maybe this is not such a good idea

[Brian W (Brian)]

Hi all, most regrettably I have developed quite an interest in the Golden Era of Visual Astronomy, in particular Sir John Herschel and his time in South Africa.

I am developing my skills and my knowledge so that in time I can do the ultimate Herschel list.

My problem is what scope do I use for this project. He used an 18" metal mirror with 67% reflectivity and a focal length of 20 FEET. Yup it was a long sucker.

How practical would it be to try to build an honest replica?

The support structure is certainly doable but is there a mirror maker that could or would grind an 18" silver coated mirror that has 67% reflectivity?

Maybe I go with store bought and simply accept that the view will be different?

Brian

[bmitchell82 (Brendan)]

I have been following your thread brian, I think if you go with the silver coated mirror you will indefinately run into problems. aka are you willing and prepared to recoat the mirror all the time? and finding somebody who still has the old tools and methods for coating mirrors as they do it in a cirtain way which i belive is with a vaccume.

For me if i was trying to replicate what the views could have been like i would try and match the telescope. That is
find out what the light gathering power for a 18" mirror with 67% reflectivity is.
then work out what a equivilent mirror would be capable of, i belive they are upto mid 90% relfectivity nowdays.

also doing some quick calculations the F number of that scope is 13.3, which would indicate that your looking at a catadiatropic style aka SCT or a Maksutof.

Thats how i would do it, but do remember that light pollution, atmospherics and the like will be different. and unless you had the telescopes side by side you couldn't 100% know that you have the same setup/conditions!

IMHO, building a replica will cost some serious type of $$$ and also the knowledge behind how to make it unless you are very skilled and have access to some good gear. ide go with a retail scope that matches the specs of the old school scope.

Brendan

[tnott]

A 16" SCT would be fairly close, depending on the eyepiece.

BTW - what type of eyepieces would he have he had?

[Brian W (Brian)]

Originally Posted by tnott

A 16" SCT would be fairly close, depending on the eyepiece.

BTW - what type of eyepieces would he have he had?

If I could afford a 16" SCT and mount I could build a full size replica!

EPs is a tough one. In one of his reports he writes of using a double lens which may have been an achromatic doublet. Ramsdens were also popular then.

However his father was renowned not only for his telescopes but for his ep's as well so he may have been using a family made one.

Always more research.
Brian