Impossible Scopes?

[Alien 13]

I have often wondered why the mainstream imaging scopes stop at around 400mm FL admittedly a few go below this but not much less than the 60mm borg at 245mm is there a technical reason for this or just a matter of cost in that a 200mm F4 that could give perfect coverage of a full frame sensor is likely to cost more than a std 80mm triplet or quad.

Alan

[ronin]

If the focal length is "short" then the surfaces are a tighter radaii, neither a parabolic or a lens has the ideal shape so aberrations are magnified, in the case of a lens it is spherical aberration. If the radaii are too small then the image becomes unusable for anything. Tighter radaii also mean more grinding and so more time and cost.

The other is image size. The "faster" the scope and so smaller focal length (to an extent) then the smaller the image.

We tend to waht an image of a workable/usable size.

[DaveS]

Also, as soon as you're below about 400 mm you're into the realm of photographic lenses, where economies of scale give them an advantage. The only astronomical players in this area are hideously expensive quadruplets and quintets.

[Alien 13]

I thought it might be a cost thing camera lenses are great but often have to be stopped down then have diffraction spike problems i just wondered why we have no 100-200mm APO scopes at F4 or even better at F1 that don't cost as much as a house.

I did say impossible...

Alan

[pete_l]

 i just wondered why we have no 100-200mm APO scopes at F4 or even better at F1 that don't cost as much as a house.

You're also down in binocular territory at those sorts of FLs

[Tiny Small]

The other is image size. The "faster" the scope and so smaller focal length (to an extent) then the smaller the image.

We tend to waht an image of a workable/usable size.

The image will be whatever size the sensor throws out no matter what lens or scope you put on it. But yes, the subject will be smaller in the image

[ollypenrice]

They are available as camera lenses and the best ones can be used fairly wide open. Camera lenses, remember, are deliberately designed down to F ratios which can never give a perfectly focused full frame because, frequently, photographers want most of the image out of focus - in portraiture for instance. The ultra fast f ratio is not designed in to obtain ultra high speed but to give the possibility of massive restriction in focal depth so a face can be pin sharp in an otherwise blurred field. ('Bokeh')

You don't have to use the diaphragm to stop down, you can make a front aperture mask.

The Takahashi FSQ85 gives a FL of 328mm and an image circle which cannot quite cover a full frame chip, so I guess this is going to define the point at which 'impossible' begins.

You also have to bear in mind that full frame CCD cameras don't, as yet, come with pixels small enough to work happily at very short focal lengths.

Olly

[Alien 13]

Thanks for the replies everyone it does appear that it is more of a technical issue in producing a scope with these kind of specifications, as i mentioned in my original post this one looks to be right on the limit http://www.firstlightoptics.com/borg-astrograph-telescopes/borg-60ed-f4-set-b06040.html.

An observation that I have made with camera lenses is that aperature masks dont quite do the same job as an iris the mask introduces vignetting and the iris doesn`t mabye one day a lens manufacturer will introduce an "astro" version optimised for wide open imaging at infinity.

Alan

[Peter Drew]

Schmidt cameras can work down to F 0.5.  

[uhb1966]

Theres a set from teleskopservice that can turn any Celestron nexstar into a f2 unit... but you need a hideously expensive CCD to really use it