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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. Temperature and focus
  16. 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!

13: Update of Ekos auto focus driver

New auto focus routine:

When I updated (January 2022) Astroberry in the commando window I noticed the auto focus routine worked a bit different afterwards.

I had some problem to get my focuser, an USB-Focuser V3 to connect to Ekos. After some information I got from INDI forum the advice was to stop the modem manager. It can "steal" the port from the focuser.

This commando stop the modem manager:

  • sudo apt-get remove modemmanager
USB-Focuser V3, remove modem manager fix

I also increased the Polling period to 1000 which was recommended in the USB-Focus V3 INDI driver, it's done in the INDI control panel. In my case it connected after this fix. The Raspberry must be rebooted after this to be active. Sometimes I need to disconnect the USB cable to the focuser and then start KStars.

If there still is a problem, read here:

There are other methods but this is the safest I have read.

Auto focus test:

Pentax 645 300 mm ED f/4 lens for astrophoto

First pass (in the image above, just started 2nd pass):
When a new auto focus is started it begins with a outward move of 1000 steps, then an inward move of 500 steps. It must do this, only focus in one direction, to get away from backlash problems. Earlier it did a lot of back and forth movements which caused problems because of the backlash. When it start doing focus test points it do it with a 100 steps interval and only in one direction. It present a polynomial fit curve that updates for every focus test point so you can follow its progress.

Beware, this outward move of 1000 steps can cause your focuser motor to move the focuser to the end stop and cause damage. Camera lenses doesn't have much extra focus travel beyond infinity.

With that the focus position should have been moved 500 steps outward, but when looking at the graph I says 59450 - 58500 = 950 steps. I think that has to do with the temperature influence of the focus position. Last time I auto adjusted the focus was when it was in equilibrium with the outdoor temperature, now it hasn't cooled down yet.

Pentax 645 300 mm ED f/4 lens for astrophoto

Second pass:
After finished pass no 1 it moves outward and do a second pass, but this time it do it with 50 steps intervals.

Pentax 645 300 mm ED f/4 lens for astrophoto

Third pass:
At last it do a third pass, now with 25 steps intervals. Compare to earlier auto focus routine this new one take much longer time, about 5 minutes. On the other hand it feels that it do a much better work so it's well worth it. Earlier typical focus HFD was 1.2 to 1.4, now more like 1.1 to 1.2, but I have to do more tests so it isn't just the seeing that is different. Total it had to use 20 focus points to find the correct focus point. That take a long time, each download from the camera takes 8 second.

Optimizing the auto focus parameters:

I will now try to optimize the auto focus to have it faster. But first some of my standard settings, this is done in INDI Control Panel. My focuser is the USB-Focus V3 and your window maybe look different.

Pentax 645 300 mm ED f/4 lens for astrophoto

USBFocusV3 Main Control:
When I use a camera lens I don't have very much focus travel after the infinity point. I must have a limit what the focus motor are allowed to travel. Here I have set a Max Position to 60000. It must be saved too, under the Options menu.

Pentax 645 300 mm ED f/4 lens for astrophoto

USBFocusV3 Options:
There is one more Max Position, I'm not sure why, but I set also this to 60000, when click on the Set button it transfer the figures to the left column. Then save.

Move over to Ekos and the focuser panel:

Pentax 645 300 mm ED f/4 lens for astrophoto

When doing the auto focus with the new driver I noticed that it move the focus more than necessary, 1000 steps outward. I think that is too big move and want to decrease it.

1st menu, Settings:
I always use the full field, but for other cameras, dedicated astro cameras, maybe it's better to use a sub field to get the download faster from the camera if they can do ROI (Region Of Interest).

Pentax 645 300 mm ED f/4 lens for astrophoto

2nd menu, Process:
The tolerance, when bad seeing it's better to increase the tolerance from the default 1% setting. With my equipment 2% works good. If too narrow it can take very long time to find correct focus, or it never find it.

Pentax 645 300 mm ED f/4 lens for astrophoto

3rd menu, Mechanics:
I haven't found any information about this new driver and how to set it up. But I think it's these two to set, at least it's the numbers that is used when doing auto focus. I keep the initial step, 100 steps. The Max Travel I want to decrease.

Pentax 645 300 mm ED f/4 lens for astrophoto

I changed Max Travel to 600 steps. There is no save button here, it will keep the new setup automatically. Now I must have a new clear night to test this new settings. It needed 20 focus points to find the correct focus point with earlier setting. A DSLR camera can be very slow to download the photos, with my Canon 6D it takes 8 seconds. If I can reduce the number of focus points it will much faster. From earlier 5 minutes to say 3 minutes will be great.

I got a clear night and could continue the tests. Sorry to say, it still go outward by 1000 steps.

Pentax 645 300 mm ED f/4 lens for astrophoto

With help from INDI forum I got the information that it's the Out Step Multiplier I should change. I change from 5 to 3. It's now expected to move outwards 600 steps and then inwards 300 steps.

Time delay when photographing:

There are a lot of things that take time when doing sequences of photographing. Focus, plate solving, download images, dithering, slew to new targets. When I doing 30 seconds exposures as I'm doing in light polluted environments almost half the time is wasted. One hour photographing only get 30 minutes of exposure.

Typical time delays in my Astroberry system:

Action Delay seconds Comment
Slew to target 10 to 30  
Plate solve 90  
Auto Focus 300  
Download image 8 Canon 6D
Mirror delay 3  
General delay 1  
Dithering 20 Internal guider

Think about it, if I can speed up this a bit it's about the same as if I had a more sensitive camera or a faster optic.

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