Turpol Neutral Density Filters

Position	Transmission	Magnitude shifts	Useful for Target brightness
0	20 %	+ 1.8 mag	V ~ 9 mag
1	50 %	+ 0.75 mag	V ~ 10 mag
2	30 %	+1.3 mag	V ~ 9.5 mag
3	10 %	+ 2.5 mag	V ~ 9 mag
4	3 %	+ 3.8 mag	V ~ 7.5 mag
5	1 %	+ 5 mag	V ~ 6 mag
6	65 %	+ 0.5 mag	V ~ 10.5 mag
7	open		V > 11 mag

Neutral Density (ND) filters must be used for bright objects. The requirement is to stay below a count rate of about 100 000. At a count rate of ~ 150 000, a mirror goes in and redirects the light in order to automatically spare the photomultiplier tubes. The brightest magnitude allowed is thus effectively V = 9 (with the attenuation of the ND filter included). If you are uncertain about target brightness, start with the densest filter (#5), check the count rates, and change to a less dense one, if possible. An easy way to check the count rates without starting an integration is to go to Options, click on Mirror: Meas and then on TV.
NB! Note that for very accurate photometry you should have count rates below about 50 000 to 70 000 for the best stability of the R and I photomultipliers.
NEW FILTERS! We have two new ND filters, namely number 6 (65%) and 0 (20%). They have been kindly provided by Svetlana Berdyugina. Thanks!

Transparency coefficients in percents. To be continued.
ND Filter	U	B	V	R	I
50%	56.9	54.2	52.9	52.0	52.1
30%	39.7	36.1	34.4	33.3	33.2
10%	11.0	10.8	10.9	11.4	12.0
3%	2.9	2.8	2.9	3.3	3.6

Last modified: 29-Sep-2009