> IR-cut filter removal improvement on Pentax K5

  • Posted on: 8 March 2019
  • By: tihomiry

I am so impressed with the K5 ability to quickly gather signal from nebulas (Quantum efficiency) and galaxies, so I decide to take one more K5 and to remove the IR-cut Filter of the old one. The same thing I have done in the past with Pentax KM and I got significant improvement of 2.5 stops. Here is a short article on that: http://planetarium-gb.eu/pentax_astrophotography.pdf
Since I have K5, I almost stop using KM as K5 has huge advantage in terms of QA and noise levels. There is no amplifier glow and no need of dark frames and cooling.
In fact K5 is one of the best DSLRs for astrophotography which is using Sony IMX071 Xmor CMOS sensor. Same as ASI071MC Pro (color) (https://astronomy-imaging-camera.com/product/asi071mc-pro)
Thus after the modification the Ha sensitivity improvement is far less then what I expected. But this camera is great unmodded, so it seems that Pentax IR-cut filter is letting most of the Ha spectrum to the sensor. Below is comparison of M42 Orion nebula with UHC-S filter of 1.Modded (without IR-cut) K5 and 2.Unmodded. There is very slight difference but well enough to show far great results, when push it to the limit with a fainter objects and stack of 100+ frames.

Recently I got one very good lens the Asahi Takumar 135mm 1:2.5. This is fast telephoto lens and from my point of view a good lens should be with aperture from 1:1,2 to 1:2,8.

Moon at 45% illumination and 70x magnification. With aperture of 200mm,14,1 stops dynamic range, color depth of 23.7 bits at this image there are slight color variation on the moon surface that reveals it’s structure.

This is the beginning of my research on how dark the sky could be and what is benefit of a dark sky for astrophotography. I was inspired from those 2 sites. First one presents mathematical model of how dark the sky is.

You can find my last DSLR Astrophotography article in Bulgarian at page 76. I am sharing my last findings for this publication, Specially written for the Institute of Astronomy.

Satellites, cosmic trash or UFOs, the moving spots are everywhere on the sky. Typical satellite behavior is to reflect sun light. It should be yellow or white and could fade and shine due to its rotation.