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  #1  
Old 12-09-2017, 10:27 AM
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FlashDrive (Col)
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Low Mag / High Mag

What do you regard as the ' cut off ' focal length between High Mag ... and Low Mag

eg ... would 20mm and up be regarded as ' low mag ' ...anything under that you start to get into the ' high mag ' eyepieces

What would you regard to this....

Col...
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  #2  
Old 12-09-2017, 11:31 AM
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Camelopardalis (Dunk)
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Depends on the scope Col

On a C8, 20mm is still 100x, but only 18x on my little Z61...
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  #3  
Old 12-09-2017, 11:34 AM
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Kunama (Matt)
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Lowest I can get with my scopes:

FOA 13x
TSA 22x
Dob 62x
TEC 75x

I regard anything less than 100x as low, 100-250 as medium and then the sky the limit..... (actually 729x)
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  #4  
Old 12-09-2017, 11:37 AM
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Camelopardalis (Dunk)
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Lowest I've tried with my Z61 was my XW40...all of 9x swallows Magellanic clouds for an evening snack
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  #5  
Old 12-09-2017, 12:14 PM
issdaol (Phil)
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Quote:
Originally Posted by Kunama View Post
Lowest I can get with my scopes:

FOA 13x
TSA 22x
Dob 62x
TEC 75x

I regard anything less than 100x as low, 100-250 as medium and then the sky the limit..... (actually 729x)

Hmmpf ......I thought people regularly use 1100x-1200x
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  #6  
Old 12-09-2017, 12:31 PM
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MortonH
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Depends on the scope and your viewing habits. Why do you ask, Col?
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  #7  
Old 12-09-2017, 12:33 PM
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FlashDrive (Col)
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Quote:
Originally Posted by Camelopardalis View Post
Depends on the scope Col
Quite right it is to .... I was speaking in a ' general sense ' ... say f5 to f6 ...Refractor

It would be ' stupid ' putting a 10mm in a f5.5 Refractor .... not a good match ... terrible view.

At what f/l eyepiece would you regard as a ' starting point ' for f/5 Achro

20mm upwards ... then 22mm / 24mm / 28mm etc etc ... anything under 20mm defeats the purpose in a ' fast Refractor .... would you agree .... or not.

Col...

Last edited by FlashDrive; 12-09-2017 at 12:45 PM.
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  #8  
Old 12-09-2017, 12:52 PM
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FlashDrive (Col)
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Let's put it another way ....

Let's say I have a F/5 Achro' with a 100mm Lens .... I have an eyepiece giving me say 20x .... what is the ' formula ' for finding the f/l of the eyepiece.

Col.....
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  #9  
Old 12-09-2017, 01:40 PM
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Merlin66 (Ken)
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Col,
If it's 100mm f5 then the focal length is 500mm.
To get x20 the eyepiece fl needs to be 1/20 of 500 = 25mm
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  #10  
Old 12-09-2017, 01:42 PM
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FlashDrive (Col)
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Quote:
Originally Posted by Merlin66 View Post
Col,
If it's 100mm f5 then the focal length is 500mm.
To get x20 the eyepiece fl needs to be 1/20 of 500 = 25mm
Cheers .... I've had a ' mental block ' today and couldn't remember ....
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  #11  
Old 12-09-2017, 06:57 PM
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100mm f/5... you eyeing up another Genesis?
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  #12  
Old 12-09-2017, 07:28 PM
Tropo-Bob (Bob)
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Col, I go on the exit pupils.

In my rough rule-of-thumb:

Low magnification is using an Eye Piece that gives an Exit Pupil of 3mm or larger.

Medium magnification is using an EP that gives an Exit Pupil from 1.5 to 2.6mm

High magnification is when the Exit Pupil 1.4mm or less.

Although, my rule may break down somewhat when using large Dobs (but I do not have one on those).

You may have noticed that I missed from 2.7mm to 2.9mm. I classs this as a "flat" zone, that is not really optimal for viewing any class of objects.
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  #13  
Old 12-09-2017, 07:34 PM
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Merlin66 (Ken)
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I have many telescopes (!) But only one Genesis.....
What ever happens in life, where I go, the Genesis goes...
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  #14  
Old 12-09-2017, 07:52 PM
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FlashDrive (Col)
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Quote:
Originally Posted by MortonH View Post
100mm f/5... you eyeing up another Genesis?
.... nah !!

Last edited by FlashDrive; 13-09-2017 at 06:43 PM.
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  #15  
Old 13-09-2017, 06:00 PM
jamespierce (James)
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It's kind of arbitary ... but a ~2mm exit pupil is generally the sweet spot between magnification, contrast and image scale (obviously some objects require a wide field, and bright objects like planets (or PN) require higher magnification) ... so lets say ... exit pupil larger than 2mm is low mag ... and exit pupil smaller than 2mm is high mag...

Or put another way ~200x plus is high magnification in my book.
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  #16  
Old 13-09-2017, 07:04 PM
Wavytone
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Quote:
Originally Posted by FlashDrive View Post
What do you regard as the ' cut off ' focal length between High Mag ... and Low Mag

eg ... would 20mm and up be regarded as ' low mag ' ...anything under that you start to get into the ' high mag ' eyepieces

What would you regard to this....

Col...
Lowest = 6mm exit pupil. The eyepiece focal length required is thus 6mm x the focal ratio of the scope. For example an f/7 refractor, this means a 42mm eyepiece. With UWA eyepieces however another limit comes into play - the diameter of the field stop - or eyepiece barrel (if there is no field stop). The result is that for a 2" barrel, a 38mm UWA eyepiece with an apparent field of view of 70 degrees will be limited only by the barrel, and a longer eyepiece is pointless.

Highest depends on the type of scope. For Newtonians and SCT's, 1X magnification per mm of aperture. Hence for say and f/5 Newtonian, 5mm.

For an achro refractor or Newtonian with a small secondary, maybe 1.5X per mm of aperture. This means an eyepiece of focal length = F/1.5, where F is focal ratio of the scope.

For a good APO refractor or very good maksutov this can go as high as 2X per mm of aperture for planetary or double stars when the seeing permits (which is rarely). For example for an f/15 maksutov in excellent seeing, 8mm is pretty much the sweet spot.
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  #17  
Old 16-09-2017, 12:27 AM
AEAJR (Ed)
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Cool

Quote:
Originally Posted by FlashDrive View Post
What do you regard as the ' cut off ' focal length between High Mag ... and Low Mag

eg ... would 20mm and up be regarded as ' low mag ' ...anything under that you start to get into the ' high mag ' eyepieces

What would you regard to this....

Col...
As noted by others it depends on the scope.

20 mm in my ETX 80 is 20X. In my 5" Mak it is 85X.

I am more of a zoom guy so I don't use fixed FL eyepieces as much as I used to.

For each of my scopes I have 1 or 2 low power wide view
  • 1.25 focuser gets a 32 mm plossl
  • 2" focuser gets a 38 mm and 25 mm 70 degree

Then I go to a zoom - 8-24 for the mid to high range. Either the Celestron or the Baader Hyperion zoom.
  • In my mak, 5"/1900 mm the zoom handles it all after the 32 mm Plossl
  • In my XT8i, 8"/1200 mm Dob it is zoom or zoom +2X barlow
  • In my ETX 80 3.1"/400 mm it is zoom or zoom + 3X barlow

Those combinations cover the entire range of the scope for 95% of the situations. I am always working at the optimum magnification for this particular target. I have many other eyepieces but mostly I use the zooms.

Last edited by AEAJR; 20-09-2017 at 03:45 AM.
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