What about using a focal reducer behind a PST etalon?

[Roel]

I've read discussions about using Barlow lenses to give short f/5 telescopes the necessary f/10 beam for the PST etalon, but what about he other way around...?

I'm currently working on the design of my next PST mod based on a 90mm f/10.1 refractor. The only 'problem' is going to be the tiny BF05 (which is mounted on an old Celestron diagonal). There is no way I can afford a larger BF, so I'm trying to get the most out my original PST parts.

So I was thinking: what about a 0.5x focal reducer somewhere between the back end of the etalon and the BF05? It will have no effect on the magic -200mm configuration, but reduces the size of the solar disk at (or near) focus. If such a focal reducer could reduce that image to less than 5mm, the whole disk would fit through the BF, just like with a f=450mm system, but with the increased detail of the larger objective.

The easiest way to do this would be to screw the focal reducer on the front of the diagonal, but I have no idea if that's a good position (I don't have any experience with focal reducers). Another option would be to mount the reducer directly in front of the BF in some way. I have a pretty short focuser so I think I might be able to reach focus if the reducer requires more inward travel.

Screw-on focal reducer: http://www.teleskop-express.de/shop/pro ... winde.html

Any thoughts?

[swisswalter]

Hi Roel

I work with the reducer after the blocking filter. I guess that is the position where it should be otherwise the beam is no longer paralell

[Roel]

Without a focal reducer the beam isn't parallel either. The only place were there is a parallel beam, is between the front corrector lens of the etalon and the rear corrector lens of the etalon. After the etalon its just an f/10 beam again.

I believe placing the reducer after the BF not only decreases the image size, but also decreases the field of view vignetted by the BF!

[Merlin66]

Roel is correct.
The screw-in x0.5 reducers (ATiK, TS etc) give the x0.5 reduction when placed about 35-45mm infront of the eyepiece/ CCD chip. These reducers were originally designed for the smaller webcams to increase the FOV.
Placing it in front of the diagonal may be too far from focus...it would have to be just in front of the BF filter to get the reduction and at this point the beam would only be slightly less than the original f10 diameter (if you know what I mean)
If you could fit a longer focal length reducer - like the ones used on SCT's then there may be an improvement.

[Roel]

Hmmm.. I think I understand.

But a screw-in reducer suggests it works as a 0.5 reducer mounted directly in front of the eyepiece, not at 35-45mm. Mounting it just in front of the BF might still keep it around 0.5, but it will indeed probably keep the light cone too wide for the BF.

Maybe it's an idea to mount it not in front of the diagonal's nosepiece, but directly onto the front end of the diagonal housing, using a 1.25" extender in front of the reducer as a nosepiece? This might be close to the 35-45mm distance Merlin66 mentiones. But as I said before, I think it only really works as a 0.5 reducer directly in front of the eyepiece...

So resuming:

-The lightpath between the barrel edge of the eyepiece and the front end of the diagonal nosepiece is around 70mm
-The lightpath between the barrel edge of the eyepiece and the front end of the diagonal housing is around 40mm.

So if there is a reducer that works as a real 0.5 reducer at one of those positions, I'm where I want to be! Any suggestions?

[Merlin66]

The focal length of the reducer would be about twice the distance to the focus..ie an 80mm focal length reducer placed 40mm inside focus would give the reduction. Note: the reducer would change the final focus position...you'll need to review the spacers.
(I think the x0.33 Celestron SCT reducer is about 88mm focal length....)

[Roel]

An SCT focal reducer is not really an option in my setup: it's simply too large to fit between the various parts of my scope. It almost has to be a screw in thingy...

I'm still thinking about placing the 0.5 TS reducer somewhere in the lightpath. (I think I understand now why those screw-in reducer will not work as a real 0.5 reducer when used visually)

The final decision to use the TS reducer depends on the following things:

- What wil be the effective reduction when placed at ~40mm inside focus
- How much extra inward travel will be required when placed at ~40mm inside focus?

Is there a way to calculate this, or will it come down to trial and error?

[Merlin66]

To do those calcs you really need to know the focal length of the lens involved..I'd hazard a guess and say you'll be close to the x0.5 35-40 inside prime focus and you may need to move inward by 40-45mm....just a guess.

[Roel]

Thanks, Ken!

I just sent an email to TS, maybe they can provide me with the specifications.
To be continued....

[Roel]

@Neo: that's interesting. Where in the optical path did you use the reducer? Have you tried to place it at the front of the BF diagonal?

[Roel]

Worst case scenario: could moving the prism (including BF) 45mm forward into the light cone result in the same amount of vignetting in the end...? And therefore totally ruin the advantage of the added reducer...? hmy:

[Merlin66]

I think you'd find it's even worse!
You want somewhere where the light beam is less than 5mm - with an effective focal length of 500mm or above this is an impossibility! The Sun's diameter will be >5mm.

[Roel]

Well, that's not absolutely necessary. With my current 70mm f/10 the full disk also does not fit entirely through the BF, but I CAN just see the full disk through the eyepiece at low power, only not fully illuminated across the entire disk. That is something I can live with (a significant part of the center still benefits from 100% illumination by the large aperture, so at higher magnifications it's a non-issue) but with the 90mm f/10 I might cross my personal line of comfort If a focal reducer could at least give a FOV comparable with my 70mm, I'd already be quite satisfied.

[Pedro]

I also tried a 2" TS 0.5x and the results were very poor. The reducer was installed in front of the 2" adaptor as suggested in the TS site:

http://www.astroshop.eu/field-flattener ... ad/p,12140

best regards

[Merlin66]

Roel,
I should have said "without vignetting of the image"

[Roel]

@Pedro, that's bad news. It is the same configuration I had in mind...
Wat did it do to the image?

[Pedro]

I had much better results with a 0.67x Astro-physics reducer (in front of the 2" adaptor). More expensive but much better.

http://www.teleskop-express.de/shop/pro ... SO-RC.html

best regards

[swisswalter]

Hi folks

that's getting very wild. I try to follow, but it is not that easy, but I have already learned a lot. Thank you very much

[Roel]

I had much better results with a 0.67x Astro-physics reducer (in front of the 2" adaptor). More expensive but much better.
That suggests it has more to do with the quality of the reducer, rather than the idea of using a focal reducer itself. Apparently it works!

Possibly the lack of contrast when using the TS reducer is caused by inferior coatings creating ghost images. In a normal PST mod the tiny ERF and the blocking filter are both slightly tilted to prevent those ghost images. When putting a reducer (which for obvious reasons can not be tilted) behind the etalon, the light can reflect between those 2 parts.

BUT! Maybe a polariser can remove these reflections (see Merlin's topic about ghost reflections) and bring back the contrast!