Skymax 150 star test

[spaceman_spiff]

Hi there SGL,

After reading the extensive and well written reports from @iPeace, I was compelled to check my 150mm Mak to see if I was getting the most out of it. I did a star test with an artificial star (a torch with pin pricked foil wrapped around it). I did the test inside my house with the torch about 15-20m from the scope. I defocused the scope until the diffraction patterns filled about 3.5cm of the camera screen. The recordings were then stacked and enhanced with Registax.

Looking at the patterns I can't see much wrong; no serious misalignments (I have never collimated the scope). I used the program aberrator to simulate different optical issues and concluded that there may be a small amount of both low order and high order spherical aberration.

The only big difference is the size of the obstruction shadow, does anyone know what this means?

Cheers! Dan

[Peter Drew]

The difference between the intra and extra focal central shadows is usually an indication of spherical aberration, yours is probably due to the proximity of the test source.   

[iPeace]

5 minutes ago, spaceman_spiff said:

After reading the extensive and well written reports from @iPeace

Too kind. Most of this is beyond me - In my own case, I didn't get past your own conclusion:

7 minutes ago, spaceman_spiff said:

Looking at the patterns I can't see much wrong; no serious misalignments

 

[spaceman_spiff]

5 minutes ago, Peter Drew said:

The difference between the intra and extra focal central shadows is usually an indication of spherical aberration, yours is probably due to the proximity of the test source.   

Thanks Peter!

I think you are right. If it was purely SA then I would expect bigger differences in the rings as well.

[spaceman_spiff]

9 minutes ago, iPeace said:

Too kind. Most of this is beyond me - In my own case, I didn't get past your own conclusion:

Credit where it's deserved . Your reports were very useful to me; I don't really have a comparable scope to test the Mak against so I never really questioned its performance until I read your posts.

I do admit a lot of the optics stuff is very hard to comprehend...I tried to read Maksutov section of www.telescope-optics.net but I think I need to go back to university to understand it. I'm happy with posting pictures and letting more qualified people tell me what's going on!

 

[spaceman_spiff]

I just noticed that there is a slight asymmetry in the extra-focal image...the right side of the doughnut is slightly narrower than the left. Interestingly the intra-focal image looks symmetrical. Am I worrying about nothing or is this a problem?

[davidc135]

Quote

 

This simulator looks fun. As Peter Drew says the artificial star may be too close and that could be causing the significant overcorrection shown although I'm certainly no expert. Did you make sure that the intra and extra focal positions were the same distance either side of focus? For a visual assessment I'd first use the focuser to establish focus with one 2.5mm shim between ep and holder. Removing that shim gives the intra position and having 2x 2.5mm shims gives the extra., these two being both 4 waves from focus (I think). Measure relative sizes of obstruction shadow. Then increase the 'star' distance and repeat to check if proximity is the culprit. There is a table which correlates ratio of the shadow diameters to spherical aberration which I've forgotten but .1 wave looks optimistic.

How does the Airy disc look?

David

[spaceman_spiff]

49 minutes ago, davidc135 said:

This simulator looks fun. As Peter Drew says the artificial star may be too close and that could be causing the significant overcorrection shown although I'm certainly no expert. Did you make sure that the intra and extra focal positions were the same distance either side of focus? For a visual assessment I'd first use the focuser to establish focus with one 2.5mm shim between ep and holder. Removing that shim gives the intra position and having 2x 2.5mm shims gives the extra., these two being both 4 waves from focus (I think). Measure relative sizes of obstruction shadow. Then increase the 'star' distance and repeat to check if proximity is the culprit. There is a table which correlates ratio of the shadow diameters to spherical aberration which I've forgotten but .1 wave looks optimistic.

How does the Airy disc look?

David

Hi David, thanks for the feedback.

The artificial star was definitely far too close to the scope (closer to 15m than 20m). This corresponds to only 8.3 times the focal length of the scope (1800mm). I read that I need a distance around 30x focal lengths (54m!) to avoid introducing spherical aberration into the star test (or cancelling out real SA). I am going to try using a bright glint off a distant car next time.

I measured the focus distance simply by measuring the diameter of the defocused star on the camera screen. Your suggestion of using shims to move the focus knob a set distance is interesting and may be worth a try if I can make them...I had the idea of counting the diffraction rings each side of the focus and making sure they are the same.

The Airy disk looks very close to the central peak. I can just about see it on the shorter exposure test (see below) but I am using prime focus video to do this so I may not be amplifying the image enough to see it properly. I have a 2x Barlow I could use...

[davidc135]

Hi Dan

I may have misunderstood you but the scope's focuser should only be used once in the star test; to establish position of focus. At this position the ep is kept proud of the diagonal or drawtube by one of the shims. When this is removed the ep can be pushed home and is now in front of focus by a known amount. Similarly, using both shims for beyond focus. A £1 coin is around 3mm or a bit less. I don't have one to hand but will measure it asap.

In the aberrator example above there are figures for waves out of focus as well as actual distance defocus. In this case .79 waves corresponds to .5mm defocus or .158mm per 1/4 wave. As 4 waves is often recommended, that comes out at 2.5mm. The £1 coins would be a bit more but I don't think it matters so long as they are the same both sides. However you'll need the right figure to feed into the program. This .158mm number is true for an F/12 setup and varies with the square of the F ratio. It may well turn out that your method of filling the screen is fine and establishes a ratio of shadows but the above is going by the book. The ratio looks to be around 67% which, unless it's explained by the source distance, isn't great. Probably is the source distance.

A ball bearing in the sun or with a torch is also handy but I'd initially do a high power visual test using the Pole star making sure the scope has cooled down. 

David

 

[spaceman_spiff]

Ahhh...Ok I understand you now.

Use a shim to push the ep out and get extra focus, then focus, then remove the shim and get equidistant intra-focus...I like it. I have been doing the star test with only a camera but it is attached to the scope by a removable holder and I think I can use a shim there.

As a test...I will try both methods (the shim method and the filling camera screen) and post them here. We can compare the results.

I also found a bunch of ball bearings so now I need to wait for the Sun.

Thanks for the help!

Dan.

 

[davidc135]

Looking forward to the results. I've been reading a few other threads on star testing and some of them mention 10 waves or more of defocus being more useful.

David

[davidc135]

There's a recent thread on CN in the cats and casses section: 'star test central obstruction size' including a graph for judging sa that may be useful. Says 10 waves defocus recommended for obstructed optics. Your lens likely to be a good one.

David

[spaceman_spiff]

On 19/03/2018 at 12:10, davidc135 said:

There's a recent thread on CN in the cats and casses section: 'star test central obstruction size' including a graph for judging sa that may be useful. Says 10 waves defocus recommended for obstructed optics. Your lens likely to be a good one.

David

Thanks for that David,

I tried the shim idea using some coins but the removable attachment didn't like it. It partly gripped but when I moved the scope the camera fell out and onto the floor . I'll send you the bill .

Luckily, I was testing the shim method indoors and the camera fell onto carpet and was not damaged.

I did eventually try to do a star test with a glint from a distant car but then my neighbour saw me and we chatted for about a hour after which the sun had set and clouds had set in . I havn't been able to get out since (long days at work) but I plan to do something later this week.

Dan.

[spaceman_spiff]

Here are a couple of stacked shots of Arcturus so no issue with distance to the light source! Turbulence was not great so the some of the details were not captured well. Also ignore the ghost artefacts in the extra-focal image (caused by incorrect stacking). I also made a scale around the focuser so I could measure the angle turned. Here I defocused by 90 degrees.

I recently bought the book 'Star Testing Astronomical Telescopes' by Harold Suiter...great book with lots of coverage on the 150mm f12 mak. The patterns here somewhat resemble the published ones with the intra focal pattern containing greater brightness inside and the extra-focal pattern having a bright ring outside. This is apparently high order SA.

I can't explain why there is a colour difference between extra and intra. Maybe the scope suffers from chromatic aberration.

[mikeDnight]

Try imaging a star nearer to the zenith which will reduce the thick turbulent atmospheric effects you'll get at low angles. Arcturus will show some atmospheric refraction due to its brilliance. Perhaps a 2nd or 3rd mag star would be more suitable. You may still see some difference in the colour of the rings either side of focus though, as Maksutovs are not entirely colour free systems. It seems something in the optical train is slightly out of alignment as the rings are not concentric, which may be the camera and not the scope, or it could just be due to some residual heat inside the tube.

[davidc135]

Looks to have fine optics. 1/8 wave overcorrected or better from that graph. Congrats. Seems there was around 1/8 wave induced SA(as well) in the first test. 

David

[spaceman_spiff]

Thanks guys!

8 hours ago, mikeDnight said:

Try imaging a star nearer to the zenith

I did!

Just stacked the images now. Here's Phad (in U Major), at 65-70 degrees altitude. This gives a much better result, clouds did dim the view so the ISO was ramped up, hence the noise. There may be some slight misalignment...rings seem slightly weaker on the right of the extra-focal and left of the intra-focal. I'm not sure if it worth tweaking the scope to correct it though.

Dan

[Stu]

Looks pretty good to me, I would leave it as you will just as likely make it worse, and that would be very frustrating!

[davidc135]

Good images. They show the difference in brightness of rings inside and outside of focus that you mentioned due (I think) to a high zone near the edge. Or even a slightly turned edge? Mel Bartels quotes John Dobson who recommended working on a mirror's zones that were bright outside of focus. Have you put this into aberrator? Looks like your original assessment of a small amount of LSA and HSA was right.

There seems to be a heck of a lot to star testing! Has Suiter's book been helpful? I must get a copy.

David

[Sunshine]

Figuring out collimation especially with a Mak and all the variables involved which can give false readings is about as much fun as a root canal!.

The last images look about as good as you’ll get, any more tweaking and you may open Pandora’s box!, easily done with a Mak as I can attest.

[spaceman_spiff]

I agree with @Stu and @Sunshine, the prospect of fiddling around with those push pull screws; turning them 1/8th of a turn) at night, remembering what ones I turned and what direction...etc does not fill me with delight. I would only attempt it if there were tangible advantages in doing so.

Hi @davidc135 thanks for your suggestions, LSA and HSA are intrinsic errors built into the Maksutov design so I am comfortable seeing them here as long as they are not greater the 1/4th wave. A zonal error or turned edge are difficult to correct without re-figuring the mirror. Do you think these errors are relatively minor or need addressing?

Suiter's Star Testing books is an interesting read (except chapter 4 on diffraction theory ) and really helps with diagnosing optics. Truth is, I have some free time since my new job starts in May so I thought I could tweak my scopes to perfection. I also have the Mars opposition in mind later this year and I want to be ready for it!

I also think that SGL should have a star testing/collimation section...

Cheers! Dan

[davidc135]

Hi Dan It could be that the zone is no more than the expression of the HSA intrinsic as you say in the design. All errors together are probably less than 1/4 wave although it would be fun to quantify if there was the kit.

David