Lensless Schmidt camera - update

[nytecam]

Today I made [out of a pizza box!] an 'aperture mask' for my piggybacked Meade 11cm f/4 quasi-Newtonian [with a spherical mirror] and converted it into a LSC.  I reused the ali strut from an earlier incarnation @  http://home.freeuk.com/m.gavin/lscmg.htm and await the next clear night to test.  I use the new  SX Lodestar-Cx2 OSC camera in brief exposure in near-live display to laptop. 

The pics below show the set-up plus schematic of the LSC.  The fixing of the aperture mask to the strut is slotted and can be rotated around this axis to provide precise alignment of the 'hole' to the optical axis.

I have in the past tested various 'aperture masks' from 70mm to 90mm diameter - in theory the smaller the aperture the sharper the images in minimising spherical aberration eg the primary optical defect.  But this makes a 'slow' system when the central obstruction is taken into account - hence my preferred 90mm aperture and nominal f/ratio of f/5. 

[nytecam]

Here's a sample M57 fov from last night via the LSC.  The image quality is much superior to the 'unmasked' quasi-Newt

[JamesF]

Interesting idea.  There are some 130mm spherical mirror OTAs about too that might make a useful donor for this kind of project too.

And I think you may just have come up with a use for those SCTs that you sometimes see on ebay with the smashed corrector

James

[Peter Drew]

The SCT mirrors at F2 would be challenging as a lensless Schmidt! The nearest I came to using some reject 8" ones was a plan to set up a multi-mirror array on a common base collectively focusing the Sun on a receptacle for sausages, Sadly...........

[JamesF]

The SCT mirrors at F2 would be challenging as a lensless Schmidt!

How about if you had, say, a C9.25 (which is f/2.3 I think?) and restricted the aperture with a mask as above to about 120mm?  There are probably easier ways to do it I agree, but just as a fun way of re-using something that was effectively useless...

James

[Peter Drew]

Unfortunately it's not that simple. To correct spherical aberration of a sphere acceptably with just an aperture mask you need to be around F7 or longer, F2.3 would still need the glass corrector plate.    

[nytecam]

How about if you had, say, a C9.25 (which is f/2.3 I think?) and restricted the aperture with a mask as above to about 120mm?  There are probably easier ways to do it I agree, but just as a fun way of re-using something that was effectively useless...

James

James - its a matter of scale that,s discussed in my linked item in my first post eg the BAA article. Optical theory says the images even in my example would be 'unacceptable' but optical theory doesn't take into account real life at the telescope eg the effects of seeing , tracking errors and post processing. Anyway I've made it work to my satisfaction whilst others do little more than 'burn ants ' and eat pizzas :-)

[nytecam]

I've tweaked the aperture mask positioning and tried for some images last night [full moon+cirrus haze] - here's an x1.5 enlargement of Vega field that shows good coma-free stars images to the edge that is very pleasing indeed - proof the 90mm aperture 'Lensless' option works. 

Some faint stars' mags added from Sloan DSS reference. No darks flats used in 1x15s exposure.

[nytecam]

Upped mask from 90mm to 100mm for more light intake and star images stil round, tight and coma-free - pics below ;-)