Fast Scopes?

[fauxpas (Tony)]

Why are lower F ratio scopes often referred to as fast?

If F5 is fast and F8 slow, if the focal length for both scopes is 1000mm but the F5 lets in more light, what does speed have to do with it?

[Kunama]

Aperture size !!!
An F5 1000mm scope is 200mm diameter
an F8 1000mm scope is 125mm diameter

The bigger aperture gobbles up more photons,
its like filling a bucket with a hose, 20cm diameter hose with constant flow fills the bucket two and a half times FASTER than a 12.5cm hose.

Much faster !!!

The terminology was particularly relevant in film photography where in order to form an image a certain number of photons are required,
they may be gathered quickly with a large diameter diaphragm (fast lens or aperture) or over a longer period with a smaller opening (slow lens or aperture)
With telescopes its the same thing, the scope is the lens, our eye is the camera and the brain sometimes does the processing and archival storage.

[Poita (Peter)]

Well, if I image at f2 with the hyperstar on a C9.25, which is around 650mm focal length, then I can capture an image 30x faster than a 650mm f10 scope.

So instead of needing a 9 minute exposure, I can get the same data in 18 seconds.

I'd call tha fast

[Kunama]

Originally Posted by Poita

Well, if I image at f2 with the hyperstar on a C9.25, which is around 650mm focal length, then I can capture an image 30x faster than a 650mm f10 scope.

So instead of needing a 9 minute exposure, I can get the same data in 18 seconds.

I'd call tha fast

Yes and no, the relationship changes somewhat due to 'reciprocity failure' with long exposure times but essentially that is correct.
CCD imaging is not my strong point so I don't know if there are differences in the relationship changes.

[Kunama]

Then of course there are the "photon-hogs" with enormous apertures like the SDM and Obsessions with fast apertures that just suck up even their neighbours photons, just not fair.

Fast scope are great for imaging, as a shorter 'exposure' time means the image is less likely to be affected by image shift, changing seeing conditions, etc etc.
Slow scope have their place though, speed is not everything....usually fast lenses have a smaller 'in focus' zone.

[mental4astro (Alexander)]

As Matt mentions, for the focal ratios you mention with the same focal lengths, the difference is the aperture.

But, if you have two scopes of the same aperture, but different focal lengths, the f/ratios will be different again. The terms 'fast' & 'slow' are photographic in origins, and their application to astronomical scopes is slightly different. Fast scopes are also called 'rich field telescopes', as they will see a wider FOV than a slow scope. Slow scopes tend to be more geared towards planetary applications in visual use.

There's another thing, get two scopes, same aperture, different f/ratios, but SAME magnification and the resulting image will be equally bright in both scopes. You'll need different focal length EPs, but that's all. No difference in brightness as both scopes take in the same amount of light.

Peter gives the photographic application in astronomical scopes.

Again following on from Matt, I've got a 'light bucket' 17.5" dob that's brilliant for deep sky stuff. But for the Moon & planets I love my C8. Different applications of f/ratios, aperture, optical design and resulting image.

[fauxpas (Tony)]

Cool cool... makes sense now...

[Terry B]

And just to complicate things even more, the f ratio only affects extended sources and not stars. Stars are point sources and are essentially not affected by the "speed" of the scope only by the aperture.

[alistairsam]

isn't it also to do with the overall surface brightness of an object due to the FOV in fast and slow scopes?
Objects in fast scopes are smaller and therefore apparent brightness higher than slower scopes where its the inverse??
So a 2 min exposure in an 8" F6 of Orion would have less detail and brightness than the same exposure in an 8" F4 due to the difference in size and area covered on the ccd, I think!!

[Steffen]

It also must have something to do with the air, because in vacuum all scopes fall at the same speed.

[Peter.M]

Originally Posted by Kunama

Yes and no, the relationship changes somewhat due to 'reciprocity failure' with long exposure times but essentially that is correct.
CCD imaging is not my strong point so I don't know if there are differences in the relationship changes.

Reciprocity failure is not generally refered to in CCD photography. Dark current in the camera may cause a similar phenomena however we usually keep that to a minimum with cooling.

[MortonH]

Fast scopes are also more likely to show up optical aberrations that slow scopes. A fast Newtonian will show coma towards the edges of the field of view. A fast refractor will likely have a lot of chromatic aberration (false colour) unless it's a very expensive triplet. That's why the cheap refractors are f/9 or f/10.

Morton

[BPO]

Originally Posted by MortonH

A fast refractor will likely have a lot of chromatic aberration (false colour) unless it's a very expensive triplet. That's why the cheap refractors are f/9 or f/10.

Or one of the many decent and inexpensive doublets in the f/6-f/7.5 range.

[OICURMT]

Originally Posted by Kunama

the relationship changes somewhat due to 'reciprocity failure' with long exposure times but essentially that is correct.

Reciprocity Failure is a non sequitur statement regarding modern electronic photography, as electronic "film" dumps the photons when the wells get "full", allowing more photons to be captured without reaching the a limit (i.e. failure). Divergence (non-linear behavior) from the well capacity of CCD's I guess could be seen as a type of RF (though I would argue otherwise).

In the old days (yes, I'm getting old), Rep Fail was a serious limitation. We overcame some of it via hypering films and cooling (I lived in Colorado, so winter time was good for shooting).

Not many of us old hypering types around... I guess the grim reaper eventually will cull us out of the general population...

OIC!

[Kunama]

Originally Posted by Kunama

Yes and no, the relationship changes somewhat due to 'reciprocity failure' with long exposure times but essentially that is correct.
CCD imaging is not my strong point so I don't know if there are differences in the relationship changes.

I am too old for this digital stuff, I was thinking of fillum, re diggital see bold.

[GraemeT (Graeme)]

Originally Posted by Steffen

It also must have something to do with the air, because in vacuum all scopes fall at the same speed.

There's one in every forum!

[Poita (Peter)]

Originally Posted by OICURMT

In the old days (yes, I'm getting old), Rep Fail was a serious limitation. We overcame some of it via hypering films and cooling (I lived in Colorado, so winter time was good for shooting).

Not many of us old hypering types around... I guess the grim reaper eventually will cull us out of the general population...

OIC!

I don't miss hypering fuji film, it was a big deal in the 80s
Not much use for those skills now. Thankfully!

[cometcatcher (Kevin)]

I used to buy hypered film from a lady in NSW so saved me doing it. Most of my scopes are from the 80's and 90's where they needed to be fast to overcome the problems of film.

Even now with digital there's advantages with fast scopes, mostly with photography. It's not so much an issue shooting unfiltered, but once filters are put in the light path, a fast scope helps make up for the extra exposure time needed to get that histogram off the left wall. With that in mind and my fascination for filtered photography, I find myself still acquiring fast scopes. Plus it's always cloudy here in summer. You gotta be fast shooting between the clouds.