In response to a comment I wrote:Near focus the Poisson point was found to be not exactly in the center, but slightly off. Which produces a coma error in the order of 0.2 wave. That little error nibbles off several hundredths of the total Strehl. (...) Perhaps in the future I will do something about it, like I did with my old f/5 150mm, which, by the way, was in worse shape at the time.
Well, this is that report.(...) What I will improve in the future is the 0.2 Wave coma. I have done that before with my former 150 f/5 too. I will make a report on that by then.
Like I wrote earlier I flocked the inside of the tube and the focusser tube. That was not really a necessity of the first order, but the easiest to accomplish. Today I decided to minimize the
I put up an improvised optical bench, a torch shielded by a card with several minimal holes pinched in it. It served as an artificial star. The telescope was some 27 feet away. ( indoors, for it rained outside) One cannot evaluate spherical aberration at that distance, but collimation and
I used the same method as Wolfgang Rohr used to do on a 150 F5 Startravel, the same as I did on my former 150mm F5. Report: viewtopic.php?p=114261&hilit=Coma+150+mm#p114261
You will get an impression with pics of his method over here. http://r2.astro-foren.com/index.php/de/ ... fotografie
It is in German, sorry, but if you are interested surely Google Translate will do the job. The pics however speak for themselves.
After establishing where on the lens I should correct the spacing, I marked the outside, unscrewed the holding ring and removed two spacing rings. I dismantled the lens by lifting the lensholder down. Made extra markings on the edges how the two lenses were situated to each other and opened it up. The spacing between the lenses turned out to be another thin plastic ring. I added one folded snippet of alufoil ( 0.02 mm) in the right spot. Last time that was enough, so I thought it was a good starting point. Then I closed the lens again.
I cleaned the inside of the lensholder, which like last time, had some very very minor pellets of black paint residue in it and lifted the holder back in place, taking care the lens slid exactly back in as it had come out, taking care that it was not knocked out of alignment. So no touching of the lens-edges and lensholder-wall. Then I put back the two spacer rings and screwed the holder-ring back in place. Then the lens holder back on the tube.
Torch with artificial stars on again and waiting for the moment of truth. It had improved. Looking in Aberrator learned that Strehl had come up from 0.79 to 0.82. That should do. According to Aberrator the perfect position should give 0.83. I decided not to be challenged by that last 0.01 ( I have learned that lesson on my former 150
The pictures below from Aberrator are for illustrative purposes only. They do not represent an optical test.
Lensholder, lens and lens-spanner
In a moment of inadvertence, it turns out the spacer was skewed in after all. The thingy blew away while breathing.... I leave it as it is since it can't be seen from the eyepiece perspective. The dust you see is on the outside by the way.
Thanks for reading!