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Project:
Pentax 645 300 mm ED f/4 lens


Contents:

  1. Overview
  2. Internal filter holder
  3. Install the lens on the 300 mm rig and setup USB-focus for reverse
  4. Temperature compensating and tilted focus plane
  5. Autofocus with APT and N.I.N.A.
  6. Move the equipment to the observatory
  7. A second try at the observatory
  8. A third try and investigating the FTDI cables
  9. Light leakage
  10. Optical analyze
  11. Fix of tilted sensor
  12. Control of aperture and deformed stars
  13. Update of Ekos auto focus driver
  14. Transport box
  15. To be continued

Related projects:

  1. HEQ5 as portable mount
  2. Pentax 645 300 mm ED IF f/4 lens
  3. KStars / EKOS and Raspberry Pi4

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!


10: Optical analyze

Now I taken some astrophotos with the Pentax 645 lens. How is the quality, is it good enough, can I get it to perform better ? To get some answer I took a closer look at the images, I even do a optical analyze with CCD Inspector.


Star quality at different areas:

Pentax 645 300 mm ED f/4 lens for astrophoto (Click on the image to get one in full resolution, opens in a new window)

This is the image I analyze, it's not cropped and full frame. M42 Nebula taken from my balcony at Bortle Class 9 environment.

The equipment:

  • Camera: Canon 6D (full frame)
  • Optic: Pentax 645 300 mm ED IF f/4 (medium format)
  • Mount: HEQ5 Sky Watcher
  • Exposure: 38x30 seconds
  • ISO: 1600
  • Aperture: f/4
  • Focus: multi star focus inside the 50% radius circle
  • Image editor: Siril

I recently took a look of the pixel matching between camera and lens:


Pentax 645 300 mm ED f/4 lens for astrophoto

This is a 400x400 crop and 2x zoom of the center of image. And I already noted earlier the stars shape are a bit triangular. They look like Mickey Mouse ears.


Pentax 645 300 mm ED f/4 lens for astrophoto

1st quadrant, crop 400x400 and 2x zoom. The same distortion of the stars but otherwise sharp and the chromatic aberration is under control (not color align processed).


Pentax 645 300 mm ED f/4 lens for astrophoto

2nd quadrant, 300x400 crop and 2x zoom. Less distortion of the stars. No color problem here either.


Pentax 645 300 mm ED f/4 lens for astrophoto

3rd quadrant, 400x400 crop and 2x zoom.


Pentax 645 300 mm ED f/4 lens for astrophoto

4th quadrant, crop 400x400 and 2x zoom. The stars are elongated.

Summary: The chromatic aberration looks to be under control, but some other things, maybe the tilt makes the star to be deformed.


CCD-Inspector:

Pentax 645 300 mm ED f/4 lens for astrophoto (Click on the image to get one in full resolution, opens in a new window)

With the Astro image editor software ASTAP there are builtin tools to analyze star images. It measure the HFD (Half Flux Diameter) values of the stars. I'm not used with this CCD-Inspector yet. But I can see from it that there is a tilt of the sensor relative the orthogonal of the optical axel of the lens. It's analyzed on the stack, maybe better to do it on a sub image.


Pentax 645 300 mm ED f/4 lens for astrophoto (Click on the image to get one in full resolution, opens in a new window)

Another presentation of the data, it looks as the tilt is diagonal relative the sensor. Can't be much tilt because the stars are in focus even in the corners of the sensor.


Pentax 645 300 mm ED f/4 lens for astrophoto (Click on the image to get one in full resolution, opens in a new window)

Even one more representation of the HFD values from CCD-Inspector.

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