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Sunday, April 17, 2011

Trying to capture Saturn with a color webcam

The planet Saturn has currently a brightness of 1.04 mag and an apparent size of 19.2". I tried to capture it using my TIS DBK21 One-Shot-Color webcam on my 21cm (f/11.5) Dall Kirkham cassegrain telescope in order to find out the most appropriate focal length for this application. The DBK21 cam uses the same chip as the popular Philips ToUcam 840k Pro. However both the electronics and the mechanical construction are excellent on the DBK21. Considering the low brightness of Saturn (compared to Jupiter, the Moon or the Sun) and the permanent bad seeing at my roof observatory, it was useful to get some experience on using this webcam to capture Saturn with my telescope. The goal is to find the "golden ratio" between the focal length and frame rate to get best possible results on Saturn.

f/11.5


Image 1: a single unprocessed frame

f/11.5 @ 27 frames per second. Focal length was 2415mm. No extenders. Notice: Quick enough to freeze the seeing. Too small planet disc but very bright. No problem to increase the frame rate.

The following data have been calculated with the free software AstroDigital.Net
Model: Takahashi Mewlon 210 (f/11,5)
Focal length [mm]: 2415
Critical Focus Zone [mu]: 290,95
Model: DBK 21AF04
Pixel size [x*y]: 5,6 x 5,6
Chip size [x*y]: 640 x 480
Chip diagonal [mm]: 6,08
Chip surface [qmm]: 18,26
Wavelength [nm]: 550
Telescope illumination power: 1,102345E-05
FOV: 0° 5' 6'' x 0° 3' 50''
Image scale: > 0,48 x 0,48


f/18.4

Image 2: a single unprocessed frame

Image 3: 5000 frames added and processed (AVIStack, Giotto, MaximDL)

Image 4: The setup

f/18.4 @ 19 frames per second. Focal length was 3864mm. The ExtenderNQ (1.6x) of a FSQ-106ED was used. It works excellent on the Mewlon! The planet disk is large and bright enough to show details under good seeing conditions. The frame rate is high enough to freeze the seeing.


Model: Takahashi Mewlon 210 (f/18,4)
Focal length [mm]: 3864
Critical Focus Zone [mu]: 744,83
Model: Takahashi Extender 1.6x
Model: DBK 21AF04/21BF04
Telescope illumination power: 4,306035E-06
FOV: 3' 11'' x 2' 23''

f/28.7
Image 5: a single unprocessed frame

f/28.7 @ 9 frames per second. Focal length was 6037mm. Televue Powermate 2.5x. Notice: Too dark on Saturn, too slow to freeze the seing. Big planet disk.

Model: Takahashi Mewlon 210 (/28,8)
Focal length [mm]: 6037
Critical Focus Zone [mu]: 1818,44
Model: Televue Powermate 2,5x
Multiplier [times]: 2,5
Model: DBK 21AF04/21BF04
Telescope illumination power: 1,763752E-06
FOV: 2' 2'' x 1' 32''
Image scale: > 0,19 x 0,19

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