[PRejto (Peter)]

I've been contemplating a new setup where I use a LP filter in place of the usual luminance filter (rather than a situation where I use a LP filter in front of all filters (LRGB). I decided to compare the Hutech IDAS to the Astronomik CLS using my Moravian G2-8300.

First I tried comparing the filters just using a light box that I use for flats. I collected the following information:

IDAS: 8 seconds = 28,510 adu
16 sec = 55,960

CLS 8 seconds = 17,270 adu
16 sec = 33,880

Comparing these numbers to a situation without any filter (no luminance filter) I calculated that the IDAS would require increasing exposure by 2X and the CLS by 3.4X.

I then selected a dim target galaxy low to the Southwest where I see considerable light pollution. I attempted to equalize adu (density) at the core of the galaxy by adjusting the exposures. The results are shown below. The photos showed little without stretching. What you see are auto stretches in CCDStack (without DDP).

Image File CLS-6 min
centroid {X=836.4806, Y=626.423}
FWHM 2.5
background 7,212.35
maximum 19,096
flux within halfMax 30,961

Image File IDAS 210 sec
centroid {X=833.3436, Y=627.2254}
FWHM 2.3
background 8,403.20
maximum 19,259
flux within halfMax 23,692

Clearly the IDAS transmits more light but the CLS does a better job of controlling the background and gradient at the expense of rather longer exposure.

However, comparing both photos further in terms of S/N it seems the IDAS may have some advantage:

Image File IDAS
rectangle {X=769,Y=561,Width=162,Height=155}
diagonal 224.21
# pixels 25,110
# rejected 0 (0.00%)
mean 8,175.46
STD 1,175.27
S/N 6.96
median 8,087.00
int mode 8,019
min 7,640.00
max 65,535.00
R:G:B ratio 1.00 : 1.00 : 1.00

Image File CLS-6 min
rectangle {X=769,Y=561,Width=162,Height=155}
diagonal 224.21
# pixels 25,110
# rejected 0 (0.00%)
mean 6,856.59
STD 1,388.67
S/N 4.94
median 6,738.00
int mode 6,697
min 6,306.00
max 65,535.00
R:G:B ratio 1.00 : 1.00 : 1.00

Perhaps I don't understand how S/N is calculated. Subtracting the background from the signal should give the usable signal over the background. For the CLS this is about 12,000 adu, for the IDAS about 11,000. It would again appear to me that the CLS is doing a better job if the price of time collecting data is to be paid.

Peter