Achieving Focus with my Lunt Herschel Wedge

[David Smith]

I was not sure quite where to post this so stuck with familiar territory!

I picked up a Lunt 1.25" Herschel Wedge some time ago now and it is fantastic for visual and  also close-up solar work with my QHY5 cam. I'd like to capture my full disk images using my 1100D and the wedge but unfortunately I cannot reach focus. I know others have solved this by using a barlow but with my Evostar 120's 1000mm FL this leaves me needing to create a mosaic to cover a full disk.

My DSLR very nearly reaches focus with the wedge so I am wandering if the following is possible as it may well solve the problem.

The wedge has a "built in" ND3 filter which is assembled as per the picture below (top to bottom - wedge, expansion ring, eyepiece holder with built in ND3)

What I am hoping I can do is do away with the expansion ring to give me  a few extra millimetres in-focus.

Is it possible to purchase an eyepiece holder that will screw directly into the wedge? I would then need to either extract the built in ND3 or purchase a separate one and attach this to the camera T-adaptor.

Thoughts / suggestions from the SGL imaging brain trust are much appreciated!

 

[xtreemchaos]

I havnt a clue mate sorry, if there isn't a fix thay work well with a st120 and ed80 with 1,5x barlow. goodluck mate.nice and clear here thismorrning but the seeings pants.. charl.

[michael.h.f.wilkinson]

My Lunt wedge has a slightly different configuration, as shown below.

The ND filter isn't placed in a little extension ring, but is screwed inside the barrel. I think it is simply a 1.25" ND3 filter, which screws into place. Maybe one fits in your barrel too

[David Smith]

Thanks Charl. I may well end up with an st102 or ed80 but for now this would be a cheaper alternative.

[David Smith]

Thanks Michael. The problem is the eyepiece holder is too wide to screw directly into the wedge hence wanting to replace it.

[michael.h.f.wilkinson]

31 minutes ago, David Smith said:

Thanks Michael. The problem is the eyepiece holder is too wide to screw directly into the wedge hence wanting to replace it.

I clearly have a different model then. Bit strange, really. Maybe some barrel from a star diagonal fits. If that is the case, you could use a 1.25"-T2 adapter, with an ND3 filter screwed into the nose piece.

[james_screech]

Are you sure you want to use a DSLR through 1.25" fittings? You will have a big  problem with vignetting with the distance from the imaging sensor to the scope side of the wedge.

[David Smith]

Hi James

Based on what I have seen so far there is no evidence of vignetting. I can all but achieve focus and the image looks OK. I used to use 1.25" fittings with this scope / DSLR combo previously without difficulty.

[michael.h.f.wilkinson]

4 minutes ago, james_screech said:

Are you sure you want to use a DSLR through 1.25" fittings? You will have a big  problem with vignetting with the distance from the imaging sensor to the scope side of the wedge.

I have done this and with crop sensors you are fine. Full frame would be a problem

[Macavity]

55 minutes ago, David Smith said:

Mine is just like the above (David Smith) version!
Oooh, I had no idea that bit unscrewed... 

The ND filter sits in the "ring bit" - The other is just an extension!
FWIW, I only have about 1mm (sic) in-focus to play with so every
little would help! Of course, if one could "ID" sizes / thread type... 

But (as we all know) such things may be easier said than done?
Guessing (even measuring) is not 100% certain re. threads.

[David Smith]

Thanks Chris. It is very very close to coming to focus. I think I have a cheap 1.25" diagonal somewhere, will see if I can get the eyepiece barrel off of that and attach to the Lunt. Would at least give me an idea whether a standard 1.25" barrel will fit.

[Macavity]

If it's VERY close, I sense you could get a "tame machinist"
to "shave a bit off" the end of that extension ring. Approx.
2mm looks to available on the (larger) female side. But! 

Competence and confidence (on both sides) I guess? lol

[David Smith]

That's not a bad idea either! Thanks.

[laudropb]

Hi David. I had the exact same problem with my ST102 and Herschel wedge. Someone on here recommended a small T adapter made by Nipon. I got one from EBay. It has a small Barlow lens 1.5x but this unscrews and allows a full disk image. Tried it out a few days ago achieved focus with a bit to spare but I am now trying to figure out how to attach my continuum filter. Unfortunately it does not fit on to the barrel. Too small. This adapter cost less than £20 pounds, can't remember exactly how much.

[xtreemchaos]

that's the size adapter I use, my one is of a old asi 120 cam I think. charl.