25.10.19 GPC v 120mm aperture stop.

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Friday: The 1:125 Baader GPC threw the focus on the 6" f/8 18cm further out compared to no GPC. By sheer luck I had just enough assorted 2" extensions to reach focus with the ZWO 120MC camera. The resulting image in SharpCap was huge and soft. Removing the GPC and extensions immediately produced a sharper and more stable image.

The weights in the image were to balance the telescopes after I removed the 2" Lacerta solar prism. It is easier to hang a weight by a cord loop than to adjust sliding weights and lose the normal balance position. Such long extensions can have a dramatic effect on longitudinal balance. These weights would dwarf smaller instruments but I have three fairly telescopes on the mounting.

Later I cut out a 120mm card ring to act as an aperture stop in front of the objective. This was to fix the focal ratio at f/10. The effect was subtle, compared to the "naked" 150mm. However, I was struggling with thick, passing cloud for most of the afternoon. The sky finally cleared after 16.00 and I captured a couple of videos of the most obvious prom at 2 o'clock on the limb. The seeing conditions were awful with obvious "boiling." So the final results from Registax very poor just as expected.

Despite being disappointed with the GPC it was hardly a fair test with so much thick cloud about. I'll try again at different spacings from the etalon when the sky is clear and the sun full strength. I should check the focus with the etalon and GPC moved away from the objective. Then adjust the etalon position to 200mm inside focus accordingly. It's a moveable situation because the focus and focal plane will change with the etalon + GPC combination.

Saturday: I have been informed that I should place the GPC closer to the PST etalon. So I bored out the baffle in the AOK etalon nosepiece to 28mm. This allows the nose of the GPC to pass through the baffle. The etalon then needs to hold the GPC in place. So I added 3 layers of rubber washers. Job done. Weather wet and windy so no chance to try the new GPC position until [perhaps] tomorrow.

Sunday: A sunny start. Time to test the GPC! 

Click on any image for an enlargement.

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