Infrared filter

I have made a cheap and simple infrared filter from 2 pieces of exposed and developed colour print film.  This was first suggested by Peter Lloyd from the QCUAIG group.

The 2 pieces of film were stapled together and cut to fit inside my Barlow lens.  Here is made up filter is shown next to the disassembled Barlow.
The filter is inserted into the eyepiece end of the Barlow (right).  The whole assembled Barlow is shown below.
Filter characteristics

Data on optical transmission of exposed film can be found on this website.  Taking this data and calculating transmissibility for two layers gives the characteristics charted on the right.  Visible light ranges from 400 - 700 nm.  This homemade filter gives a very sharp cutoff at about 750 nm and has peak transmissibility at about 850 nm, in the near infrared.

Incidentally this range of wavelengths is also that used by household remote control devices for TV's, VCR's etc.  Shining a remote control's IR LED through this filter at a webcam demonstrates that the filter is transparent to IR light.

Images

So far, I have taken two planetary images through this filter.  The first was of Jupiter, rushed off after dusk, before it disappeared behind my roof.  Focus was not very good as a result!  My 2x Barlow was used at prime focus of my 8" f/5 Newtonian.

The satellite visible to the left of Jupiter is Io.  Usually images that are exposed for Jupiter will not show the satellites, however Io looks very bright in IR.  I don't know why this is so.

Lower right of the disk, is the Great Red Spot, usually very apparent in visible light images.  In IR, the GRS is hardly visible.

Another image was taken of Mars.  I was hoping to be able to do an LRGB, with this IR as the luminance image, but unfortunately the clouds rolled in.

A 2x Barlow was used.  The image was then enlarged another 3x in processing with the Iris 'DRIZZLE' function.  The planetary disk was 18 arcsecs across.  North is up.

The dark feature top right is Syrtis Major.  To the left is the light coloured expanse of Arabia.  The Southern icecap is visible, and above that the impact basin Hellas.  The icecap is less bright than in visible light images.

For comparison, the simulated view from the JPL solar system simulator is shown below right.

 

 

 

 

 

© Copyright 2003, 2004, 2005 by TG Tan.  All rights reserved.  Copyright exists in all original material available on this website.  This material is for your personal individual, nonprofit use only.  Redistribution and/or public reproduction of this material is strictly prohibited without prior express written permission from the author.