DIY quadruplet achromat

[jnp]

I fancied a go at wide field astrophotography for a change, but I only have long focal length scopes. I could go and buy another scope, but that would be a waste of a good bodging opportunity

Here's a cheeky quadruplet. The outer objective lens is a Meade 90mm F9 (bought of an SGL member). There is also a Meade 70mm F8.5 halfway inside. The 'quadruplet' is therefore two cheapish achromat doublets. The combination of doublets is behaving like a focal reducer. By changing the distance of the lenses, via a tube within tube & hose clip arrangement, I can vary the effective focal length according to the formula in this Focal Reducer Calculator link. It's giving about F5.3 at the moment.

This didn't cost me any extra as I cannibalised my dormant long FL drainpipe refractor and already had the 70mm F8.5 lens and plastic tubing handy.

My first light astro full frame image is here: http://stargazerslounge.com/imaging-deep-sky/129716-rosette-first-light-diy-quad-acro.html

The vignetting/coma with a DSLR sensor and CA performance is better than I thought, but I need to try this without the Baader UHC-S filter to be sure.

It's not going to match an ED apo performance, but this will open up targets otherwise two large for an 8" Newt.

thanks for looking

John

[Rob]

Very interesting indeed.. takes a nice shot also. well done

Rob

[msinclairinork]

Very clever bodging!

And with a great result!...Job's a good'in!

Well done,

Michael

[ollypenrice]

Ah, well done. Great bit of improvising.

Olly

[michael.h.f.wilkinson]

Brilliant! Really like the shot of the Rosette as well.

A colleague of mine mentioned he once picked up a collection of all sorts of (mainly doublet) lenses from some car boot sale (about EUR 10 for a box full of old lenses). We have been thinking of building various bits of kit with it. Maybe this is the way to go

[philj]

I like that alot, something to think about for a couple of objective sets I have hanging around. It takes a nice image as well the FOV is excellent to get all that Rossette on a frame.

Philj

[jnp]

Thanks for the encouragement guys.

It would be good to develop this further to modify the F ratio on the fly.

cheers

John

[michael.h.f.wilkinson]

Thanks for the encouragement guys.

It would be good to develop this further to modify the F ratio on the fly.

cheers

John

Aiming for a quadruplet varifocal lens?

[Glasspusher]

Very interesting, worth further investigation. Nice image.

John

[beamish]

I love stuff like this ! , pretty sure I said that last time

[philj]

Thanks for the encouragement guys.

It would be good to develop this further to modify the F ratio on the fly.

cheers

John

Now that would be something, a zoom scope instead of a zoom eyepiece, just adjust the FL of the scope to the object your imaging:cool:

Philj

[catburglar]

Very interesting & a great photo. Can I just check- what's to stop someone with an ED100 and an ED80 mounting the lens cells 500mm apart and ending up with something akin to a 100mm F5.4(not too dissimilar to a Tv NP101?). Are there other aberrations that come into play?

[jnp]

just adjust the FL of the scope to the object your imaging

I can sort of already do that by reducing the gap between the lenses and extending the focuser out. The snag is that the R&P focuser is happiest when its not extended very far. A genuine zoon scope that keeps focus is probaby beyond my bodging!

Are there other aberrations that come into play?

I am not sure if this will compromise the performance of using premium ED optics, but I certainly hope you will top of performance of the £20 of lenses I used! I would expected this amplify coma a bit. Colimation is also critical. Messing about with ED optics also sounds a risky too.