Calculate EP size to fill view? FL / 43 still work?

[iwatkins]

This is an old one, but I don't know if it still holds true.

With a given telescope, take its focal length in millimetres and divide it by 43. This then becomes the size of EP you would need to fill the view looking at the moon (or sun with suitable filtration).

Seems to hold up on my telescopes with the EPs I have.

Any comments/people find it not working?

Cheers

Ian

[DOBBY]

Using a 45 deg. FOV plossl ? or 70deg. FOV eyepiece ?

[ronin]

For a plossl it would seem to be FL/100, or pretty close.

The eyepiece would need to have a 25 degree field of view (or is it 100) for 43 to work.

The Moon/Sun is ~ 0.5 deg.

So Apparent FoV = 0.5.

Using a 50 deg plossl.

This gives AFoV = 50/Mag

Mag = SFL/EFL (Scope FL/Eyepiece FL)

AFoV = 50/(SFL/EFL)

AfoV = 50*EFL/SFL

So:

0.5 = 50*EFL/SFL

SFL/2 = 50*EFL

SFL/100 = EFL

The Eyepiece FL has to be the Scope FL/100 using a plossl to get the moon/sun fully in the field of view.

So no cannot see how FL/43 gives what you say.

[iwatkins]

Ok, thanks guys.

Not sure why I remember this or where it comes from. But I guess doesn't hold true after all.

Cheers

Ian

[John]

Maybe it's 42 ............

[RikM]

Sorry Ian, I don't think this is quite right. Using a white light ST80 today 400mm / 43 = 9.3 and a 10mm is just about right to give a nice view. But with my Dob 1200 /43 = 27mm and actually a 14mm is about right.

[iwatkins]

Thanks John

Thanks Rik. I'm racking my brains where this came from. I remember the numbers but not why. Maybe it only applied for certain scopes/certain situations/certain EP type.

Did you get much sun at Slimbridge today? Had about five minutes of total sunshine in Stroud today

Cheers

Ian

[ismangil]

I just go to TeleVue calculator page...

Perry on mobile (so blame strange words on text prediction!)

[RikM]

Did you get much sun at Slimbridge today? Had about five minutes of total sunshine in Stroud today

Cheers

Ian

Probably not much more than that We had a couple of clear spells but nothing to write home about. Better than I had feared given the weather forecast. We had two PST's, a projection solar scope, my Baader film ST80, and a Lunt 60THa so a good spread of kit available. The breaks in the clouds were just about enough to get the scopes on target and focussed and then let one Joe Public have a quick look before clouding over again. Frustrating but fun.

[iwatkins]

Tough one Rik.

I kept seeing blue sky and was hoping you guys were having clearer weather.

Cheers

Ian