
  • Eyepiece(s) Three (3) 1.25" MA (26 mm, 9 mm & 6.3 mm)
  • Minimal Useful Magnification (CA/6) 15
  • Maximal Useful Magnification (CA*2) 180
 
  Therefore maximal useful magnification 180× is achieved with ocular 5 mm, which is just a not much smaller then that supplied 6.3 mm.
  However market offers oculars 4 mm that theoretically deliver magnification 225×, but it exceeds maximal useful magnification 180×.
 
  Do you suggest me to purchase a 4-mm ocular for this model of telescope?
  What optical artifacts can I experience with a 4-mm ocular?
 
  Thank you for a detailed reply.


#2 SBacon

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Posted 26 July 2018 - 05:42 AM

Hi Andrey,

 

Here's a link from Sky and Telescope that has a lot of valuable information concerning your question.

 

https://www.skyandte...-magnification/

 

Clear skies!

 

Steve



#3 SBacon

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Posted 26 July 2018 - 05:43 AM

Hi Andrey,

 

Here's a link from Sky and Telescope that has a lot of valuable information concerning your question.

 

https://www.skyandte...-magnification/

 

Clear skies!

 

Steve



#4 Andrey

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Posted 01 August 2018 - 01:27 AM

Dear Steve,

 

  Thank you --- I will read it and get back.



#5 Andrey

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Posted 21 June 2019 - 03:40 AM

Greetings!

 

  I've got the theory.

 

  However it would be great to read about practical some user experience.

  Can anyone share selection of oculars and filters optimal for Meade Polaris 90 please?

 

  Thank you.



#6 MistrBadgr

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Posted 21 June 2019 - 11:52 AM

Hi Andrey,

.

There are a lot of good brands and designs that are very good, but for this post, I am going to stick with the Meade 4000 series Plossles.  Plossles tend to fit the description of getting 80% (or more) of the benefit for 20% (less actually) of the possible costs.

 

What I would most likely use with a Polaris 90 is as follows:

 

1.  I would use a 32mm Plossl as a finder eyepiece.  I have a lot of different eyepieces to use, but dollar for dollar, a 32mm Possl is just about optimum for the widest piece of sky that you can pull into that scope.  A 40mm can get just a touch wider, but with a limit of about 44 deg field of view, compared to 50deg for the 32mm, it is a tiny difference.  The higher magnification of the 32mm seems to help a bit with contrast and detail.

 

2.  Something like the 12.4mm Plossl eyepiece that Meade has will give a 1.25mm pupil diameter, which seems to provide an over-all light level that helps in seeing faint objects with some light pollution.  At first, the view may seem surrealistic and may take some time over months to really train your eye and brain to deal with it.  But, working at these light levels, using some patience, has helped me see many galaxies I would never have thought I could see at all from my suburban back yard. For me, with a scope like yours, an eyepiece in the 9.7mm to 12.4 range ends up being optimum for most of my planetary viewing.

.

3.  Dropping from 32mm all the way to the 12.4 is a really big jump.  Having something in between, maybe 20mm will help when working you way down from the finder.  It will work well with many open clusters and some nebula.

 

From what I have read, below about an 8mm focal length, the quality of an image in a 4 element Plossl begins to drop.  Below the 9.7mm eyepiece, I would use 2x Barlow for higher magnifications.  Higher magnifications are useful for lunar and tight double star splitting.  The #126 2X Barlow has done a nice job for me in long refractors like yours.

 

Though not used as much, a 15mm gives some intermediate magnifications that are useful at times, either by itself or in combination with the Barlow.  There are three of the magnifications that I do not end up using at all with long refractors, 40mm, 26mm, and 6.4mm.  Not using the 26mm is a shame, because it can be one of the best they make.  However, I find that I simply do not need it.

 

With more expensive eyepieces, there can be other options.  I find the HD 60 4.5mm eyepiece as my go to eyepiece for high magnification of the Moon.  I think it would work well with your scope.  Very rarely will a 4mm eyepiece give you any more information in the view of a lunar crater than a 4.5 or 5mm.  There is almost always something going on in the atmosphere to mess up the view of the eyepieces in the 4-5 mm range, with the 4mm being more easily affected. 

 

Hope this helps,

 

Bill Steen


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma




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