36 replies to this topic

[piotradam]

I recently did a star test for my FS-60Q and would like to know if someone who is more experienced than I am could take a look at the results and make some comments.

I find it a little strange that both sides aren't identical, so I'd like to hear your thoughts. Images attached below are near focus (top image) and very far out of focus (below). Both are roughly equal distance since I lined the disks up against a grid so that they are equal size.

The test was done on Capella, which from my location is at around 65 degrees of altitude. The one on the left is inside focus, and the one on the right is outside focus. Apologies for some images being noisy, I was messing around with exposure times.

This one is near focus:


And this comparison is quite a large distance from focus:




 

[Jim Waters]

I will let others comment.  FYI - http://aberrator.astronomy.net/

[eyespy]

How are the images when in actual focus. In that scope at say 50x per inch, you should see a very definite sharp edged Airy disk and maybe one diffraction ring as I see in my 60CB.

 

Doug…..

[Spikey131]

The FS-60Q is a fluorite doublet.  It is normal that the pattern will be different on opposite sides of focus.  I see nothing wrong.

 

How do objects look, in focus?  Have you noticed any problem with the optics?

[civis]

I recently did a star test for my FS-60Q and would like to know if someone who is more experienced than I am could take a look at the results and make some comments.

I find it a little strange that both sides aren't identical, so I'd like to hear your thoughts. Images attached below are near focus (top image) and very far out of focus (below). Both are roughly equal distance since I lined the disks up against a grid so that they are equal size.

The test was done on Capella, which from my location is at around 65 degrees of altitude. The one on the left is inside focus, and the one on the right is outside focus. Apologies for some images being noisy, I was messing around with exposure times.

This one is near focus:
startest_closetofocus.jpg

And this comparison is quite a large distance from focus:
startest_farfromfocus.jpg



 

Looks like spherical aberration under-correction to me.

"Over-correction is when the concentric rings inside focus are diffuse and not well defined, while those outside focus are sharply defined.

Sharp rings inside focus, together with misty rings outside of focus, shows under-correction."

 

https://www.skyatnig...est-a-telescope

[kgb]

Looks like it could be under corrected. An in-focus image would be helpful, but are you sure that these are defocused by the same amount on each side of focus? There does appear to be some SA, but the extra focal image appears smaller and since there appears to be some under correction, that extra focal image should, at a minimum, be the same diameter if not larger than the intra-focal.

Edit: I am referring to the “near focus” image, the second one is too defocused to be helpful. You want there to be only about 10 waves of defocus. At f/10, this should be - 0.1732”.

Edited by kgb, 23 March 2025 - 08:34 PM.

[Oldfracguy]

When you were doing this star test, had the scope and other accessories been outside for very long before you took the pictures of "near focus" in Post #1?

[peleuba]

The FS-60Q is a fluorite doublet.  It is normal that the pattern will be different on opposite sides of focus.  I see nothing wrong.

 

When its said that a doublet will have different appearance of the intra vs. extra focal Fresnel pattern, one is speaking of the chromatic affects that will muddy and smear the extra focal pattern.  This often gives the appearance of under correction but in reality its spherochromatism.  To neutralize spherochromatism and determine the sign of SA, a monochromatic test is needed, usually with a green filter or green and yellow filters stacked.

 

To me, there is something else going on.  Notice the very strong/bright/distinct outer rings in the extra focal image?  I have seen this when either SA, or a wide rolled edge is present.  Its causes may be thermal, but honestly, it does not take a 60mm lens very long to settle down.

 

Also, this scope has a sub-aperture corrector - Extender Q - adding a layer of complexity.  Best to test without that and compare the test with it in place.

 

Not sure where the OP is located, but I would be glad to test the scope if it helps.

Edited by peleuba, 24 March 2025 - 09:38 AM.

[C. Evangelista]

To me, there is something else going on. 

Agree.

 

A nearly perfect star test (with artificial star) has been achieved and published for a Tak FS-60 by German optic tester Wolfgang Rohr:

Attached Thumbnails

• 

[Kitfox]

The size of the objective helps, but I still envy the seeing you must have had to throw smooth images like that!!!

 

I have found the more elements there are in an optical train, the more likely there is a substantial difference in the intra- and extra-focal images.  This is not necessarily bad, just an observation.

 

I agree with Paul, you need to repeat with a green filter and just the objective at first, and see how much energy is going into that outer ring at a narrow wavelength band.  Lots of guesses would be premature before you rule out spherochromatism.  But it sure looks like some kind of zone with all the energy in that outer ring...yes, I am guessing!  Take Paul up on his offer (or someone else local to you with a DPAC setup), a double pass ronchigram would complement your star test quite well...and narrow down the "guessing".

[piotradam]

I really appreciate all the help, I certainly did not expect my post to get this much attention. I apologise if any information I provide is unhelpful, I am admittedly relatively a bit of a noob.

 

Looks like it could be under corrected. An in-focus image would be helpful, but are you sure that these are defocused by the same amount on each side of focus? There does appear to be some SA, but the extra focal image appears smaller and since there appears to be some under correction, that extra focal image should, at a minimum, be the same diameter if not larger than the intra-focal.

Edit: I am referring to the “near focus” image, the second one is too defocused to be helpful. You want there to be only about 10 waves of defocus. At f/10, this should be - 0.1732”.

That being said, I redid the star test without the extender-Q module and got better results. I have to preface by saying that I live in Norway, an area of exceptionally, often surprisingly terrible weather all year round. Ironically when weather is great during the summer its bright outside 24 hours a day. 

I setup hoping for a window of scattered clouds instead of overcast because its the best I'm going to get for the next week at least, and I managed to do a star test on Polaris through a hole in the clouds. But given the limited time I did a rather poor job. The image is defocused to 2-3mm either side, one being slightly more defocused than the other, and shot through a layer of clouds so the inside focus image (to the right) is slightly more stretched and more defocused. Besides, you can judge for yourself since I haven't scaled the images. Results further down.
 

 

When its said that a doublet will have different appearance of the intra vs. extra focal Fresnel pattern, one is speaking of the chromatic affects that will muddy and smear the extra focal pattern.  This often gives the appearance of under correction but in reality its spherochromatism.  To neutralize spherochromatism and determine the sign of SA, a monochromatic test is needed, usually with a green filter or green and yellow filters stacked.

 

To me, there is something else going on.  Notice the very strong/bright/distinct outer rings in the extra focal image?  I have seen this when either SA, or a wide rolled edge is present.  Its causes may be thermal, but honestly, it does not take a 60mm lens very long to settle down.

 

Also, this scope has a sub-aperture corrector - Extender Q - adding a layer of complexity.  Best to test without that and compare the test with it in place.

 

Not sure where the OP is located, but I would be glad to test the scope if it helps.

Unfortunately I don't have the equipment to do an artificial star test nor do I have colour filters for testing for spherochromatism. To my eye at least the star test at f/6 looks much better. It would suggest that the extender-Q module is to blame. I let the scope cool down outside for roughly 45 minutes, and for the previous star test it was around 30 minutes. I don't think it would make a difference for a 60mm. The image to the left is outside focus, and on the right is inside focus. It is attached below.


 

 

How are the images when in actual focus. In that scope at say 50x per inch, you should see a very definite sharp edged Airy disk and maybe one diffraction ring as I see in my 60CB.

 

Doug…..

Visually at f/10 I can see 3 clear diffraction rings at 120x, progressively dimmer the further out they are, but I wouldn't describe the airy disk as sharp, it looks fuzzy and doesn't really have an edge. At 40x however nothing really stands out to me as out of the ordinary, but I am not experienced in visual observing, and I've only looked visually through a dob at the moon and planets a handful of times and so have very little experience.

Piotr

[Mike W]

That looks fine! 

[Steve Allison]

My FC60 shows a nearly identical diffraction pattern inside and out.

[kgb]

That looks really nice. Focus is a tad off on the inside, but you can plainly see that, after scaling this side proportionally with the other, that they appear almost identical. Wonder what is going on with that extender.

[peleuba]

Unfortunately I don't have the equipment to do an artificial star test nor do I have colour filters for testing for spherochromatism. To my eye at least the star test at f/6 looks much better. It would suggest that the extender-Q module is to blame. I let the scope cool down outside for roughly 45 minutes, and for the previous star test it was around 30 minutes. I don't think it would make a difference for a 60mm. The image to the left is outside focus, and on the right is inside focus. It is attached below.

 

Piotr - very nice job taking the photos.

 

Are you 100% sure these images are on opposite side of focus?

 

Both of the images you posted look to be inside of focus at slightly different waves of defocus.  Please double check this.

 

Typically on opposite sides of focus the center spot will be bright on one side then dark on the other.   Both of yours look identical.  I would doble check this as this does not look like the star test of an F/6 doublet.  It looks like two images on the same side of focus.  You may have gotten confused with the many images you've taken.  Its very east to get confused.  I have done it myself more times then I care to admit.

 

Also, you would also see slightly different chromatic affects on the outer ring.  I see what looks to be the same.

 

Nevertheless, I applaud you for wanting to test your lens.  Its one of the most educational and enlightening things you can do.  And yes, I am not surprised the Extender Q may be causing some issues. 

Edited by peleuba, 26 March 2025 - 07:54 AM.

[kgb]

Paul. If that is indeed the case, that would be a really good catch.

[Psion]

I don't think this is a diffraction image of inside and outside focus. It would look different and the subtle red around the diffraction image would not be red, but green and the center would be inverted.

[peleuba]

Paul. If that is indeed the case, that would be a really good catch.

 

Thanks.  

 

 

I don't think this is a diffraction image of inside and outside focus. It would look different and the subtle red around the diffraction image would not be red, but green and the center would be inverted.

 

Yes, I am nearly certain this is the same side of focus, just at different waves of defocus.

 

Its really easy to get confused - especially in DPAC.  In the star test, its a little easier (then DPAC) to see the nuances of the intra and extra focal images. But, in these two images, none of these differentiators that are typically present between images at opposite sides of focus are visible.  I would bet my next pay check that these images are from the same side of focus.

Edited by peleuba, 26 March 2025 - 12:33 PM.

[piotradam]

Piotr - very nice job taking the photos.

 

Are you 100% sure these images are on opposite side of focus?

 

Both of the images you posted look to be inside of focus at slightly different waves of defocus.  Please double check this.

 

Typically on opposite sides of focus the center spot will be bright on one side then dark on the other.   Both of yours look identical.  I would doble check this as this does not look like the star test of an F/6 doublet.  It looks like two images on the same side of focus.  You may have gotten confused with the many images you've taken.  Its very east to get confused.  I have done it myself more times then I care to admit.

 

Also, you would also see slightly different chromatic affects on the outer ring.  I see what looks to be the same.

 

Nevertheless, I applaud you for wanting to test your lens.  Its one of the most educational and enlightening things you can do.  And yes, I am not surprised the Extender Q may be causing some issues. 

You are 100% correct. I can't tell you how (I don't know), but I managed to take pictures on the same side of focus twice.

Due to unpredictable weather the forecast changes practically every 10 minutes, and a 30 minute window of dark and clear skies opened up, in between the sunset and clouds rolling in, and I managed to get a solid star test in this time around.

However, I took the time to improve my method a little bit, and since the FS-60 does not have millimetre markings on the draw tube I took a piece of masking tape and drew on my own millimetre markings that I measured out and which I stuck to the drawtube. I took photos 1, 2, and 3 millimetres out of focus on each side, which I've labelled in the picture. Note: I ended up shooting the 3mm photo outside of focus through clouds as time was running out.

I setup before sunset, and the telescope got to cool down for around 30 minutes. I'm still taking pictures at f/5.9 (thought it was f/6 but turns out the scope is slightly faster), meaning that I only have the doublet lens and no other optical elements, no flatteners or reducers. Figured I should mention that.

I've split up the test below into two images, since it was behaving weirdly when I tried uploading a very wide image. Here they are:




The reason I did a star test to begin with is that I noticed my stars developing small halos around them and I could not fix it in post processing. This is what my images looked like, the example is a recent image of mine at f/10 with this telescope of m81. I've cropped it due to upload limits



I've only done a light stretch to the image, nothing else.

Piotr
 

[peleuba]

You are 100% correct. I can't tell you how (I don't know), but I managed to take pictures on the same side of focus twice.

 

I thought so!   Please don't  worry about it.  It gets very confusing when you have so many intra and extra focal images.  Its very easy to make mistakes.

 

Than you for clarifying this.

 

I am very glad that you are posting your test results.  It really does help us all.  

[Psion]

This test is fine. To find out more, we'd need a Ronchi test. Anyway, the FS 60Q will show a slight CA at higher magnifications, and that's OK.

[Lagrange]

I've had a look through my FS-60Q to see what the star test looks like. Unfortunately I don't have the means to photograph it and the seeing here is pretty bad at the moment but I was able to get a reasonable look at the diffraction patterns.

 

Inside and outside focus were fairly similar with clearly visible diffraction rings, but inside focus seemed to be more defined as the rings were very sharp and clearly visible.

 

There certainly wasn't the softness that I can see in your images, especially the outside focus patterns which look very different to the ones I saw.

 

I don't know enough about this stuff to give you any definitive advice, but it looks like there might be something odd about your scope. Hopefully other FS-60Q owners can do their own tests to give us some more data.

 

In focus, the Airy disc and first couple of diffraction rings were very clearly defined from what I could tell through the less than ideal seeing.

Edited by Lagrange, 26 March 2025 - 06:13 PM.

[piotradam]

I thought so!   Please don't  worry about it.  It gets very confusing when you have so many intra and extra focal images.  Its very easy to make mistakes.

 

Than you for clarifying this.

 

I am very glad that you are posting your test results.  It really does help us all.  

Are the results of the test something I should be concerned about? Someone here earlier posted a nearly perfect star test of a 60mm Takahashi scope. It seems like that is the expected standard for Takahashi telescopes. How does my star test compare?
 

 

This test is fine. To find out more, we'd need a Ronchi test. Anyway, the FS 60Q will show a slight CA at higher magnifications, and that's OK.

I haven't heard of any places locally where someone with the equipment and experience in optics testing could take a look at it, so I'd like to hear your thoughts on what might be visible in the star test that I was able to do on my own.

Piotr

[davidc135]

This test is fine. To find out more, we'd need a Ronchi test. Anyway, the FS 60Q will show a slight CA at higher magnifications, and that's OK.

Can you get a Ronchi eyepiece or grating? That should reveal anything significant. Even if the tests aren't split up into separate colours it should tell you a lot. As you have other excellent refractors, they can be used to produce a collimated beam indoors.

A double pass test using an optical flat is most sensitive but single pass is still very good and easy to do.

 

David

 

PS I should have said that the Ronchi ep would be used on a bright star.

Edited by davidc135, 26 March 2025 - 07:10 PM.

[Oldfracguy]

Can you get a Ronchi eyepiece or grating? That should reveal anything significant. Even if the tests aren't split up into separate colours it should tell you a lot. As you have other excellent refractors, they can be used to produce a collimated beam indoors.

A double pass test using an optical flat is most sensitive but single pass is still very good and easy to do.

 

David

 

PS I should have said that the Ronchi ep would be used on a bright star.

This is the one I have:

 

https://www.teleskop...ith-a-star-1812

 

Be sure to click on the link to the PDF manual at the bottom of the webpage.

 

I last used it to check a new AT70ED a few weeks ago using Capella.  Perfectly straight lines at 4-5 across.  I also used it to set the spacing of the primary and secondary mirrors in a Classical Cassegrain.

Edited by Oldfracguy, 26 March 2025 - 09:31 PM.