TheSkySearchers

In my review I wrote about some flaws in my new 102mm f/5 Startravel.

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.

In response to a comment I wrote:

(...) 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.

Well, this is that report.

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 coma. In order to do that one must open up the lens.

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 coma can be done that way. Used magnifications were 200X and 400X. In order to focus properly one has to introduce some extra extensions in the prism.

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 f/5 , moreover even W. Rohr did not achieve that) and thought it was fine like this.

The pictures below from Aberrator are for illustrative purposes only. They do not represent an optical test.

Before:

After:

Lensholder, lens and lens-spanner



Three rings



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!

With a refractor, the focal plane tilt can ruin the overall image from the tilt of the mirror or prism in the diagonal.
So that is one more place to check for tilt.
Good job on the correction, by the way.
Have you had a chance to see if it improved the actual images in the field?

Don Pensack wrote: ↑Tue Mar 07, 2023 2:44 pm With a refractor, the focal plane tilt can ruin the overall image from the tilt of the mirror or prism in the diagonal.
So that is one more place to check for tilt.
Good job on the correction, by the way.
Have you had a chance to see if it improved the actual images in the field?

Thanks!
Yes I ruled that out.
For the coming week the weather will be horrible, so I'll have to wait how it turns out in the field.

Well done John!

Well, I showed my "weak side" and still allowed myself to be challenged by the residual coma that was still in the instrument.

After some fiddling with the matter, during which the results did not exactly match my expectations, I decided to undo my corrections in the spacing and concentrate entirely on the correct initial position of the lenses in relation to each other: bringing the optical axes into line with each other as much as possible. And placing the standard 0.25mm spacing ring exact in the middle. To my surprise, this allowed me to reduce the residual coma of 0.1 wave to approaching zero. ( about 0.04 wave) Small enough to no longer be noticed in the star image.
So it appears that potentially the instrument can be qualified a bit higher than the way they are put together by the factory. It appears to be down to the assembly and not the optics themselves. In doing so, unfortunately, I must also reach out to myself.

This is how it turns out in the field.
I used Alnitak, the eastern star of the belt of Orion.
Size and orientation of the second image differs, but it is the principle I want to show you.
Before and after optimization.

Looking good John!

Hard work pays off, btw how's the focuser on that scope?

I always had fun improving those Synta focusers, with proper cleaning and adjusting and/or shimming they can be tuned up nicely.

Ylem wrote: ↑Fri Mar 17, 2023 12:25 pm (...)btw how's the focuser on that scope?

I always had fun improving those Synta focusers, with proper cleaning and adjusting and/or shimming they can be tuned up nicely?

Thanks!
I is a very simple focusser, good enough for visuaI observations.
I have to monitor the focusser after a session; I use those two Allen-screws for it. Maybe you have a suggestion about the shimming?

John Baars wrote: ↑Fri Mar 17, 2023 1:37 pm

Ylem wrote: ↑Fri Mar 17, 2023 12:25 pm (...)btw how's the focuser on that scope?

I always had fun improving those Synta focusers, with proper cleaning and adjusting and/or shimming they can be tuned up nicely?

Thanks!
I is a very simple focusser, good enough for visuaI observations.
I have to monitor the focusser after a session; I use those two Allen-screws for it. Maybe you have a suggestion about the shimming?

I have had a couple of ST80 focusers that have been too stiff (this was after removing the Synta snot).

So I loosened the 4 small philips screws a bit and that made it nice and smooth.

I didn't like leaving those screws loose of course, so I put thin washers between the plate and the focuser body and then tightened the screws down.
As buttery smooth as a Synta focuser can get, I also use a white lithium grease called Lubraplate.

@Ylem
Thanks!
I'll be looking out for those washers and applying the white Lithium in the morning.

@Ylem
I did my homework this morning.
The new lubricant works well!
I took the opportunity right away to use the Philips screws and the Allen screws to align the focuser-tube nice and straight in the holder. It all works exemplary!

~

Nice work to improve your scope.

If the aberration outside of focus appears, not as coma, but as an elliptical galaxy on one side of focus, shifting 90° other side of focus, would this be considered astigmatism?

If so, is there any hope for correction by a shade-tree mechanic?



.

Nakedgun wrote: ↑Sun Mar 19, 2023 1:42 am ~

Nice work to improve your scope.

If the aberration outside of focus appears, not as coma, but as an elliptical galaxy on one side of focus, shifting 90° other side of focus, would this be considered astigmatism?

If so, is there any hope for correction by a shade-tree mechanic?

Yes, that is astigmatism.

It is mostly due to a ingrinded saddle shape in the lens, and sometimes caused by internal tensions in the glass.
Don't put your hopes up high for it is almost impossible to correct it, not being an optical worker. Optical workers reshape the lens by grinding and polishing ( sometimes even by re-heating) but even so residues of astigmatism will still be visible most of the time if it is caused by internal tensions. Sorry.

What you could try:
- rotating the front-lens bit by bit ( mark the original setup on the edges) , let's say 45 degrees each time and retest after every correction. While hoping that it might do some good. It is quite a painstaking job though and it takes a considerable amount of precision for the remounting of the front-lens over and over again.
- Easier: Making a ring of 0.75 cm wide, to mask off the edges of your lens. ( takes away 1.5 cm of the total aperture, so that it is a considerable light loss). While hoping that it shades the top of the "glass mountains" on the surface of the glass, which are in many cases most evident at the edges.

In both cases there is absolute no guarantee it will work. But you could try the latter one quite easily.
Succes!

P.M. To reassure you, moderate astigmatism lowers the Strehl ratio by only the root of an equally large coma error and only the third power root of a spherical aberration error.

Well done with the correction John.
I have spent a few evenings 'spinning lenses' with various achromats.
Enjoy the views

Lady Fraktor wrote: ↑Mon Mar 20, 2023 6:42 pm Well done with the correction John.
I have spent a few evenings 'spinning lenses' with various achromats.
Enjoy the views

Thanks.
The spinning, remounting, testing, spinning etc. is a time-consuming and grimly accurate job. My compliments!

@Lady Fraktor
Were you trying to enhance the general performance or trying to minimize a specific aberration?
I am quite curious what you did, how you did it and if it worked out. I know that even the tiniest improvement can be very satisfying. And even if it didn't help at least one has got the knowledge that everything possible was done. Quite satisfying too.

I was trying to fix a situation of over corrected with spherical chromatism and lateral colour.
Checked wedge, spacing, lens centering, orientation and slowly worked on getting a good star test.
Many times star testing with the end result being, that is not right
Sheer stuborness paid off in the end though, the refractor puts up a good visual image though a camera may show some needed work still.

Yikes! I'm not brave enough to try such surgery on my ST 102, but I did swap out the focuser for 2 speed Crayford. Not top of the line, but neither is the scope. For colour fringing I've had good results with a Baader Contrast Booster filter. Pretty cool how the optics can be tuned by rotating the lenses. While my first love is looking through them, posts like yours are rekindling my fascination with physics.

Sometimes small changes can make good differences.
Being a Fraunhofer doublet, these refractors are quite a good instrument to learn how the optics work.
Just always record your starting position and progress if you need to return to a past point.

Ylem wrote: ↑Fri Mar 17, 2023 12:25 pm Looking good John!

Hard work pays off, btw how's the focuser on that scope?

I always had fun improving those Synta focusers, with proper cleaning and adjusting and/or shimming they can be tuned up nicely.

One thing I do with these focusers, if they have the ribbed teflon sliders is to replace with solid PTFE strips. The ribs can compact and wear down over time allowing the drawtube to shift.
I also replace the hard plastic slider under the 2 grub screws and tension screw at the top with a piece of PTFE.
You can usually get these focusers operating fairly well for visual observing.