Ultra-short Focal Length Telescopes?

[Gib007]

Hello all! Sometimes I get the urge to image a very large portion of the night sky, with or without mosaics. I've got a Borg 77EDII with a focal length of 368mm to handle widefield imaging and an Altair Astro 8" RC with a focal length of 1103mm to handle narrowfield imaging (both use appropriate focal reducers). What I'm looking for is an ultra-short focal length telescope for ultra-widefield imaging. 

I've seen the Borg 60ED telescope, which works at f/4 to provide a focal length of just 245mm. However, I've compared the view of this vs my Borg 77EDII on Stellarium and it's pretty similar. I'm therefore looking for focal lengths of around 100mm or less. The obvious answer of using DSLR lenses is not going to work out, I'm afraid, since I'm upgrading my QSI 660i to a 660wsg-8 soon and the backfocus on the 660wsg-8 is just over 50mm, going over the DSLR lenses' 45mm requirement. I could upgrade to the 660ws-8 (no guide port, but can work with DSLR lenses via a QSI adapter) but the guide port is advantageous enough to not pass up (no more need for separate OAG!). 

My question is simple - what could I use as an ultra-short focal length telescope (lens) if not a DSLR lens? Thank you for any suggestions you may have. 

[swag72]

I'd be interested in this too as I use a QSI-wsg and so cannot easily use camera lens'. I suppose that one alternative would be medium format lens' as they have a much longer back focus requirement as far as I know - But am willing to be corrected if this is not the case.

[Astrobits]

You really are asking for objectives in the camera lens focal lengths arena. Even binocular objectives are ( mostly )longer than 100mm. I have just checked a binocular objective ( ex 10 x 50 bins ) and it is approx 170mm fl.

I suspect that you will have to find a suitable objective and have it built into a dedicated OTA for your purpose. You could try Ian Poyser for a suitable lens and he might be able to turn up an OTA to fit.

http://www.irpoyser.co.uk/

Alternatively, as Sara says, look at camera lenses for the larger formats.

Nigel

[DaveS]

Pentax 67 lenses look to be reletivly inexpensive, have a look here

http://www.ffordes.co.uk/category/Medium_Format/Pentax/67_Lenses

As for adapters, you may need to find something like a Pentax to Canon then a Canon mount CCD adapter, else try to have something machined bespoke.

[zicklurky]

Pentax 6x7 lenses need ~85mm back focus I think, so much more space to play with than regular lenses.

[Gib007]

All excellent information to chase up. Thank you very much. If Pentax 6x7 lenses have an 85mm backfocus requirement, then that would be absolutely perfect as I just need to add 35mm to the QSI 660wsg-8's existing 50mm and it's done! Thank you again. 

[zicklurky]

I've been investigating the Pentax lenses as well, seems the ED IF versions are very high quality, although at a higher price. I've seen amazing images from the 300mm M* ED IF. I'm just curious how to connect the Pentax K mount to T thread...

[perfrej]

I would consider this more an issue of CCD chosen than scope chosen. Many good CCD cameras have too small sensors to provide any significant field of view. Try entering your scope's focal length in Stellarium with a full frame, or even larger, sensor. The difference is staggering. I like wide-field as well but I am betting on a new camera in order to increase the field of view. I am going to wait a while and save up for an FLI 50100

/per

[swag72]

Actually Per I think that you made a very valid point indeed - But sadly sometimes we have to make compromises

[Psychobilly]

This is the website I use when looking for lens register info...

http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html

Peter...