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Pentax 645 300 mm ED f/4 lens


  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

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

12: Control of aperture and deformed stars

Pentax 645 300 mm ED f/4, aperture analyze:

At bright stars it could be rays of light around them, something like if the aperture is closed down. I take a look inside the lens how it looks.

Pentax 645 300 mm ED f/4 lens for astrophoto

Here I examine the aperture, set to f/4, fully open as I had it at last photo. Still I can see the blades protruding into the opening. This is what cause the rays you can see around the oversaturated stars, the opening isn't a perfect circle. In the background you see the full frame sensor, much room left for a bigger sensor, it's a medium format lens ! It's hard to see here, but a closer look and you see the camera's mirror shadow the sensor, it steal a couple of hundred rows of the sensor.

Pentax 645 300 mm ED f/4 lens for astrophoto

Rotate the aperture ring hard to the end stop and I got it to open a little bit more. Much better but not perfect. I'm not sure it will stay in this position, maybe have to built some lock mechanism for it.

Pentax 645 300 mm ED f/4 lens for astrophoto

Aperture set to f/5.6, a big portion of the photons is blocked to enter the sensor, better quality of the stars, but ...

Deformed stars:

Pentax 645 300 mm ED f/4 lens for astrophoto

Oversaturated stars are a bit deformed, not round, more triangular. Especially in the 1st quadrant, upper right corner. I can't remember I had this strong effect in the beginning when I did my first astrophotos. A 1:1 800 x 800 pixel crop of the corner.

Pentax 645 300 mm ED f/4 lens for astrophoto

This is my first photo and at the same corner, 1st quadrant. No problem with the stars' shape here. Something must have changed. It could be the stepper motor focuser. It has a timing belt that wrap around the lens barrel. To not get any backlash the tension of the timing belt is relative strong, it cause a side force to the lens barrel. The lens have an inner focus construction and shouldn't get problem with this, but the only I can figure out for the moment that could cause these deformed stars shape.

Later I have think that it could be that the aperture ring had slipped away to f/5.6 and I didn't notice that.

List of things to do:

To sum up what I have to do with the lens to have it to be really good:

  1. Get the aperture all open
  2. Correct the tilt
  3. Find what cause the deformed stars

1. Get the aperture fully open:

Pentax 645 300 mm ED f/4 lens for astrophoto

My first idea was to 3D-print a clamp around the aperture ring and lock it in correct position. But then I found a simpler solution so I could do a test first. A spring that force the ring against the end stop.

Pentax 645 300 mm ED f/4 lens for astrophoto

On the aperture ring there is an knob that I can attach the spring, the spring force the aperture to hold it at f/4 and not slip away. Good enough to do some test.

2. Correct the tilt:

Pentax 645 300 mm ED f/4 lens for astrophoto

I continue width my aluminum foil shims until I found the correct position and thickness.

3. Find what cause the deformed stars:

Pentax 645 300 mm ED f/4 lens for astrophoto

This problem is a little bit more difficult, I don't even know what's causing these but I know it wasn't there from the beginning. I'm at least think it has something to do with the focus motor and its timing belt, that it act to much force on the lens barrel. Here I move the stepper motor closer to the lens and then lower the timing belt tension. Later test will show if the stars get a better shape.

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