16.07.2021 PST Etalon + Baader 1.25x GPC as collimater.

 *

 

 Friday 16th. 75F, clear, calm and bright.

I have spotted a loophole in the PST optical limitations in Christian Viladrich's discussion on etalons. Spreading the sweet spot with a longer focus collimation lens behind the PST etalon.

I had a weak, negative lens in the form of as Baader 1.25x GPC. The rear, AOK Swiss etalon adapter has a baffle. The opening is slightly too small for the Baader GPC. So I opened it out slightly, to 28mm, in the lathe. Only a few tiny cuts were necessary.

This allowed the Baader GPC to drop neatly into place. It is then clamped securely in place by re-inserting the PST etalon body.

I have not added any camera GPC in the first, trial captures. Just the Baader GPC right behind the etalon. The seeing is very soft so far. Finding best focus was a problem. I'll keep trying different etalon adjustments in the hope of improvement.

10.22 The seeing is improving slightly. A more even spread of brightness?

 

 

10.29 1.6x GPC added to camera nose.

11.03 The seeing will not support a 2.6x GPC but I am trying anyway. I have now rotated the dome to allow things to cool down in ventilated shade. The ASI174MM Gets very grainy from noise at 50C! The farmer chose that moment to spray the field outside the hedge. So I retreated indoors to sit in the  blast from a fan.

These tests are not just about sharpness. So much as looking for even distribution of brightness across the entire frame. I have been using 800x800 frame size today. The scale changes only as a result of the GPCs placed on the nose of the camera. The higher the power the more the seeing is exaggerated. I never saw any real detail on the monitor. The processed image on the AS!314 screen was little better. ImPPG pulled out what you see here.

12.45 I went over to have another try but the seeing was too awful to capture anything. 

It reached 85F in the afternoon with all day, solid sunshine, from a cloudless, blue sky. 

17.30 I shall try again in the hope of better seeing conditions.

18.10  Started to capture today's interesting surface details. 

The brightness is  not even over the whole frame but is much improved over the naked, PST etalon.

All of these images were with only the Baader 1.25x just behind the etalon.

 

 

 

 

 

 

 

 

 

18.54 Last worthwhile image as the trees covered the sun. 

 

The filter stack is now about 12cm longer. Just enough room for me to fit the 2" helical focuser. 

Now I can set the etalon at 200mm. Then use the helical focuser to bring the sun to focus. Final focussing is done with the motor driven FT focuser over fractions of a millimetre. Being remotely controlled I have no need to touch the telescope for adjusting the last degree of sharpness.

 

*