9 replies to this topic

[Kokatha man]

Ok, I appreciate the 0.965" long-running thread but I'd like a bit more info on ep types as well as specific replies to a couple of questions...most of my knowledge (what little there is! ) comes from "Olden Times" when I actually looked through scopes - my OCD by-product of CCS (Classic-Collector Syndrome) now has me trying to understand these a bit better as well as the other questions here!

 

I understand that Huygens, Huygens-Mittenzwey & Ramsden ep's are all non-cemented types...correct..?

 

I always thought "SR" signified "Simple Ramsden" (plano-convex, convexes-facing) - but does it mean a "Symmetric Ramsden as in aspheric symmetrical..? 

 

I know a little about Erfles...but what about the "AH" - is this an "Achromatic Huygens" or an "Airy Huygens"..? (a little knowledge really is dangerous!)

 

I'd like to use my old-time Herschel Prism - I understand an ND filter is a good pre-requisite but what value ND should be used to give best advantage to the HP's qualities...I think it was Dave from NZ or perhaps JonH gave me some forgotten advice a year or so back...

 

I know about the "Terrestrial Erecting" & standard 90° diagonal devices btw.

 

Lastly the Carton device - I haven't checked the thread sizes but an initial (& possibly stupid!) thought is that it is an ep projection fitting for a camera: the knurled element allows turning from "K" to "H" (Kellner to Huygens?) & the top view shows where an ep can click-fit into said device - any answers..?!?

 

I have a few others including the Tasco/Towa "Zoom" barlow (6mm-18mm) (last pikky) which did seem to function ok using one of the ep's the other night: I was always told to view any of these old barlow-type units with extreme circumspection...unsure atm whether it is a compound lens element or one of those old "single" nasties...once I find my 0.965" to 1.25" adaptor I guess it'll be unlikely that I'll continue to use any of these ep's but learning a bit about them is always good!

 

 

 

 

 

  

[sdedalus83]

SR is symmetric Ramsden, meaning both plano convex singlets are identical in diameter and curvature. It would only aspheric due to poor figuring. HM should indicate that the field lens is a meniscus, but I haven't disassembled any to verify. A Huygens has teo plano-convex singlets with both convex surfaces facing the objective.

 

AH is achromatic Huygens, basically a Kellner. The larger, singlet field lens has its convex surface facing the objective. An AR, achromatic Ramsden would have the field lens reversed. Both have a cemented doublet eye lens.

 

I've had a 40mm AH like that one. It's a good eyepiece with the largest tfov you'll get in .965. The others are mostly interesting for completing a set or for nostalgia 

 

One thing you can do with a .965 scope, if you have a diagonal with a slit-tube friction holder and a full clear aperture, is wrap a layer or two of tape around the holder, cut it at the slit, and slide 1.25" eyepieces over it. Be careful not to damage the field lenses. I've got two diagonals which give full illumination and fov with a 20mm RG konig or other 24ish mm field stop eyepieces.

Edited by sdedalus83, 20 November 2019 - 08:46 PM.

[Kokatha man]

...any other ideas - especially regarding the Herschel prism re the ND filter rating required...plus the Carton (ep projection?!? ) device please..?

[CharlieB]

There are two different naming conventions for the ND filters.  The ones I have are labeled by percentage of light transmission.  The three that came with my Optica b/c wedge are labeled 1.0, 0.1 and 0.01.  The photographic ND filters are numbered by the number of f-stops they reduce transmission by - ND2, ND4, etc.

 

The Herschel prism is pretty much useless without the proper ND filters.  With a 40-60mm scope, I usually use the 0.1 filter along with a polarizing filter.  The light transmitted by the Herschel prism is polarized, so a single POL filter will let you fine-tune the remaining brightness.  For a 76mm scope, I might use the 0.01 instead.

[CharlieB]

The 0.1 is equivalent to a ND3 and the 0.01 is equivalent to a ND4.  A nice guide to ND photo filters is here.

[Stew44]

I certainly agree with CharlieB on the ND3 and .1 transmission.  I have the Baader Hershel wedge prism in 1.25 and use it often.  Depending in image brightness I increase ND filter effectiveness.   In my Gotos with the convertible prism you change eyepiece and nose to either transmit through glass or reflect off the unsilvered back side for a significant reduction in sunlight.  That prism diagonal is meant to be used safely with the Goto sun filter.  Also the pair is augmented on some scopes with an aperture reduction mask.    But in all honesty I use a mylar filter to view the sun on my refractors.  Then truly I don't worry about things very much except the integrity of the mylar.

[Kokatha man]

Thanks to both of you, Charlie & Stew!

 

I appreciate the comment re mylar film which I have used for several different scopes & camera lenses...I had heard from some folks that the Herschel prism does give a somewhat different view to mylar filters ( though not the same as the modern "wedge" types such as Baader, Lunt) hence my interest in trying it. (without blinding myself hopefully!)

 

Still a bit confused after  a cursory read of that link Charlie: using your ND3 suggestion the ND3 line states the following <"10 ND-3.0 ND 110-1024 (a.k.a. ND1000)-1/1024>"

 

To me this suggests an ND3 filter reduces the light throughput to 1/1024th that of the total, or roughly 0.001.

 

Similarly the ND4.0 reduces light throughput to 1/10000th or 0.0001.

 

Clearly I have hold of the wrong end of the stick here somewhere - any clarifications please..?

 

 

 

 

[CharlieB]

The 0.1 is 1/10%, light transmission, which is 1/1000 and 0.01 x 1/10% = 1/10,000.  That equals ND3 and ND4 light transmissions.

[Terra Nova]

I agree, an ND3 (post-wedge- typically a thread-in in the eyepiece barrel) is what’s needed for safe reduction. Even so, the image will still be a bit bright for most viewers. You can stack filters in that case. A single polarizing filter (behind) the ND3) works well. Then you can rotate the eyepiece to adjust brightness (the wedge itself accomplishes polarization in one plane). In lieu of the polarizer, can also use a Wratten 56 (green) or a 15 (yellow) which can accentuate photosphere contrast and bring out detail, (or a Baader solar continuum filter). A medium density Moon filter can also be used (again, behind the ND3). If you are worried about UV and IR getting through, you can use a good UV/IR blocking filter like the Baader as a pre-filter ahead of the wedge.

 

B+W (Germany) makes an excellent ND3 filter if you can find one.

Edited by Terra Nova, 22 November 2019 - 10:19 AM.

[Kokatha man]

The 0.1 is 1/10%, light transmission, which is 1/1000 and 0.01 x 1/10% = 1/10,000.  That equals ND3 and ND4 light transmissions.

Thanks Charlie, that explains it...basic maths has never worried me but with the stock market lingo & "basis points" etc I get a bit confused with some things nowadays!

 

I agree, an ND3 (post-wedge- typically a thread-in in the eyepiece barrel) is what’s needed for safe reduction. Even so, the image will still be a bit bright for most viewers. You can stack filters in that case. A single polarizing filter (behind) the ND3) works well. Then you can rotate the eyepiece to adjust brightness (the wedge itself accomplishes polarization in one plane). In lieu of the polarizer, can also use a Wratten 56 (green) or a 15 (yellow) which can accentuate photosphere contrast and bring out detail, (or a Baader solar continuum filter). A medium density Moon filter can also be used (again, behind the ND3). If you are worried about UV and IR getting through, you can use a good UV/IR blocking filter like the Baader as a pre-filter ahead of the wedge.

 

B+W (Germany) makes an excellent ND3 filter if you can find one.

Thanks also Terra, I have some ND filters but they are all 50mm I think. (I found them the other day while digging out my 0.965" to 1.25" adaptor that I made a year or two ago...)

 

Hopefully I can find something on the market to see whether those claims about Herschel prisms (old ones) having some qualities is true or not.....I could insert my adaptor into the Herschel prism, screw in the polariser & then the ND3 into a 1.25" ep & stick all this into the 1.25" adaptor.

 

I'll have to work out the ability to have an ir/uv cut in the train because I'd hardly expect to lay my hands on a 0.965" one to go before the H. prism (plenty of 1.25" ones) but I suspect its position isn't that important - too much thinking this early in the day for me atm!