20.08.2022 Lunt 60MT etalon mod. Main tube shortening: First H-alpha light.

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  Saturday 20th 58F, speckled cloud with mostly veiled sunshine. 

My existing 6" f/10 iStar H-alpha telescope has a focal plane 33cm from the backplate.

The 2.5" FT focuser is 11cm long at minimum inward travel. Its full travel is 8.9cm.

The etalon extension uses up 4cm of optical path length.

33 - 15 [11+4] = 18cm.

The desired position of the etalon lens [from focus to the face of its housing] = 26.4.

26.4  - 18 =  8.4cm. This is the minimum length which must be removed from the main tube.

However, a bit of leeway is extremely valuable. To allow for miscalculations or precise etalon tuning. The mid position of the focuser adds 4.4cm.

8.4 + 4.4cm = 12.8cm.  This much is the minimum length which must be removed from the main tube. To bring the etalon to 26.4cm from the focal plane. Having to remove and dismantle the entire telescope twice. Just to saw a bit more off the main tube. Is a serious problem. So cutting off a little extra length would probably be wise. To be safe, I think I shall saw off 15cm rather than 13cm. Done.

First I checked the focal plane. Which was 48cm from the backplate. iStar objective only. No other optics.

So, 48-26.4 = 21.6cm from the backplate to the black face of the 60MT etalon housing. Check.

It was 13.03 [CET] before I captured my first tuned image with the new etalon. Some more tuning to do. The brightness is very asymmetric. I can't blame the thin high cloud for this. Only a general softening of the image.


 14.00 The cloud is thickening into clumps but lower now. With clear blue in between instead of hazy white. I shall continue to experiment with tuning and focusing this afternoon.

 The third image shows the effect of deliberately boosting contrast.The ring shaped "sweet spot" is clearly visible. Can I tune out this ring by adjusting the position of the Lunt etalon? Tuning, vis the pressure knob, did not improve the uneven brightness.

14.21 Further tuning did nor help to spread the sweet spot. The presence of Ellerman bombs suggest the etalon is not centred on H-alpha.

 Cloud is thickening.  Making it impossible to continue.

 

 

 

 

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