Why do I get full solar disk with Binoviewers?

[Moonshane]

Why do I get full disk with Binoviewers?

I am not complaining by the way but when using my 100mm PST mod (with 5mm standard PST blocking filter) I cannot get full disk no matter what eyepiece I use. This is usually a 15mm plossl which provides 66x magnification and a true field of 0.75 degrees so it is the BF that is vignetting the field.

However, when I use my BVs, with a 1.7x GPC, I can get full disk with my 32mm BCP (53x and 0.94 field), my 25mm TV Plossl (68x and 0.73 field) and even my 18mm BCO (94x and 0.53 field).

Obviously the optical train behind the etalon is shorter with the BVs and maybe this is the reason I get full disk but maybe it’s the GPC changing the angle of the light cone allowing more of the view to be seen through the BF?

Anyone know?

Cheers

Shane

[Peter Drew]

Good question,I have the same result and pondered why. It's the reason I now exclusively use binoviewers for solar viewing. I would have thought that the optical train behind the etalon to be longer with binoviewers due to the significant length of light path through them, this being the reason that a Barlow is needed to reach focus. Without thinking about this particularly deeply, my suspicion is that the Barlow somehow magnifies the blocking filter exit aperture thereby reducing the vignetting. Whatever the true reason, the effect is significant.  

[alanjgreen]

try a separate barlow & single eyepiece, see if you get the same effect.

 

[Moonshane]

I have Alan and strangely not as far as I recall. I'll try again though when I get chance.

[alanjgreen]

Then that only leaves the prism (in the binoviewer)

Don't know much about prisms though except they bend light!

[Moonshane]

I am delighted to report that after another kind sgler made me an adapter allowing me to connect my pst eyepiece holder directly to a Baader t2 prism I can now get full disk with bvs without without a gpc needed with my pstal 100mm mod. This means full disk with quite some room to spare at 40x with the 25mm plossls and and a respectable space around the full disk even with with the 15mm plossls at 67x.

I am so happy I went down the bv route not the bigger bf route.

I still want to know why the bv seems to expand the bf diameter.  might try adding extension between the bf and mono eyepieces next to see if I can replicate in cyclops mode.

[Moonshane]

I can confirm that it is the position of the bf that affects availability of full disk views with my bf5. Whilst my etalon remains at the correct position I have added extension after the bf to take up the in focus requirements of the bvs and whilst cumbersome allows cyclops viewing at full disk with the 15mm plossl.  try it!

[Merlin66]

Hmmm

A very interesting experiment, with a very interesting outcome.....

As you know the focus of the PST etalon assembly is 200mm behind the etalon. The blocking filter (in the standard PST eyepiece holder) is approx 30mm inside this final focus. The added optics (?) of the BVS system must be acting as a relay system and refocusing the original "prime focus" image.

I can only assume in this configuration the blocking filter is sitting much closer (than the original 30mm) to the first focus which is giving the larger FOV.

It would be interesting, in the set-up you show, to only remove the BVS attachments, and add an eyepiece to the eyepiece holder and confirm that it no longer comes to a focus.

 

 

[Moonshane]

Hi Ken

I'll check your suggestion although I doubt it will focus as in the photo above there are no optics In the extension tubes other than the single eyepiece.

 

[Merlin66]

Hmmm...

There must be something transferring the prime focus (at 200mm beyond the etalon) to the new eyepiece position???????

[Moonshane]

Weird isnt it? The only thing that I can think of is that I am using a t2 prism diagonal but surely this would not have such a dramatic impact on light path?

Unfortunately I have sold the mirror diagonal I used previously so cannot check until the end of March as I'll see the mate I sold it to then.

[Merlin66]

roughly what is the length of the spacers from the diagonal to the "new" focus???

I would still like to check if the prism diagonal allows you to focus (as normal?) behind the etalon.....

[Stu]

Really interesting results Shane. I'll be trying to replicate these, once I see some sunshine!!

[Moonshane]

Hi Ken

The etalon rear lens is about 10mm from the diagonal front face. The adapter connecting the original bf eyepiece holder from a pst is about 10mm. Then there's a t2 to 1.25" adapter (2mm) plus 2*15mm and 1*40mm plus an eyepiece holder 35mm.

So all in all it's just taking up the 200mm behind the etalon including the prism.

[Merlin66]

OK

So the eyepiece is still about 200mm from the etalon, but the 5mm bf is much closer -say 60 mm (maybe 70mm?) behind the etalon???

 

[Moonshane]

Yep that's spot on :0)

I do wonder if I am losing effective aperture but surely the solar image size in mm is always 10mm approx at focus for a 1000mm focal length?

[Merlin66]

Hmmm

Time out!

The solar disk will be approx 1/100 the effective focal length....

Let me give it some thought.

 

 

[Moonshane]

My donor scope is a 100mm f10 Tal in case this helps.

[Merlin66]

Well I've walked around the garden a couple of times (someone else did that sort of thought process - Darwin?)

At this stage it still doesn't make any sense to me...............

Just as a sanity check - remove the eyepiece and set a piece of paper with a 10mm circle across the holder and confirm the size of the solar image at focus.

(with the current diameter of the sun, it should be close to 9.4mm diameter.)

 

 

[Moonshane]

Cheers Ken

will do if we ever get any sun in the UK

[Moonshane]

Just an off the wall idea......could it be an effect like eyepiece projection?

Still no sun to allow me to check

[Merlin66]

As there are no additional optics (other than the  diagonal) between the etalon and the final eyepiece/ focus, seems highly unlikely ;-)

[Moonshane]

Thought not but it occurred to me earlier. 

Hopefully I can get a confirmation of the image scale at some point this week.

[Moonshane]

hi Ken

I checked the disk size with a piece of paper with everything removed from the drawtube and it was 10mm just under at focus so there's nothing weird there.

I then put the PST bits back in and tried to get an image with the paper but there image was too dim to be seen.

It was a bit cloudy today so maybe this did not help (although I tried at a time when the sun was bright). I looked directly into the holder and could just about see the whole disk through it (I think). Still at a loss to explain how the set up with or without BVs creates a full disk image with lots of space around it with a 25mm plossl and with a small amount of space around it with a 15mm plossl. If I were using the scope as originally made I am at 40x magnification and therefore 1.25 degree true field with the 25mm plossl. I'd estimate off the top of my head that the field being shown through the PST mod is close to this but maybe just a bit less.

[Merlin66]

OK, the size of the prime image is as expected.....

Could it be that the vignetting as caused by the smaller BF is, due to the brightness, not as noticeable as anticipated????