Lowest practical magnification

[25585]

While advice on eye piece and scope OG usually centres around highest practical magnification, are there any restrictions on lowest?

I assume refractors would be least affected, but would Newtonians, Maksutovs, and Cassegrains have a lower limit on account of their secondary mirror obstruction? 

Are any eye piece designs less well suited to low magnification in certain types of fl and/or scope?

[John]

I think your assumptions are correct with regard to the various scope designs.

The figure I think of is to divide the aperture (in mm) by 7 to get the lowest useful magnification. I can't recall where I got that from though

[wookie1965]

5 minutes ago, John said:

I think your assumptions are correct with regard to the various scope designs.

The figure I think of is to divide the aperture (in mm) by 7 to get the lowest useful magnification. I can't recall where I got that from though

Is that right John, that would mean my 200 mm / 7 would be 28.5 does not look right.

[John]

10 minutes ago, wookie1965 said:

Is that right John, that would mean my 200 mm / 7 would be 28.5 does not look right.

A 35mm eyepiece would give you that and a 7mm exit pupil which is generally regarded as the maximum that the human eye can accommodate. I'd not use it myself because I'm ancient (well 57) and my pupil probably does not dilate beyond 6mm. Then there is secondary shadow plus the pale background sky if there is any LP about. I guess under ideal conditions it's feasible though

[wookie1965]

7 minutes ago, John said:

A 35mm eyepiece would give you that and a 7mm exit pupil which is generally regarded as the maximum that the human eye can accommodate. I'd not use it myself because I'm ancient (well 57) and my pupil probably does not dilate beyond 6mm. Then there is secondary shadow plus the pale background sky if there is any LP about. I guess under ideal conditions it's feasible though

I`m 52 John I have only used a 32mm but mostly the BST 25mm probably that`s why not noticed anything.

Paul

[YKSE]

38 minutes ago, John said:

I think your assumptions are correct with regard to the various scope designs.

The figure I think of is to divide the aperture (in mm) by 7 to get the lowest useful magnification. I can't recall where I got that from though

Thats is correct

It's from the definitions of the following quantities and rule of thumb that 7mm as largest exit pupil (=lowest magnification).

Lets denote FL as telescope's focal length D as telescope's diameter, f as telescope's focal ratio. and Fe as eyepiece focal length

f is defined as f=FL/D                                                        (1)

Magnification M is defined as M=FL/Fe                                (2)

Exit pupil Ep is defined as Ep=Fe/f                                     (3)

As 7mm is the largest, we get from (3)

7=Fe/f                                                                             (4)

from (2), we get   Fe=FL/M                                              (5)

Replacing f in (1) and Fe in (5) in equation (4), we get

7 =(FL/M) / (FL/D) =D/M

therefore M=D/7

[Galen Gilmore]

57 minutes ago, wookie1965 said:

Is that right John, that would mean my 200 mm / 7 would be 28.5 does not look right.

I seems that it is right. Looking at the Orion website (US), under the specs of any telescopes it shows lowest useful and highest useful magnification. The numbers about match up with the calculations. 

https://www.telescope.com/Telescopes/Dobsonian-Telescopes/Dobsonian-Telescopes-with-Free-Shipping/Orion-SkyQuest-XT8-Classic-Dobsonian-Telescope/pc/1/c/12/sc/398/p/102005.uts?refineByCategoryId=398

[Stu]

I normally find the eyepiece which gives me a 7mm exit pupil by multiplying the focal ratio of the scope by 7. I guess this is ultimately derived from the same equations Yong quoted, but is basically rearranging 

Exit pupil = Focal Length of eyepiece/focal ratio of scope.

[triton1]

My lowest I use is 28mm so seems about right.

[wookie1965]

I read the post wrong I thought it was saying the lowest eyepiece I.e a 5mm but now I have read it again I understand what John and everyone else has said. I thought it was wrong when I thought lowest mag eyepiece was 28mm knowing full well I have used a 5 mm not thinking this was higher mag not eyepiece. 

[Stu]

23 minutes ago, Stu said:

I normally find the eyepiece which gives me a 7mm exit pupil by multiplying the focal ratio of the scope by 7. I guess this is ultimately derived from the same equations Yong quoted, but is basically rearranging 

Exit pupil = Focal Length of eyepiece/focal ratio of scope.

Goes without saying that this works for any exit pupil, so if you want to find your optimum smaller DSO observing ep (e.g. 2mm exit pupil) then it is 2 x focal ratio.

[YKSE]

43 minutes ago, Stu said:

I normally find the eyepiece which gives me a 7mm exit pupil by multiplying the focal ratio of the scope by 7. I guess this is ultimately derived from the same equations Yong quoted, but is basically rearranging 

Exit pupil = Focal Length of eyepiece/focal ratio of scope.

Absolutely It's much easier to think it in terms of exit pupil, it's, after all, an eyepiece of suitable focal length we're trying to get.

[Moonshane]

Personally it depends where I am observing from. At darker sites I can use bigger exit pupils, maybe 7mm but from light polluted home I find 5mm about the max or the sky seems washed out.

My longest eyepiece is a 27mm which gives me an averaged 6mm in my big dobs.

 

[25585]

With my 100mm Genesis 500mm FL, the 35 Panoptic gives an exit pupil of 7 & mag of 14x in round figures. So ideal. 

Arithmetically the 35 in my 254mm dob (also f5) fl 1270mm also has a 7mm exit pupil - but of course higher power & less TFOV. 

My 200mm C8 1000m fl using the 55mm gives roundabout the same mag as 35 in the dob but with an exit pupil of 5.55. 

 

[Stu]

55 minutes ago, 25585 said:

My 200mm C8 1000m fl using the 55mm gives roundabout the same mag as 35 in the dob but with an exit pupil of 5.55. 

2032mm focal length? Everything else makes sense f10 gives 55/10 = 5.5mm exit pupil

[TSRobot]

54 minutes ago, 25585 said:

 

M 200mm C8 1000m fl using the 55mm gives roundabout the same mag as 35 in the dob but with an exit pupil of 5.55. 

 

Isn't a C8 FL of 2000mm at F10 or FL of 1280 reduced to f6.3? I used to see the central obstruction with a 32 plossl at f6.3. Now i have a 130 F5 newt and can see the central obstruction with cheep swa 32mm 70 degree eyepiece. It's hard to know when the central obstruction will interfere before you buy ?

[25585]

Shorter Fl a newt is the larger it's mirror I believe. Larger focusers also need larger mirrors. 

Not sure if there is a ratio, or what the cass/mak is by comparison. 

[Louis D]

16 hours ago, TSRobot said:

Isn't a C8 FL of 2000mm at F10 or FL of 1280 reduced to f6.3? I used to see the central obstruction with a 32 plossl at f6.3. Now i have a 130 F5 newt and can see the central obstruction with cheep swa 32mm 70 degree eyepiece. It's hard to know when the central obstruction will interfere before you buy ?

That's going to depend a lot on the percentage diameter of the central obstruction and the dilation of your exit pupil.  Let's say the SCT has a 35% by diameter central obstruction and your eye's pupil is only dilated to 5mm.  The exit pupil will be 32/6.3=5.1mm while the central obstruction will appear to be 35% of that or 1.8mm.  This is starting to become noticeable.  However, let's say it's early in the evening and you just came out of your house and you're only dilated to 2mm.  You're basically going to be seeing nothing but central obstruction when viewing exactly on axis.

Let's say that f/5 newtonian was using a 25% obstruction by diameter.  Now you'll have an exit pupil of 32/5=6.4mm and the CO will appear to be 25% of that or 1.6mm.  Quite similar, so I can see where you have problems with both.

I would recommend more time for dark adaptation to maximize the difference between CO size and your entrance pupil size.  You might also try a slower Newt with an undersized secondary.  My f/6 uses an 18% obstruction, and I've never noticed the CO except on the full moon when using 40mm eyepieces.  Even then, it's manageable for brief periods of time because the CO appears to be 1.2mm across.  There's still 0.8mm entrace pupil left when dilated to 2mm because of the brightness of the full moon.

[Merlin]

For the lowest magnification, multiply the aperture ( in inches ) by 4. So, lowest practical mag for an 8-inch reflector = X32.

[Stu]

56 minutes ago, Merlin said:

For the lowest magnification, multiply the aperture ( in inches ) by 4. So, lowest practical mag for an 8-inch reflector = X32.

I'm not sure how that can work, or the working behind it? In an 8" (f10) SCT for instance, even with a 55mm Plossl the lowest power you can achieve is x37, and in an 8" f4 scope it would be x28.5 with a 28mm eyepiece to keep the exit pupil at 7mm.

Can you explain a little more? Thanks  

[Louis D]

2 hours ago, Stu said:

I'm not sure how that can work, or the working behind it? In an 8" (f10) SCT for instance, even with a 55mm Plossl the lowest power you can achieve is x37, and in an 8" f4 scope it would be x28.5 with a 28mm eyepiece to keep the exit pupil at 7mm.

Can you explain a little more? Thanks  

Since lowest power is generally determined by exit pupil being below 7mm or so.  I'll go through the derivation below:

Assuming an f/6 8" reflector, that would be 6*7mm=42mm for a lowest power eyepiece.  Thus, the lowest power would be (6*8"*25.4mm/inch)/42mm = 29x.  The 32x recommendation is very close to this.

Rearranging the terms of the two equations and cancelling the focal length in both the numerator and denominator, you get 8*25.4/7 = 29x.  Take 25.4/7 = 3.6, so 8 * 3.6 is the exact answer and the rule of thumb rounds 3.6 to 4.

This is true of any 8" theoretically to achieve a 7mm exit pupil for lowest power.  In the case of an f/10 8" SCT, that would be a 10*7mm=70mm eyepiece.  The mentioned 55mm Plossl only yields a 55mm/10=5.5mm exit pupil, so not really lowest possible power.  Pick up a Russell Optics 72mm XL Super-Plossl to try it out in an SCT.

[Stu]

1 hour ago, Louis D said:

Since lowest power is generally determined by exit pupil being below 7mm or so.  I'll go through the derivation below:

Assuming an f/6 8" reflector, that would be 6*7mm=42mm for a lowest power eyepiece.  Thus, the lowest power would be (6*8"*25.4mm/inch)/42mm = 29x.  The 32x recommendation is very close to this.

Rearranging the terms of the two equations and cancelling the focal length in both the numerator and denominator, you get 8*25.4/7 = 29x.  Take 25.4/7 = 3.6, so 8 * 3.6 is the exact answer and the rule of thumb rounds 3.6 to 4.

This is true of any 8" theoretically to achieve a 7mm exit pupil for lowest power.  In the case of an f/10 8" SCT, that would be a 10*7mm=70mm eyepiece.  The mentioned 55mm Plossl only yields a 55mm/10=5.5mm exit pupil, so not really lowest possible power.  Pick up a Russell Optics 72mm XL Super-Plossl to try it out in an SCT.

Thanks Louis, appreciated.

I was sticking to normally used eyepieces, and from what you are saying it is very much an approximation, a rough rule of thumb? I think I'll stick to the accurate equations discussed earlier  

[25585]

Using cameras as viewing optics you can go lower. For a 35mm SLR, 1000mm fl would be 18.18 (1000/55). 55mm was normal althogh most lenses were 50mm.

Medium format 120 roll film would be less, taking 6x6 as standard 12.5x (1000/80). 6x4.5 and 6x7 would be respectively larger and smaller magnification. 80mm normal for 6x6.

Back to the hypered Tech Pan 2415 days!

 

[LukeSkywatcher]

Interesting question and thread. My main scope is a 200mm SCT and i normally use EP's between 8-32mm. If i am feeling lucky, i will venture to 6mm, and i once used a 4mm Omni (on the Moon).

I guess it all depends on the type of scope and EP in use?. 

Highest practical mag is easy to work out...........its usually 1.5 times the aperture...............so on a 200mm scope you can happily and easily get about 270-300x.

 

 

[Louis D]

7 hours ago, LukeSkywatcher said:

Interesting question and thread. My main scope is a 200mm SCT and i normally use EP's between 8-32mm. If i am feeling lucky, i will venture to 6mm, and i once used a 4mm Omni (on the Moon).

I guess it all depends on the type of scope and EP in use?. 

Highest practical mag is easy to work out...........its usually 1.5 times the aperture...............so on a 200mm scope you can happily and easily get about 270-300x.

 

 

So that rule of thumb assumes a 0.67mm exit pupil to be the smallest useful exit pupil (200mm/0.67mm=300x).  I usually say 0.7mm is my limit, so for me, I'd agree with it.