Energy
Rejection Filters (ERFs)
Energy
Rejection Filters (ERF) are the first thing the sun's rays strike on a Ha
telescope. It's function is to block ultraviolet, infrared and other
unwanted wavelengths. This helps the filter to function more efficiently
and slows the degradation of the blocking filter. Both front mounted and
rear mounted filters use some sort of ERF.
The ERF I use is the same one sold by Daystar. On the AP155,
it's a 140mm
(5.6"), half
inch thick, plate of #25 Wratten red glass. Depending
on the scope, I've had to stop down the aperture to achieve darker views.
The Smaller ERF on the Megrez 80 is 110mm in diameter.
The white
cells are WWII vintage Aero Ektar aerial lens
cells made by Kodak.
This particular one fits the 12 inch lens, which
has a 12 inch focal length at F2.5 with an internal iris. The original
set up used 6 inch by 6 inch film.
It's possibly from the K17 or
K22 Metrogon wide angle cameras on the RB-29A or F-13A aircraft. The larger ERF
cell is mounted on a black Kendrick Kwik Focus made for the AP155.
I also now own a
Baader "Cool ERF". This ERF is made with optically flat glass and coated
to minimize IR energy.
Adjustable Iris
The Edmund iris
allows me to enhance disk contrast . The opening is adjustable between
120mm - 6mm and is made of high quality aluminum. I attached it to the
lens cell with white
industrial strength Velcro. With it set at full open, prominences are at
their brightest while disk detail
is slightly washed out. For disk detail, I decrease the opening to about
95mm. This darkens the view
and allows you to see more of subtle features. The exit pupil also
decreases and makes it harder to
center your eye but, once your there it's a better view.
Iris Data
|
Aperture |
f/# |
Exit Pupil |
Resolution* |
| 120mm |
f/35 |
1.64mm |
1.37 |
| 110mm |
f/38 |
1.50mm |
1.50 |
| 95mm |
f/43 |
1.30mm |
1.74 |
| 75mm |
f/55 |
1.02mm |
2.20 |
| 60mm |
f/69 |
0.82mm |
2.75 |
* = in arc seconds
This is the difference in image
brightness from full open to 75mm. It's a photo of the suns image
on white paper without a Ha filter (only the ERF).
For more info or to
purchase this type of ERF contact Jim Sweeney at
The Baader Cool ERF:
http://www.baader-planetarium.de/zubehoer/zubsonne/energie.htm
Information on the Edmund Iris is at :
http://www.edmundoptics.com/IOD/DisplayProduct.cfm?productid=1461
