What focal ratio best suits a 40mm plossl?

[bendiddley]

As the title says, I want to know what focal ratio is best for a 40mm plossl. I know some say f/6 or slower  ie.Celestron 40 Omni. Is this the case in practice or would an even slower scope be better? Cheers.

Edited March 25, 2021 by bendiddley

[scotty38]

Slower scopes are easier on eyepieces than fast(er) ones so I have no reason to think it'd be anything other than just fine.

 

BTW I am assuming you have a scope slower than f6

Edited March 25, 2021 by scotty38

[John]

If you have not already purchased the 40mm I would urge you to consider the 32mm instead.

Looking at your scopes, a 32mm eyepiece will show pretty much as wide a true field of view and have a more effective exit pupil (the diameter of the bundle of light that exits the eye lens of the eyepiece). The optical quality of the 32mm will be the same as the 40mm.

 

[vlaiv]

In fact, we could argue that it is not suited to faster than F/6 because of exit pupil.

Say you have F/5 scope and want to use 40mm Plossl. You'll get 8mm exit pupil and since odds are that your pupils can't dilate that much - you'll see less light than your scope is capable of delivering. Your eye will act as aperture stop.

[bendiddley]

23 minutes ago, scotty38 said:

Slower scopes are easier on eyepieces than fast(er) ones so I have no reason to think it'd be anything other than just fine.

 

BTW I am assuming you have a scope slower than f6

Thanks, but I'm wanting to know how it fairs on slower telescopes. I am curious. I have an f5 and know it wouldn't suit that. Celestron says it suits f6 and higher, just wondering how it fairs for slower scope and how slow it needs to be.

[John]

6 minutes ago, bendiddley said:

Thanks, but I'm wanting to know how it fairs on slower telescopes. I am curious. I have an f5 and know it wouldn't suit that. Celestron says it suits f6 and higher, just wondering how it fairs for slower scope and how slow it needs to be.

The slower the scope, the better the correction at the edges of the field of view will be. I presume that Celestron feel that F/6 is where the correction will be tolerable for most observers but it is still not likely to be perfect. In the centre of the field of view eyepieces generally perform pretty well regardless of scope focal ratio.

At F/10 and slower these plossls will be pretty much sharp right across the field of view.

 

 

Edited March 25, 2021 by John

[bendiddley]

3 minutes ago, John said:

The slower the scope, the better the correction at the edges of the field of view will be. I presume that Celestron feel that F/6 is where the correction will be tolerable for most observers but it is still not likely to be perfect. In the centre of the field of view eyepieces generally perform pretty well regardless of scope focal ratio.

At F/10 and slower these plossls will be pretty much sharp right across the field of view.

 

 

Thanks. Is this the same for a 32mm plossl too ie. need much slower optics, what's the baseline for this? Or can faster scopes benefit from these as well, say f5?

[bendiddley]

5 minutes ago, bendiddley said:

Thanks. Is this the same for a 32mm plossl too ie. need much slower optics, what's the baseline for this? Or can faster scopes benefit from these as well, say f5?

Also when you say correction at the edges, what sort of corrections do you mean ie. better light transmission, sharper stars, less coma etc?

[John]

2 minutes ago, bendiddley said:

Thanks. Is this the same for a 32mm plossl too ie. need much slower optics, what's the baseline for this? Or can faster scopes benefit from these as well, say f5?

I have not used all the Omni plossls but I have used quite a lot of similar quality ones. Optically they perform similarly across the the range of focal lengths. In the shorter focal lengths (10mm and less) the eye relief starts to get pretty tight.

In the central area of the field of view they will be fine in an F/5 scope but the optical correction may well suffer a bit in the outer parts of the field of view. There is not a sudden focal ratio cut off where an eyepiece simply won't perform, it is a gradual thing.

By correction I mean distortions to star images, mostly caused by the optical issue astigmatism which causes stars to take on an elongated "seagull" shape towards the outer parts of the field of view. At F/5 coma can also be an issue in newtonians but that is usually generated by the primary mirror of the scope.

This article gives more information:

http://umich.edu/~lowbrows/reflections/2007/dscobel.27.html

 

 

[vlaiv]

21 minutes ago, bendiddley said:

Celestron says it suits f6 and higher, just wondering how it fairs for slower scope and how slow it needs to be.

I think that confusion comes from terms faster / slower and lower / higher.

I believe that terms lower / higher are referencing actual number in F/number expression - hence F/6 is lower than F/10 because 6 is lower number than 10.

Phrase F/6 and higher should be therefore interpreted as F/6 and slower

[bendiddley]

1 minute ago, vlaiv said:

I think that confusion comes from terms faster / slower and lower / higher.

I believe that terms lower / higher are referencing actual number in F/number expression - hence F/6 is lower than F/10 because 6 is lower number than 10.

Phrase F/6 and higher should be therefore interpreted as F/6 and slower

Ah yes, good point!