Why do I get full solar disk with Binoviewers?

[Moonshane]

Hi Ken

Previously the vignetting was very obvious in cyclops mode with a standard mirror diagonal and with spacing between the BF and the etalon. The image was quite bright too but with anything more than about 12mm eyepiece focal length, the image did not fill the eyepiece. E.g. with a 15mm plossls, about 2/3 of the solar disk was visible.

Now with the spacing between the BF and the eyepiece, the image is (I think) just as bright in cyclops mode as before but I will check this (I can easily put the spacing between the etalon and BF again as all connections are now T2). The difference though is that there is around 0.25 degree of field all around the disk in the 25mm plossl.

The image with BVs is definitely simmer than in cyclops but with approximately the same field of view.

I’ll check sometime soon whether the placement of the spacing affects apparent brightness in cyclops mode. I might even be able to do this today although I have the 60mm version with me today as I was going to have a look in white light primarily.

More testing to do but the results are certainly interesting. I am still seeing the same levels of detail as captured on GONG

 

[Moonshane]

Still trying to work this one out. I am hoping that I can try all permutations in my available fittings etc. I can say that as originally set up, the BV view with the 25mm plossls and 1.7x GPC is a lot dimmer and less detailed than with cyclops and the equivalent 15mm plossl at native focal length. Therefore I do suspect that whilst it's nice to see full disk, the cost is that the aperture is somehow being clipped reducing brightness and resolution.

I tried a differing route today after my usual set-up (with the 1.7x GPC inside the nose of the maxbrights) which with some effort does bring nice levels of detail - see sketch and GONG comparison below:

I tried screwing the 1.7x GPC into the diagonal itself, therefore taking it closer to the BF by about 25-30mm. To account for the shortening of the light path due to increased barlow effect, I had to add 25mm of extension before the BF and I think this may be an answer as although it still just allowed full disk, the magnification increased and more detail was very apparent. I'll be having a more thorough trial this weekend at DIYPSP and will report back.

 

 

[Peter Drew]

My current, and preferred setup is my 6" F10 mod with Triband ERF, BF5, Skywatcher 2x Barlow lens screwed on to the nosepiece of a standard Revelation binoviewer fitted with either 40mm, 32mm, or on a really good day 25mm Celestron Plossls. This combination has given me my "holy grail" for Ha viewing namely high resolution bright  images and binocular vision. I have spent a significant amount of time not to mention expense to reach this goal and nothing that I have so far seen tempts me to change. For so long I avoided the use of a Barlow lens presuming that used in conjunction with a binoviewer to reach focus in the belief that the resulting high magnifications would not be usable, I have to thank my friend "Paulastro" for encouraging me to give it a try. I've never looked back!.   

[Moonshane]

I still don't understand why I get full disk but have solved the issue of dimness (of the views before you start) thanks to the help of Stu, Peter and Ken. The views through the BVs today even though they were through pretty thick cloud at times were simply sensational at full disk and 67x. Probably the best views I have had since getting the bits 2.5 years ago.

 

[Stu]

12 minutes ago, Moonshane said:

I still don't understand why I get full disk but have solved the issue of dimness (of the views before you start) thanks to the help of Stu, Peter and Ken. The views through the BVs today even though they were through pretty thick cloud at times were simply sensational at full disk and 67x. Probably the best views I have had since getting the bits 2.5 years ago.

 

How did the views compare with the Vixen Shane? Probably difficult to compare, but I imagine they were brighter due to the lack of ITF?

[Moonshane]

Hard to say for now really as I had such brief and poor views as such. The view was definitely 'too bright' without the Baader Ha but d where it normally serves as an erf.n't forget I was using a lot less power and getting full disk so I is going to be a bit brighter. I may even need to take off the Baader Ha when observing with the BVs and barlow element - I don't normally use it and forgot it today! I tried with the 15mm plossls at 113x but seeing was poor with a bath (well the water in it) floating through. Was absolutely astonished at the level of detail. Truly staggering. I think I'll have to do a rough outline of the full disk and then pick a couple of bits in future when sketching. I'll be updating this thread though as I go. I think I may eventually invest in one of the replacement IYFs recommended by Ken as I can then leave the Baader filter out of the equation and in my 60mm mod where it normally lives, serving as an erf.

[Merlin66]

I think you may find adding a polarising filter before the eyepiece may help "tone down" the brightness - I use it on my PST mod.

(It can also have some benefits is suppressing ghosting - reflections of the rear of the etalon)

Worth trialing.

 

[Moonshane]

cheers Ken

I agree that further filtration is required. I tried a ND 0.9 and it was a bit dark so might try the polariser as suggested. I think I might have an ND0.6 somewhere too but a polariser will allow variability of course.

[Peter Drew]

I used a ND6 filter on my setup prior to using binoviewrs and that seemed to be the ideal unit. Now with the binoviewers and 2X Barlow on the nosepiece of the binoviewers the high magnification takes care of the excess brightness. 

[Moonshane]

I suppose though I have no itf (but am using a baader 35nm ) so maybe I have a brighter starting point. I'll be doing more experiments over coming weeks but suspect I'll be funding a Maier replacement.

[Moonshane]

On 2017-3-31 at 00:14, Merlin66 said:

I think you may find adding a polarising filter before the eyepiece may help "tone down" the brightness - I use it on my PST mod.

(It can also have some benefits is suppressing ghosting - reflections of the rear of the etalon)

Worth trialing.

 

I now have the appropriate adapters to use a polarising filter between my etalon and blocking filter. I have a two part polariser but do I only need the top part? Suppose I should just try it!

[Merlin66]

Yes, you just need one of the polarising elements.

 

[Stu]

Shane, Sorry if this has been posted earlier in the thread, but did you ever get to a conclusion over why you get full disk in your mod? Is there something specific in the arrangement/spacing used?

[Moonshane]

Not really buddy but I am delighted to say I am now getting bright full disks with 100mm aperture and at 68x! The polarising filter will simply reduce the brightness / increase the contrast whilst retaining the full disk. I have also recently used 170x when seeing allows so I am delighted. The high power views are still not quite a match for the ones I recall through yours but in truth I prefer the wider view I am getting so I consider it a win / win :0)

[Moonshane]

I suspect it's due to the shortening of the path between the etalon/lens pair and the BF. I have also found that changing this creates different mags like when changing the position of the barlow lens and suspect that it's narrowing the cone. Whatever the reason it works

In a way it would be good to try imaging (not me naturally) in cyclops with more space between etalon and bf and then with the spacing like I have for BVs and see if there's a resolution difference but I cannot be mithered  ;0) 

[Merlin66]

OK,

I have been thinking about this "increased FOV" issue......

Based on the details given in the thread I can now only assume that the bino viewer is also acting as a reducer...this may be due to the spacing and the optics acting as a transfer lens with a positive effect ie the effective focal length is reduced and hence the FOV increased.

Does this make any sense??????

[Moonshane]

I think it must be something along these lines Ken but my knowledge of optics is minimal. What I can say though is that the view through my 20mm TV plossls with a 1.7x GPC (ostensibly 85x) the view is apparently the same as in cyclops mode with my 11mm TV plossl (90x) when the extension is all added after the BF. The difference is that with the BVs and 20mm the full disk plus space (I'd estimate the true field is close to the 0.6 degree field that this calculation equates to in normal circumstances 50/85 = 0.588 degree true field)  and with the 11mm plossl it reverts to the expected vignetted disk views.

Very confusing. I wish I could show you