Max magnification question

[tripleped]

Have a Skywatcher flextube 250p 10” GOTO DOB.  Company told me the avg max mag is 300x.  Live in a suburban Midwest metro so pretty significant light pollution. What have others found to be their max useful mag? Have tried 8mm astromania ep with celestron xcel LX 2x Barlow to give me 300x but so far underwhelmed. Image gets fuzzy. Even on what appears to be a good viewing night with clear views without the Barlow. Suggestions?

[johninderby]

Try the Televue eyepiece caculator. 

https://www.televue.com/engine/TV3b_page.asp?id=212&plain=TRUE

[Mr Spock]

It all depends on the seeing conditions.  I used to have Skywatcher 250mm Newt. I mainly used to keep it to around x200. I had a few good nights where I could use a 4mm eyepiece to get x300. One very memorable night looking at Mars at x300 not repeated since.

[Les Ewan]

The usual rule of thumb is X2 the apature of the primery in millimetres this is also what is used by manufactures. This means your 250P's upper limit is 500X.As Spock says everything  depends on  seeing. I have the 400P which in theory goes up to 810X but Ive seldom gone over 300X with it,and this seems to be the upper limit of seeing the vast majority of the time. So the upshot of all of this is you get large telescope for resolution and light grasp not magnification and if a telescope provides a comforable image quality at 300X in good seing thats all thats required as far as magnification is concerned.

[tripleped]

Thanks to all for the responses. Gave me the perspective I needed. Now I know what to expect and what not to expect.  Thanks for the help!

[hoss]

Hello tripleped

One thing I have learned over the years is that I have wasted A LOT of money buying expensive  eyepieces when cheap ones work better!!!! Generic plossl  32mm for $30.00 is as good as my Televue Panoptic 19mm for $275.00. Why? Because of the focal length. A 15 mm plossl is not as good as the 19mm panoptic. A plossl less than 9mm is junk. 9mm is the shorted fl plossl usable in my book.

 

So be careful trying to fix an unfixable problem by chasing expensive eye pieces.

 

You need need use your optical instrument with its capability. A newtonian is best used for wide field of view, at lower power gathering a lot of light. Of course newtonians can be used for all types of viewing. 

 

Now maximum usable magnification can be effected by the condition of the atmosphere (there is seeing conditions and there is also transparency), was your telescope cooled down to ambient temperature, was there dew on the mirror, was the scope columnated, do you need parabolic correction,   how accurately is the mirror figured, what image were you looking at? Jupiter, the moon, a double star, nebula, open cluster, globular cluster, galaxy? Each target has different requirements of field of view, magnification, dark sky, steady seeing. Every night will be different.

I have had my Nexstar 11 at 2000x viewing mars in 2005 one time, never again. I have viewed Saturn  in 2004 at  1400x one time.  But I just use various eyepieces to get the best image.  Mostly that means using 32 mm plossl in a Denkmeir bino viewer with a power switch in and F10 SCT with a 2800mm Focal length. This is all I ever really use for any type of viewing. I have no  idea what this magnification is, all I know is that this works for me.

I don't know how to put this, but the idea of a maximum magnification is not of importance.  There is really no real rule of thumb that matters in my book.  Some writers will state that a very good optical system can handle magnification of a certain amount per inch of aperture. I think that for very high end refactors such as a Takahashi or AP or TEC is something like 50 x per inch.

But that really does not matter.  All that matters. is what you have in front of your eyepiece.  To put things in non technical terms, when I was in school we had movie projectors or slide projectors.  If the slide projector was close to the screen the image was small but it had very bright colors and the image was crisp. If the projector was moved back, the image got bigger, but the colors were not as vivid.  The same thing happens with a telescope.  At low power, say a 32 mm plossl the image will be very crisp and bright. Now when you use say a 7mm Pentax XL, the image is slightly bigger, but the colors as not as vivid. Also with a barlow you are losing some of the light cone, called vignetting.  Your telescope is a Newtonian at F 4.7.  I am not an expert, but most  Newtonians are not know as  superb optical instruments. There are very good mirrors out there, but by their design capabilities, they usually are not near what a very expensive apochromat refractor is or what a Takahashi Melon  Dall-kirkham is.  

To learn about how eye pieces and telescopes  work search the youtube for "how to choose you eye piece" with david nagler televue optics.

 

 

 

 

 

 

 

 

[Alfian]

A useful idea to work with is exit pupil size, that is the aperture divided by magnification. Your 250p (1200) with say a 20mm EP will give 60x and this will give just over a 4mm exit pupil. An eyepiece yielding 300x will give 0.83mm exit pupil. I find with my slightly aging eyes that anything much below 0.5mm and I run into problems with seeing floaters but by this time the image is notiecably dimming. The point here is that pushing magnification to the max is not always a good thing and in terms of seeing detail although object image size is smaller, often less is more so to speak. That said for a scope like yours with a sizeable aperture pushing magnification to 300x (+) is more likely to be limited by conditions and in practice will be significantly less. The good news is that with all that aperture DSO hunting should be rewarding.

[tripleped]

Good to know thanks for the valuable insight. Had  really good DSO viewing the other night even though planetary viewing wasn’t ideal. It seems in my limited experience that a larger aperture scope is more forgiving of less than ideal conditions for DSO’s as opposed to planets. 

[John]

With Jupiter and Saturn where they are, viewed from the UK (ie: low down) smaller aperture scopes seem to be able to cut through the atmosphere better than larger ones.

When they are higher in the sky though, the additional aperture shows what it can do.

300x is going to be too much for Jupiter much of the time. Might be OK for Mars and Saturn if your scope is in good collimation and is cooled down. I was using 300x last night with my 100mm refractor on Mars and the views were pretty good. Excellent optics in that scope though.

 

 

Edited October 15, 2020 by John