Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Herschel Wedge Prism Glass Composition

This topic has been archived. This means that you cannot reply to this topic.
3 replies to this topic

#1 KarlL

KarlL

    Apollo

  • *****
  • topic starter
  • Posts: 1341
  • Joined: 14 Dec 2012

Posted 07 April 2014 - 04:26 PM

Is it true that, as I have read, the prism glass of a Herschel Wedge absorbs UV and IR? If so, what is the glass composition? How did John Herschel know that UV and IR were so dangerous and how to address this?

Regards,

Karl

#2 icecreamcat

icecreamcat

    Vendor (Skybender)

  • *****
  • Posts: 769
  • Joined: 12 Nov 2013

Posted 07 April 2014 - 06:49 PM

I think all glass blocks UV light to a certain extent. What makes the hersch prism special is the reflection angle.

This angle produces multiple bounce inside the prism, and involves two or three more surfaces than a standard mirror. 10 percent of a 10 percent reflection and then reflected at 10 percent again.

Uncoated surfaces(non mirrored) tend to reflect less intensity. Each surface reduces the trasmission.


I do not think the infrared is blocked at all. The transmission is just reduced to an acceptable level at the eyepiece, and the rest dumped out in the "dump zone"


How did hersche know this? He probably noticed that uncoated surfaces reflect light differently, and that prisms refract light at varying intensity. He just used his common sense of the day, and combined the two into one experiment..

#3 icecreamcat

icecreamcat

    Vendor (Skybender)

  • *****
  • Posts: 769
  • Joined: 12 Nov 2013

Posted 07 April 2014 - 06:59 PM

http://en.wikipedia..../Herschel_wedge

Well I guess they do absorb some of the infrared too. He was a genius for sure :)

#4 The Ardent

The Ardent

    Skylab

  • *****
  • Moderators
  • Posts: 4393
  • Joined: 24 Oct 2008

Posted 07 April 2014 - 08:36 PM

Even so, I use a UV-IR cut filter on top of the ND3 inside the Baader Wedge. I know I don't really need it, but it makes me feel better.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics