Herschel wedge effect on focal train?

[Giles_B]

Having persuaded @bomberbaz to part with his Scopetech f12.5 refractor, I'm looking for some advice to make sure I don't run into compatibility problems when using a Herschel wedge.

I've read elsewhere that Japanese refractors can have problems with diagonals as they are often set up to be used straight through. I'm hoping to use this scope mainly for solar with a Lacerta Herschel wedge. Steve reports he has used the scope with a prism diagonal with no problems, but it seems prudent to check whether I can expect similar performance from a solar wedge - in other words is the effect of a herschel prism similar to a normal prism diagonal - and do all prism diagonals (and all herchel wedges) have a similar effect on the focal train?

[Giles_B]

A bit more info - from the 365 astronomy page on the Lacerta wedge: "The Herschel-LAC2s version takes up about 110mm of back focus."

Can't for the life of me find data on the Takahashi 1.25" wedge that Steve is using.

[Franklin]

The Tak prism has 64mm light path.

Ask him how far out he had to rack the focuser to get focus with the Tak prism, if it was over 2" then the wedge should work.

Edited December 12, 2022 by Franklin

[Franklin]

Data on Tak prism from Bill Paolini's excellent article on Mirror versus Prism diagonals.

[Giles_B]

Thanks, that's a great article

https://www.baader-planetarium.com/en/downloads/dl/file/id/160/product/2871/testreview_mirror_vs_dielectric_vs_prism_diagonal_comparison_w_paolini.pdf

- interestingly the light paths of the prism diagonals are all clustered in a 63-67mm range, whereas the mirror diagonals are anywhere from 73-112mm

I'm hoping that Lacerta might have some definitive data on their Herschel prism.

 

[Louis D]

Prism diagonals will always have a shorter optical path than mirror diagonals of the same size.  It has to do with the physics of how light travels through glass versus reflecting off of a surface.

The 1.25" diagonals will all have shorter optical paths than their 2" equivalents due to physical dimensions.

If there isn't enough back focus for a 2" Herschel wedge, and your scope's aperture is 100mm or less, you could look into getting a 1.25" version.  I use a 1.25" Hercules solar wedge myself.

Edited December 12, 2022 by Louis D

[Franklin]

A prism has a shorter light path than a mirror if the same size fitting. Obviously 2" diagonals have a much longer light path than a 1.25". I use a 1.25" Lunt solar wedge for white light viewing and it has a similar focus point as my 1.25" regular star-diagonal. Those Japanese refractors usually have very long drawtubes to accommodate straight through viewing.

[Franklin]

Was that an echo?

What Louis D said🤣.

[Giles_B]

I do have the 1.25" wedge - so this is better news - 110mm is a lot of focus.

[Franklin]

You have the Lacerta? Isn't that the funny shaped prism one? Not sure but maybe it's design gives it such a large light path, 110mm is long for 1.25".

Whatever, I think it will work in the long achro.

[Franklin]

Brewster-angle prism???

[wookie1965]

I have the 1.25" herchel wedge and I have noticed I have more focuser length than just my diagonal. 

I had to buy a special 2"-1.25" adapter with a 1mm collar so I can focus with a 4mm eyepiece, with the wedge I have 10mm to play with so I can easily get the 4mm focused. 

Paul 

Edited December 12, 2022 by wookie1965

[Giles_B]

@wookie1965 is that the Lacerta wedge?

 

The 110mm is quoted for the 2" version - I googled in haste.

Not sure if the Brewster angle on its own would necessitate a longer light path. Anyway, the 1.25" should be less.

I rarely get good enough seeing to get to a 4mm eyepiece - my garden faces west, so the sun gets fairly high, and the seeing us usually poor, before I can start observing.

[Franklin]

I think the Scopetech 80mm F/12.5 refractor will make a superb solar-scope and your 1.25" wedge will be just fine.

[wookie1965]

16 minutes ago, Giles_B said:

@wookie1965 is that the Lacerta wedge?

 

The 110mm is quoted for the 2" version - I googled in haste.

Not sure if the Brewster angle on its own would necessitate a longer light path. Anyway, the 1.25" should be less.

I rarely get good enough seeing to get to a 4mm eyepiece - my garden faces west, so the sun gets fairly high, and the seeing us usually poor, before I can start observing.

Yes it is the herchel wedge 

[Giles_B]

@wookie1965 Okay - it looks like you have the Lunt Herschel Wedge - mine is the Lacerta Herschel Wedge, which has a "Brewster angled" eyepiece holder (which Lacerta claim helps polarisation).

 

So I surmise the long light path is probably not down to the Brewster angle.

 

[Giles_B]

17 minutes ago, Franklin said:

I think the Scopetech 80mm F/12.5 refractor will make a superb solar-scope and your 1.25" wedge will be just fine.

I hope so - it's a lovely scope. I'm going to try to work out the light path tomorrow by focusing with and without the Wedge on the Meade (if I can find something bright enough to see, even without the ND3.0)

[Stu]

@Giles_B this might be of use

This was with the f12.5 and a Zeiss T2 prism, and the remaining inwards focus doesn’t look that much.

Also, and even more helpfully is this thread from @johninderby who unfortunately has been MIA for too long, I do hope he is ok. Anyway, he did get the Lunt wedge to work but inwards focus was very tight so I think he ended up shortening the tube a little I think to accommodate eyepieces which needed more inwards focus.

[Carbon Brush]

I have a 2" Lacerta Herschel wedge. Yes 110mm sounds about right - from memory.
I went for 2" so I would have a more robust means of hanging a heavy eyepiece or DSLR on the back.
Choosing the 1.25" would have given a significantly shorter light path. And saved a few ££.
I have tried it on different refractors and in one instance had to swap the focusser. The alternative being to hacksaw the tube.
Fortunately I had a decent focusser sitting unused so that part was fairly easy.😁

Don't forget that if you want to add another filter (or two) after the wedge, the light path is extended again.
Same goes if you fit a 2" to 1.25" reducer - the added height.

HTH, David.

[Giles_B]

Thanks @Stu I knew I had read about having to shorten the OTA on a Japanese refractor, but couldn't remember where! The inward focus might just do it - Steve thought the light path might be reduced by about 20mm on the 1.25".... still it does sound like it will be touch and go, and as @Carbon Brush says, there will be not one but three filters in the light path if I'm using a continuum filter (and I'm a real convert to that piece of kit) - so some definitive measuring of the light path is in order - I'll report back after some testing tomorrow.

[Franklin]

13 minutes ago, Giles_B said:

there will be not one but three filters in the light path if I'm using a continuum filter

What filters are you using? With my 1.25" Lunt, there is the ND3.0 which stays in the wedge and a Baader SC on my zoom eyepiece. 

[Giles_B]

Polarising filter, UV/IR cut and a Contiuum. Actually it's the altair contrast booster version, not the Baader continuum, so a bit thinner, but all 3 are still a good few mm all told.

The Lacerta also has the ND3.0 built in.

I've just now found on Teleskop-Express for the bare T2 version of the 1.25" Lacerta wedge - "Light path from T2 to T2 is 61 mm" - so a bit less than the Tak diagonal. Still I'll test tomorrow to be sure.

Edited December 12, 2022 by Giles_B
Updated information

[Franklin]

I tried a polarizing filter and it was good to be able to "tune in" the intensity with it, but in the end I found the continuum filter alone to work well enough for me. I have read about the use of a UV/IR cut filter, for safety, but I don't understand why because the continuum cuts all that out as well, or so I believe. Sorry for flying off on a tangent in the middle of your thread btw, I'll shut up now🙂.

[Giles_B]

No worries, this has been a lot of hekp.

For me the uv\ir cut is better safe than sorry - the forums have a lot of conflicting opinions although I can see the logic that a narrow band filter should do the job, i know there is a lot I don't know - so I'll stick with the uv\ir cut.

On the other hand i've never tried observing without the polarising filter, so i might try without - although I don't see anything in the continuum that would reduce the intensity of the light - but like I say, there's a lot I don't know...

[bomberbaz]

1 hour ago, Giles_B said:

Polarising filter, UV/IR cut and a Contiuum. Actually it's the altair contrast booster version, not the Baader continuum, so a bit thinner, but all 3 are still a good few mm all told.

The Lacerta also has the ND3.0 built in.

I've just now found on Teleskop-Express for the bare T2 version of the 1.25" Lacerta wedge - "Light path from T2 to T2 is 61 mm" - so a bit less than the Tak diagonal. Still I'll test tomorrow to be sure.

If this is right Giles it sounds like you'll be fine. I will test my overall tak diagonal plus remaining back focus distance tomorrow.

However, all the praise the scopes receiving is making me regret the revised selling on decision now, haha 😉