645z astrophotography, and ampglow

Started Mar 8, 2016 | Discussions thread
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weihaowang Forum Member • Posts: 70
645z astrophotography, and ampglow
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Hi,

Want to share with you a few of my astronomical pictures taken with my new 645z:

https://www.flickr.com/photos/weihaowang/24532290754
https://www.flickr.com/photos/weihaowang/24795271809
https://www.flickr.com/photos/weihaowang/25044708232
https://www.flickr.com/photos/weihaowang/25044708692

Overall, after the astro modification (removing the IR blocking filter in front of the sensor) its performance is as good as any astro DSLRs based on the recent generation of Sony CMOS, such as the D810A. Unlike Nikon and Sony, Pentax doesn't seem to be doing anything stupid on the raw files that may prevent accurate image calibration in astrophotography.  So everything is good.

One thing I noticed is some ampglow-like light in the bottom of the images.  Please see the attached pictures.  In all the M-mode exposures (with lens covered), you can see a band of light along the bottom edge.  It's there no matter what exposure time I use in M mode.  However, it goes away in images taken with Bulb mode.  So, somehow, Pentax cleverly turns it off in Bulb mode, but leaves it on in M mode.  In addition, the brightness of the light band does not seem to increase linearly with exposure time.  (Otherwise the band will be 4000x brighter in the 1-sec image than in the 1/4000-sec image.)  So whatever is producing the light, it is only turned on for a fixed period of time, regardless of the actual exposure time.  This puzzles me a bit.  If this can be turned of in Bulb mode, why does it have to be there in M mode?  Anyone has any ideas?

Anyway, since this goes away in Bulb mode, all I need to do is to take all my images in Bulb mode.  This is not very convenient, but acceptable.

Cheers,
Wei-Hao

Pentax 645Z
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