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Herschel wedge effect on focal train?


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28 minutes ago, bomberbaz said:

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 😉

Yes I did worry that might happen Steve :)

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5 hours ago, Franklin said:

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

Line filters can leak light from outside the visible spectrum without it affecting visual performance.  Take a look at this thread which specifically tested two generations of the BSC filter.  One version seriously leaked in the NIR:

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That could cause a serious amount of discomfort to one's eye while observing.

Here's what it looked like combined with a UV/IR filter:

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While another BSC version did not leak NIR light:

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6 hours 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.

I put the UV/IR cut filter ahead of the wedge as someone on here suggested, and nothing bad happened.  It didn't even get hot.  Now I just leave it on there full time as a safety precaution for my eyesight.

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For the record, the results of testing are: 130mm straight through with no wedge vs 43mm with the Herschel wedge - so an 87mm real world light path - quite in excess of the T2-T2 light path quoted on Teleskop Express.

The eyepiece was with a 30mm Baader Plossl plus three stacked filters (to simulate the continuum, UV/IR cut and the polarising filter). The wedge had the ND3 removed (necessitating holding the eyepiece on with tape as the ND3 had a male thread both sides), but the filter stack was a little thicker than usual (for transparency I used coloured filters and a skyglow), so it would be a fair approximation.

Edit - I had this reply from Lacerta:

"the backfocus needs of our Herschel wedges:

1.25" Version: 61mm from T2 to T2 ("S"-Version ca. 89mm)
2" Version: 81mm  from M48 to M54 ("S"-Version with Rotationadapter ca. 106mm)"

"S" Version - I guess the version with a nosepiece and eyepiece holder - is pretty near to my measured light path. In fact (lightbulb moment) the 2mm shortening could be due to removing the ND3.0

Edited by Giles_B
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3 hours ago, Mr Spock said:

Bad idea - you'll be subjecting it to the full focussed rays of the sun! Filters need to come after the wedge.

Daystar actually suggest using a UV/IR filter in front of the diagonal with a Quark in scopes up to 150mm, so should be no problem with a Herschel wedge. The light path is not fully focused at that point, and I believe that at least the Baader UV/IR filter is reflective rather than absorptive so won’t overheat. I have done this too with no problems.

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6 hours ago, Mr Spock said:

Bad idea - you'll be subjecting it to the full focussed rays of the sun! Filters need to come after the wedge

 

3 hours ago, Stu said:

Daystar actually suggest using a UV/IR filter in front of the diagonal with a Quark in scopes up to 150mm, so should be no problem with a Herschel wedge. The light path is not fully focused at that point, and I believe that at least the Baader UV/IR filter is reflective rather than absorptive so won’t overheat. I have done this too with no problems.

The plot thickens!

I'd have thought that the likes of Lunt and other solar equipment manufacturers would have made issues like this a bit clearer, from a safety aspect at least.

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10 hours ago, Louis D said:

Line filters can leak light from outside the visible spectrum without it affecting visual performance.  Take a look at this thread which specifically tested two generations of the BSC filter.  One version seriously leaked in the NIR:

spacer.png

That could cause a serious amount of discomfort to one's eye while observing.

Here's what it looked like combined with a UV/IR filter:

spacer.png

While another BSC version did not leak NIR light:

spacer.png

Observing with the SC filter and Herschel wedge didn't ever cause me any discomfort with my 80mm F/6, as the wedge plus ND filter remove enough of the energy anyway. For bigger scopes there may be an issue. For imaging, you really need to insert a UV/IR cut filter. I must say the new SC filter looks an interesting proposition. 

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36 minutes ago, Franklin said:

I'd have thought that the likes of Lunt and other solar equipment manufacturers would have made issues like this a bit clearer, from a safety aspect at least.

Worst case, the UV/IR filter cracks or shatters for whatever reason, you clean out the mess, and try a different UV/IR that is reflective rather than absorptive.  It's not like you're in any danger of shattered glass getting in your eye or being exposed to the full force of the sun's light.  It's not much different than the more focused light cone after the wedge (4.6% of the pre-wedge energy) hitting the ND filter, which is absorptive rather than reflective.  The ND filter is very near to the scope's focal plane.

I will say that any finger grease on the UV/IR could lead to differential heating, and that might lead to an issue with cracking.  Thus, keep it spotless.

Edited by Louis D
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7 hours ago, Mr Spock said:

Here's my combination - Lunt wedge and Baader continuum. Works perfectly.

723981324_DSC_0142_DxO1200.jpg.34b61a6b7fbb673b6b0909d76247ce77.jpg

That is the same set up I've been using and the solar views are great. Don't feel the need to turn down the brightness with a polarizer as well, though I only use it in my 81mm and 103mm refractors. Regards the UV/IR debate, I might just piggyback one on the continuum, better safe than sorry.

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19 minutes ago, michael.h.f.wilkinson said:

Observing with the SC filter and Herschel wedge didn't ever cause me any discomfort with my 80mm F/6, as the wedge plus ND filter remove enough of the energy anyway. For bigger scopes there may be an issue. For imaging, you really need to insert a UV/IR cut filter. I must say the new SC filter looks an interesting proposition. 

By the looks of it, the newer BSC filter is indeed NIR safe.  If it didn't cost so much, I might try it.  I do have a vintage Optica b/c green line filter with ~30% transmission somewhere in the green part of the spectrum.  I tried it briefly with my wedge, but I wasn't convinced the view was any better than without it, so it did nothing to whet my appetite for the BSC.

I started using the UV/IR cut with my 8" Dob and Visual ND5 Baader Solar Film.  It always felt like my eye was getting cooked over time.  Based on the logarithmic spectrum of BSF shown below, it's no wonder I felt that way.  Clearly, it's ~ND5 at visible wavelengths, but only ~ND3.7 to ~ND4.5 in NIR.  That is more than a 10x increase in NIR transmission over visible transmission which is enough to cause discomfort for me.

image.gif.c70bd9753c2880fe361379fea83bd7f0.gif

The Baader UV/IR filter (similar to my Meade photographic UV/IR filter) has the following transmission characteristics on a log scale:

Ba-UVIR-log.gif.1aaac614df1b86e8e33e639d12ee8657.gif

So it will definitely help with the BSF in my Dob where I need it most, in the NIR.  That extra ~ND2.5 in NIR really helps make things more comfortable when solar observing.

Interestingly, notice how a Baader ND3 starts to go off the rails toward ND2 above 1500nm in the IR part of the spectrum:

Ba-ND3-log.gif.770ad27d7af5bb91e53eaf131ab38bbc.gif

Thus, a 10x increase in IR transmission in that region.  I wonder how similar the ND3 filters packaged with Herschel wedges are to the Baader.

Here, notice how crossed polarizers (similar to those packaged with my Hercules solar wedge) never block IR at 900nm and above regardless of crossing angle.

image.gif.b0831e91d1fe4758c2fc32d2ec195846.gif

That's just downright scary!  I'll take the UV/IR cut filter as one more line of defense for my aging eyes, thank you very much.

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30 minutes ago, Louis D said:

I'll take the UV/IR cut filter as one more line of defense for my aging eyes, thank you very much.

I'm sold as well now.

I've got a Baader 1.25" UV/IR cut filter around somewhere, I picked it up ages ago after having some crazy idea about trying my hand at imaging.

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  • 2 weeks later...

Final update on the original Herschel Wedge question as it may be of interest to someone with a similar question in future.

I've now taken delivery of the scope (thanks @bomberbaz) and finally had some sunshine - but I cannot bring the Lacerta Herschel wedge to focus with any eyepiece in my collection, not even the simplest Baader Plossl or Circle T orthoscopic - there is just not enough in-focus. This is despite doing what I can to shorten the focal train I e. Removing the spacer between the eyepiece holder and the ND4, using only the UV/IR cut in addition to the ND4 and using a vixen to T2 adaptor to directly connect the wedge to the focuser.

I will have a play over the next few sesions to see if there is any way focus can be achieved - some posts on cloudy nights about gaining more in-focus on the Baader Herschel wedge suggest that adding a Barlow after -or before- the wedge may help, although I'm cautious about placing one before the wedge for a sustained period.

Ultimately my suspicion is that the Lacerta wedge is just not going to be a long term good choice in this setup. Having been advised by FLO that the Lunt Herschel Wedge will come to focus with simple eyepieces with this scope, I'm now looking out for a Lunt (wanted ad posted!). It's a shame because I really like the adaptability of the T2 connectors on the Lacerta. But my experience of astronomy so far us that the lure of one thing inevitably seems to lead to the necessity of buying another thing to get the first thing working / better!

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On 27/12/2022 at 02:14, Giles_B said:

Having been advised by FLO that the Lunt Herschel Wedge will come to focus with simple eyepieces with this scope, I'm now looking out for a Lunt (wanted ad posted!).

I went with the Hercules solar wedge.  Being 1.25", it doesn't take up that much back-focus.  It works perfectly well.  I ordered direct from them before China cracked down on retailers not going through approved marketplaces like AliExpress.  It looks like their online store may be back up again.  They're also available under Hercules and no-name versions on AliExpress here, here, here, and here.  They're low cost, but work very well.

662542773_SolarFinders5.thumb.JPG.064f5566be8a462573406b65c40eb445.JPG675291827_SolarFinders2.thumb.JPG.bd771f630152ff3fb7b5e2eca19e1a6b.JPG

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Thanks for the recommendation - I have the Lunt wedge now, although I haven't seen the  sun yet to try it out. Do you have the Hercules paired with a Scopetech refractor? Certainly the design looks quite similar to the Lunt.

 

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That's a TS-Optics 90mm FPL-53 Triplet APO I bought a few years back.  I've read it may be a Sharpstar scope.

To me, the Hercules/Antlia 1.25" wedge looks like a 1.25" version of the 2" Meade, Starfield Optics, and Altair Astro wedges:

Hercules Herschel Wedge Sunspot.JPGpost-301285-0-10719300-1631704083.jpgandrew_hw_1024x1024.jpg?v=1521422452sol-wedge.jpgaltair-imaging-ready-2-inch-solar-hersch

Coincidence?  I think not.

 

Edited by Louis D
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