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Lowest Useful Magnification

eyepieces
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#1 Ray_T

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Posted 05 July 2020 - 11:56 AM

The specs for my 130mm, 650 focal length (f/5.0) reflector indicate that the lowest useful magnification I can have is 18x. I calculated a 36mm eyepiece would give me this magnification.

 

So, for example, if I went to say a 40mm eyepiece (16x) would I not obtain focus? What is the manufacturer getting at? Thanks.  



#2 Hesiod

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Posted 05 July 2020 - 12:15 PM

You would obtain focus, but may see the shadow of the secondary mirror.

The "lowest useful magnification" parameter is usually related to the "exit pupil" parameter: the exit pupil is the size of the bright circle the optical system focus in (you can estimate its size by dividing the eyepiece focal length by the telescope's focal ratio).

Typically the "low magnification limits" coincides with an exit pupil around 7mm as this is on average how much the human eye's pupil can widen in the deepest darkness.

You could still use an eyepiece/telescope setup providing larger exit pupils but will not get advantages in term of overall brightness and the disadvantage of lower magnification so may even end by having an harder time to see the object.

 

I have a 130/650 Newtonian as well and a 32mm Plossl is a nice eyepiece to observe very large objects from dark sites; however, a 20mm/70° is IMHO a more versatile option


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#3 WarmWeatherGuy

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Posted 05 July 2020 - 12:16 PM

I can make a guess. The size of the exit pupil is the size of the main objective divided by the power. Given that the power is the focal length of the scope divided by the focal length of the eyepiece and the f-number is the focal length of the scope divided by the size of the main objective you can do some substitution to find that the exit pupil is the focal length of the eyepiece divided by the f-number. Your f-number is 5. With a 36mm eyepiece you would have 36/5 = 7.3mm exit pupil. With a 40 mm eyepiece it would be 40/5 = 8mm. Your pupil would never be that large so light would be wasted on the white of your eye. You would see a smaller image without the benefit of it being brighter.



#4 sg6

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Posted 05 July 2020 - 12:18 PM

With a reflector the image of the secondary can become apparent, and I suspect that that is different between people.

 

Usually the "minimum" is defined by maximum exit pupil and that is how big your pupil gets, again different from person to person. Suppose easy is assume 6mm or 7mm and work a magnification backwards for your scope.

 

However can argue that even if exit pupil is 8mm and your pupil is 7mm but if the object is rendered good who cares about the lost light. I wouldn't, as long as the rest is good.


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#5 LDW47

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Posted 05 July 2020 - 06:16 PM

The specs for my 130mm, 650 focal length (f/5.0) reflector indicate that the lowest useful magnification I can have is 18x. I calculated a 36mm eyepiece would give me this magnification.

 

So, for example, if I went to say a 40mm eyepiece (16x) would I not obtain focus? What is the manufacturer getting at? Thanks.  

In 99.9% of scopes you wouldn’t have a problem !  Clear Skize !



#6 kathyastro

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Posted 05 July 2020 - 06:57 PM

The "standard" lowest magnification giving an exit pupil of 7mm is based on a young person's dark-adapted eye.  Everyone's eyes are different.  If you are older, your pupil will likely not open up to 7mm.  If you cannot get dark-adapted because of local light pollution, your pupil will not open up to 7mm.  In those cases, the longest focal length  you can use may be shorter than the standard.

 

When the exit pupil is larger than your eye's pupil, you run the risk of seeing the shadow of the secondary.  This is particularly true with bright targets like the Moon, and can be easily detected in daytime viewing.  The proportion of light entering your eye will decrease, resulting in a dimmer image than you would otherwise expect from that magnification.


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#7 Tony Flanders

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Posted 05 July 2020 - 07:59 PM

The specs for my 130mm, 650 focal length (f/5.0) reflector indicate that the lowest useful magnification I can have is 18x. I calculated a 36mm eyepiece would give me this magnification.
 
So, for example, if I went to say a 40mm eyepiece (16x) would I not obtain focus? What is the manufacturer getting at? Thanks.


Setting theory aside and looking at the practical aspect, in a telescope with a 1.25-inch focuser and a reasonably short focal ratio (say f/6 or faster), there is no benefit whatsoever to using any eyepiece with a focal length longer than 32 mm.

The main reason to use low magnifications is to achieve wider true fields of view. And a 32-mm Plossl already maximizes the field of view possible with a 1.25-inch focuser. You could, if you desire, use a 40-mm Plossl, but you would still get the same true field of view -- just at lower magnification, with less detail. That's because in order to fit into the 1.25-inch format, the apparent field of view of a 40-mm Plossl is limited to something around 40 degrees, compared to roughly 50 degrees for a 32-mm Plossl.

But as everyone else has said, the rule of thumb is that the useful range of exit pupils is 0.5 mm to 7 mm. Translating that into magnification per inch of aperture, it is anywhere from 3.6X to 50X per inch of aperture, which works out to 18X to a bit over 250X for your (slightly bigger than) 5-inch telescope.

 

Neither the lower nor the upper bound is absolute, but in practice just about everyone does more than 90% of their viewing within that range.


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#8 aeajr

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Posted 08 July 2020 - 04:37 AM

The specs for my 130mm, 650 focal length (f/5.0) reflector indicate that the lowest useful magnification I can have is 18x. I calculated a 36mm eyepiece would give me this magnification.

 

So, for example, if I went to say a 40mm eyepiece (16x) would I not obtain focus? What is the manufacturer getting at? Thanks.  

As others have stated, this specification is based on a 7 mm exit pupil.  Pretty much a standard in how the manufactures provide this spec.

 

                   FL eyepiece / FR scope = exit pupil 

 

35 mm eyepiece / F5 scope = 7

 

On some reflectors you can go larger than 7 mm.  On my Orion XT8 I used a 38 mm that worked fine.

 

38 / 5.9 = 6.44 mm exit pupil.  No shadow of the secondary

 

I now have an Apertura AD12 - FR F5

 

38 / 5 = 7.6 mm exit pupil.   Works fine

 

However on my little 100 mm table top Orion SkyScanner 100 I do run into a problem.  F4 Newtonian reflector  If I go any shorter than 25 mm on the eyepiece the secondary mirror shadow becomes an issue.  Even the 25 mm shows a hint of the secondary mirror.

 

25 mm / F4 = 6.2 exit pupil.   Should be fine but if I go longer I get the shadow.  I normally don't go any shorter than 20 mm on this scope.

 

On your scope, with that short focal length, I would not go longer than a 32 mm Plossl eyepiece.  You gain nothing and you invite problems.   See if you can borrow a 32 mm Plossl from someone to try to be sure you don't get the shadow, but I would expect you would be OK. 


Edited by aeajr, 08 July 2020 - 04:39 AM.

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#9 vtornado

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Posted 08 July 2020 - 10:36 AM

Hello Ray and welcome to CN.

 

A 32mm plossl is the cheapest, largest true field of view (TFOV) you can get. 

 

If you have a lot of light pollution, you may want to consider getting 25mm 60 degree eyepiece. 

This offers about the same TFOV as the 32mm, but the background sky will be darkened more, because of the higher mag.

 

The reverse effect occurs if you choose a 40mm over a 32. That background sky will be brigther and will make seeing faint

objects more difficult.

 

There are 24mm 68 degree eyepieces that are better yet but the prices are much larger than the 25/60.  It is up to

you how much you want to spend.

 

VT


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#10 phillip

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Posted 08 July 2020 - 11:15 AM

Exactly brighter background is exactly what I notice using 40mm in my XT8i.

 

However I often use the 40mm for search and most often when I lose an object in high power, easy maneuver with the 40mm. Use mine extensively even with all its Issues! 

 

Clear Sky

 

WUCIWUG


Edited by phillip, 08 July 2020 - 11:28 AM.


#11 gnowellsct

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Posted 08 July 2020 - 12:22 PM

Hello Ray and welcome to CN.

 

A 32mm plossl is the cheapest, largest true field of view (TFOV) you can get. 

 

 

VT

Well nominally.  If he has a 2 inch focuser he might be able to squeeze more field with a 2 inch 70 degree or similar ocular.

 

But it is iffy with a 5 inch Newt design.   The constraints of field illumination are significant under 6 inches of Newt aperture.

 

Some people have 6 and 7 mm night time exit pupils and can do better than others with wide exit pupil configurations. 

 

I note that there are a lot of way to look at this one is simply what is being sold.  The manufacturers know this game.  It is hard to find oculars above 40 mm though there are a few (a very few: 55 mm TV plossl, some of the Russells, the Pan 41 just nudges over 40).

 

Most fast Dob owners I know seem happiest in the 17 to 20 mm ocular range for general purpose viewing.  That's 3.8 to about 4.4 mm exit pupil (on an f/4.5), which is about where most people like to be most of the time, except for unusual high magnification applications.   In an f/10 C14 my favorite oculars are the 30 and 40 mm which is the same spot in terms of exit pupil.  


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