Magnification Calculation

[Greg6498]

Hello all!

iI know there is a way to calculate this, but I’m not sure what it is. I have a Skywatcher Evostar Pro 80mm f7.5 What is the highest magnification eyepiece I can use? I would like a 5mm but not sure it would work. I even saw a 4mm!

Thanks!

[Gfamily]

Usual rule of thumb is that using an eyepiece shorter than your f ratio will need 'better than average' skies. 

There are refractor users who will tell you this is rubbish and you can go to half the f ratio. I don't know about that, and for compact refractors like yours you may well be able to use a 4mm and get good results more often than not.

I have a f/10 SCT and I generally get decent views from my 9mm EP, but shorter than that and results tend to turn to mush. However, if the jet stream is directly above me, then the 9mm gives too much magnification for the conditions. 

 

[JOC]

8 hours ago, Gfamily said:

Usual rule of thumb is that using an eyepiece shorter than your f ratio will need 'better than average' skies. 

Having a reflector I've never paid much attention to refractor 'numbers', but found this ^^^ an interesting statement.  Even with my F6 reflector the conditions have to be very good to get a good view with my 5mm Pentax and its generally 'happy point' under most skies is around 8mm.  I've seen 'rule of thumb' statements which refer to a multiplication factor somewhere along the line, but it would seem that even for my 200P flextube this F ratio statement seems about right in the UK.  Perhaps as a 'rule of thumb' it works for many types of instrument?

Given that, it sounds like the OP might be limited to around 7mm or 8mm.

[laudropb]

Hi Greg. The magnification  is calculated by dividing the focal length of your scope by the focal length of your eyepiece. With a 4 mm ep this is 600 /4 = 150x. Your scope is capable of this given good seeing conditions. This is a useful magnification for viewing the planets and the moon. I would stress that the seeing  conditions are very important and also that high magnification is not always the be all and end all of observing. It very much depends on the target you want to observe. Remember that increasing magnification will decrease the field of view and darkens the image you get.

  

 

 

 

 

 

 

[JOC]

2 minutes ago, laudropb said:

darkens the image you get.

I notice this a good deal when I get the 5mm Pentax out, but it is good for things like double stars.

[Ricochet]

There are really three things to consider here:

1. The maximum magnification your skies will support
2. The maximum magnification that your telescope will support
3. The smallest exit pupil that you can comfortably use.

The first one, I can't be a great help with as i have no knowledge of your skies. However, in the UK  , under the jet stream, we often say 200x-250x is about the limit and I don't know of any reason why you can't also reach at least this. I see you have an 8" dob listed so you should have a pretty good idea of the magnifications that you have used with that. Anything that has been useful with the dob is a magnification that your skies can support (at least on some occasions).

Moving onto the magnifications that the scope can support, which is what you were really asking about, we can start with the "traditional" limit, which is 50x-60x per inch of aperture. These days everything about your scope is measured in millimetres so it is probably more useful to convert this limit to 2x-2.4x per mm of aperture. However, we can't just take this as gospel and apply it to all scopes, but remember that it was devised using long focus achromatic refractors solely for the purpose of splitting double stars. At the top end of magnification, where the exit pupil is small, the image is dominated by diffraction and the Airy disk is visible around stars. Any scope design with a central obstruction has more diffraction and so I suggest that telescopes with a central obstruction, and short focus achromats which are dominated by CA and SA, have a lower limit. Additionally, most people want high magnifications for viewing the planets for which the traditional limit is often revised as the more conservative 25x-30x per inch, or 1x-1.2x per mm, of aperture. I'm not sure quite when this new limit came about, but suspect that it is partly a result of the increasing use of larger, centrally obstructed designs that have more diffraction and are more likely to hit atmospheric limits. For your ED refractor (a design superior to long focus achromats) I suggest that you are looking for a planetary limit somewhere between these two given limits.

Finally, we have to consider the exit pupil and your eye. The exit pupil size can be found easily by dividing the focal length of the eyepiece, by the focal ratio of the telescope it is being used in. Converting the limits above gives us corresponding exit pupil limits of 1mm-0.85mm and 0.5mm-0.42mm, which equates to a eyepiece focal lengths approximately equal to the focal ratio of your telescope, or half of it for quick in the field selections. As well as the image becoming more blurred due to diffraction as you decrease the exit pupil, you will also notice an increase in the visibility of "floaters" in your eye(s). This will vary from person to person and there is no real way to determine where your limit is, aside from trial and error. If you want to reduce the effect of floaters at high magnification then using binoviewers will help, as your brain will use the signals from both eyes to filter them out.

Your current 6.7mm eyepiece gives an exit pupil of 0.89mm, so right in that lower limit, so with an ED refractor, which I think can push those limits I would definitely give a 5mm a go.

[LukeSkywatcher]

Easiest way to get to the useful magnification for any scope is to double the diameter of the lens. In this case its 80mm, so double that is 160x.

On night of great seeing, you could push the magnification a little higher. 

I had a 90mm refractor and was happy using a 9mm EP for viewing planets. The f/l of the scope was 1000mm.

[Stu]

14 hours ago, Gfamily said:

There are refractor users who will tell you this is rubbish and you can go to half the f ratio.

I certainly won't say its rubbish, but I do think the 'rules' are different for newts and smaller apo refractors. With, say, an 8" newt, you run up against sky limitations before you get to exit pupil limits, so a 1mm exit pupil guideline with a 200mm scope ie x200 makes some sense, certainly in the UK.

With a small 80mm apo for example, this would leave you limited to x80, which would mean you miss out on a whole range of available performance. A good 80mm ED or triplet apo will cope with at least x160, likely more if the skies cooperate. That means using exit pupils down to 0.5mm or even less, something which I do regularly for lunar, planetary and solar observing. It does mean floaters get to be more intrusive which is why I generally use binoviewers for high power these days.