12 replies to this topic

[Jupiter71]

Hello all- I’ve searched and not really found a single answer.

 

Although based on my own limited experience as well as what I’ve read I’m pretty sure I know the answer…

 

Based on 400x maximum usable magnification (200mm aperture x 2 formula) the eyepiece needed would be approximately 5mm.

Im in a bortle 4 sky location in Texas- and I realize that has nothing to do with “seeing” conditions, but is a 5mm usable? The math says yes but I have my doubts…

 

Currently have a 10mm Ultima Edge and a 9mm X-Cel Celestron eyepieces (plus a 24mm Ultima Edge).

 

I know I need something in the 12-18mm range, and I’ll get them in time.

 

just curious of everyone’s thoughts regarding best magnification for planetary views - and if it’s worth getting anything in the 5mm-8mm range specifically for use in an EdgeHD 8…I do not currently have.

 

thanks in advance and happy star gazing!

Edited by Jupiter71, 13 March 2025 - 08:19 AM.

[Martinbruce]

In my experience the short answer is no. 5mm in your scope is 406x way too much. I have a Evo8 Non-HD scope and typically don’t go higher than the 150 - 200x range. At 406x you would see a fuzzy, indistinct blob which you would be unable to focus. As you note, rightfully so, 12 - 18mm range would be the sweet spot, perhaps a little higher magnification if the seeing permits.  Besides, the Field of View for a 5mm would be awful Small. Play with this calculator. https://astronomy.tools
 

hope this is helpful. 

[Jupiter71]

In my experience the short answer is no. 5mm in your scope is 406x way too much. I have a Evo8 Non-HD scope and typically don’t go higher than the 150 - 200x range. At 406x you would see a fuzzy, indistinct blob which you would be unable to focus. As you note, rightfully so, 12 - 18mm range would be the sweet spot, perhaps a little higher magnification if the seeing permits.  Besides, the Field of View for a 5mm would be awful Small. Play with this calculator. https://astronomy.tools
 

hope this is helpful. 

Yes- very helpful and confirms my suspicions, as well as my own experiences.

 

thanks for the detailed response!

[SeattleScott]

It would certainly be pushing it. Generally I recommend 7mm or so for max magnification with a C8. Now, if your location has particularly good seeing, it could work. But probably only rarely, if at all. There is also the question of whether your mass produced optics can handle that much magnification. And finally, you would only be cranking up that high on the Moon, and maybe Jupiter, with that scope.

My rule of thumb: 50x per inch for refractors, 40x per inch for obstructed scopes. And even then you can run into limits from the atmosphere.

[Sincos]

Think of it this way , might make more sense . Just because the speedometer in your automobile goes up to 180mph does not mean that it can achieve that speed nor that it is controllable at that speed . The field of view appears to narrow much like the space between the lines seems much smaller at terminal velocity.

[hfjacinto]

I used to have a Celestron 9.25 and on many days the best I could do would be 9mm or 260 Magnification, but on a few great days, I was able to pull out a 5.5 which got me 430 magnification and those are the days you remember. Considering I have other scopes that a 5.5MM is fine I still have it but its not often you can use that high of a magnification. As to FOV, there are objects that merit that limited field of view (like planets, planetary nebula, close double stars, looking for Sirius B)

[Notdarkenough]

I have the Original Nagler line of eps, 4.8, 7, 9, 13mm. I do get to use the 4.8mm several times each year. Granted, I do live in a high mountain desert, but maybe you, too, have the skies to support it. The misuse of the Bortle Scale has nothing to do with Seeing, which is what determines how much magnification one can effectively utilize on any given moment. And Seeing isn't a whole-night thing. On nights of Excellent Seeing, when I ramp up the magnification on any given planet, the clarity shifts  from moment to moment. 

 

The best source of information on what your skies can, and cannot, support, isn't here on Cloudy Nights, it is in your local astro club.

[T1R2]

Certainly your scope is capable of 400x, but usually sky conditions limit us to much lower than that unless you are in the south east and its summer time with a lazy high pressure dome surrounding you, many of us use high power of that mag when we can, and double stars like Antares need/ benefit from it. Most the time 150x -200x being normal powers. I routinely use my telescope theoretical max 254x in the SE at least 50% of the time, but sometimes the sky just wont cooperate and i'm stuck to a measly 165x. And usually if 250x doable then so will be 300x+

[GGK]

It depends on location.  Where I live now, I was able to use a 5mm in my C8 maybe 10% of the nights.  When I lived in Pennsylvania under the jet stream, I usually hit max magnification at around a10mm, and went to 7mm maybe 30 to 40% of the time.

 

A good zoom (with a Barlow if needed) for the 5 to 10 range is nice to have when riding the Seeing limit.  When right at the edge, the image will go from sharp to blob and back to sharp in little time, and I like the ability to increase and decrease the power quickly as needed.

 

Gary

[Jay_Reynolds_Freeman]

You might use 400x occasionally, when seeing permitted, but probably not often even if seeing were always perfect.

 

I second the recommendation for 7 mm, unless you know that you have good seeing and are particularly interested in objects that can benefit from high magnification, in which case a 6 mm might be reasonable.

 

At f/10, most zoom eyepieces will work well, and there are several that would give you a range of medium to high magnifications, notably the SvBONY 3-8 mm zoom.

 

"Try before you buy" (if possible) is perhaps the best advice.

 

 

Clear sky ...

[ABQJeff]

Keep the 5mm for the rare occasions that seeing is 400x excellent (or you move to Florida).   300x is the average best seeing at most locations, worse in New England much better in Florida (where 1000x with big scopes is possible).

 

Next state over in Albuquerque, 400x capable seeing is pretty common.

[Starman1]

Hello all- I’ve searched and not really found a single answer.

 

Although based on my own limited experience as well as what I’ve read I’m pretty sure I know the answer…

 

Based on 400x maximum usable magnification (200mm aperture x 2 formula) the eyepiece needed would be approximately 5mm.

Im in a bortle 4 sky location in Texas- and I realize that has nothing to do with “seeing” conditions, but is a 5mm usable? The math says yes but I have my doubts…

 

Currently have a 10mm Ultima Edge and a 9mm X-Cel Celestron eyepieces (plus a 24mm Ultima Edge).

 

I know I need something in the 12-18mm range, and I’ll get them in time.

 

just curious of everyone’s thoughts regarding best magnification for planetary views - and if it’s worth getting anything in the 5mm-8mm range specifically for use in an EdgeHD 8…I do not currently have.

 

thanks in advance and happy star gazing!

Whether you can use such a power depends on the 3 "C"s: Cooling of the optics, Collimation of the optics, and Conditions of the sky.

I had an 8" SCT for 11 years and only saw seeing that allowed a 5mm eyepiece once.

A more practical high power would be that of an 8mm (~250x).

A decent set for an 8" SCT would be:

40mm (2"), 20mm (2"), 13-14mm (1.25" or 2"), 10mm, and 8mm.

If your skies are light polluted, a 30-31mm low power becomes more practical.

If you use a focal reducer, your eyepieces focal lengths will correspond to the new f/ratio and a low power might be a 22mm eyepiece.

Ordinarily, though, 250x makes a practical high power in that scope, though even that will depend on Seeing.

[Jon Isaacs]

My 2 cents:

 

There will be nights when 300x - 400x is doable for viewing the planets and more may be needed for close doubles if you enjoy such things. 

 

Invest in a decent quality 2x Barlow and you won't be left disappointed on those nights when the seeing does support 300x-400x.  And more importantly, you won't be left wondering what might have been on those nights when the seeing supports 250x or so.

 

Jon