29 replies to this topic

[Jeff B]

I thought you might all find this test interesting as there are so many of these scopes out there.

 

This sample is an older one with the golden tube.  Both the main and focuser draw tubes were cut back to make it "bino-friendly", something that it excels at.  I knew this was a good sample lens but the DPAC results just backs up my visual assessment of a fine lens.

 

One thing though, the fully illuminated FOV on this sample is small due to the fact that the OTA's inner diameter is not that much larger than the objective's clear aperture, but, mostly, from the positioning of the internal light baffles in both the main tube and the focuser draw tube.  I eliminated or moved a couple of the draw tube baffles but left the ones in the main tube pretty much alone, except for the aft most one which was eliminated as a result of cutting back the main tube.  Fortunately, the slower F7.5 focal ratio means that light fall off out towards the edges of the visual FOV is fairly low at maybe 20%.

 

Regarding the DPAC photos, I've only displayed the inside and at focus images since the outside of focus images don't really tell me anything extra about this lens.  Also, note the very dim red images.  As the inside side of focus images were all derived from the white light image, I had to really lower the cell phone's camera ASA and shorten the exposure times so as not to over saturate the cell phones blue channel.  This is because my phone's sensor as well as the "white" LED are both a bit "hot" in blue.  But my camera also seems a bit low in red sensitivity too and to compensate, I had to boost the brightness of the red images.  But the red images are still dim relative to the green and, especially, the blue.  

 

As you can see this is a fine lens with a very smooth figure, good edge and really very good spherical correction.  You can see a little, normal, spherochromatism by looking at how the lines in the red and blue bend differently but in this regard, I consider this to be a well "balanced" lens.  Now looking at the blue bands relative to those of the green and red, you can also see there is a slightly different focus position of the blue relative to the red and green.  

 

So this is a really very good lens and it does indeed give sharp views up to high magnifications.  

 

Jeff

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[SandyHouTex]

Looks perfect to me.

[Tyson M]

It seems to confirm the ubiquitous reports that Skywatcher apo doublet scopes are some of the best astro buys, for the price.

 

I have had a 120 and several 100ED's.  Still have one which provides amazing views despite being slightly overcorrected. 

 

Excellent scopes that are lightweight, good stock focuser (needs a better visual back though like scopestuff offers), great optics all in a hard case for a fair price.

 

Thanks for sharing the DPAC of this. 

[Steve Allison]

Jeff-

 

Theoretically, in what ways could a more expensive Takahashi be better, if at all? 

[Jeff B]

"..(needs a better visual back though like scopestuff offers).."

 

Agreed and if you look closely, I have the Scopestuff VB.  I was surprised that I could adjust the focuser to be pretty tight and smooth with the bino-viewer load.  Saved some money not having to replace it, but I still may at some point.as the draw tube is what really limits the fully Illuminated FOV the most.  Notice the black bump at the bottom of the in focus images?  That's the very top of a flattened screw  I had to install in the draw tube to keep it from sliding out of the focuser's body.   A nice FT or ML 2.5" focuser would work well.  But for now the stock focuser is fine.......for now.

 

Jeff

 

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[Jeff B]

Jeff-

 

Theoretically, in what ways could a more expensive Takahashi be better, if at all? 

Theoretically?   Maybe eliminate/reduce the small difference in the blue focus, better focuser and larger fully illuminated FOV.   The later two are related and fixable on this sample.

 

So let's not go too far down that path on this thread please.

 

Jeff

[areyoukiddingme]

Theoretically?   Maybe eliminate/reduce the small difference in the blue focus, better focuser and larger fully illuminated FOV.   The later two are related and fixable on this sample.

 

So let's not go too far down that path on this thread please.

 

Jeff

 

 

Is there a DPAC on a Tak TSA-120 to compare to? It's an interesting question, assuming it doesn't degenerate into the usual nonsense. . . 

[Doug Culbertson]

Is there a DPAC on a Tak TSA-120 to compare to? It's an interesting question, assuming it doesn't degenerate into the usual nonsense. . . 

Paul Leuba posted his DPAC results on a TSA-120 a little while back.

https://www.cloudyni...a120/?p=9884614

[Steve Allison]

Thanks, Jeff.

 

I am just trying to learn about optical testing and from what little I know so far, your DPAC images look about perfect. I probably just should have asked if an even better DPAC result was realistically possible. I certainly wasn't trying to get in to a "which is better" type debate.

 

Steve

[fate187]

As you can see this is a fine lens with a very smooth figure, good edge and really very good spherical correction.  You can see a little, normal, spherochromatism by looking at how the lines in the red and blue bend differently but in this regard, I consider this to be a well "balanced" lens.  Now looking at the blue bands relative to those of the green and red, you can also see there is a slightly different focus position of the blue relative to the red and green.  

Could you elaborate the difference between the two. Is there one at all? Spherochromatism leads to different focus position of the colors, hence the diffence on how the lines bend? Is that correct? How would one see lateral color in DPAC?

 

Regarding the SW vs Tak: I have a nice Equinox 120ED, which is optically identical to the SW120ED. I am considering swapping the scope with a smaller Tak FC100DC or DF. I will obviously loose aperture, but gain better correction...

 

In any way, thanks for the DPAC Jeff!

[John Huntley]

I've owned a "gold tube" Skywatcher ED120 for around 7 years now. I've always thought that it had fine optics but this was confirmed to me when I acquired a Takahashi FC100- DL and a TMB/LZOS 130mm F/9.2 triplet back in 2016.

 

I thought that the ED120 might be outclassed by the more renowned and expensive (much more expensive !) Takahashi and LZOS optics but that was not the case. The Synta-made ED120 doublet holds up extremely well and remains a very valued member of my scope "fleet"

Edited by John Huntley, 22 February 2020 - 05:59 PM.

[Jeff B]

Could you elaborate the difference between the two. Is there one at all? Spherochromatism leads to different focus position of the colors, hence the diffence on how the lines bend? Is that correct? How would one see lateral color in DPAC?

 

Regarding the SW vs Tak: I have a nice Equinox 120ED, which is optically identical to the SW120ED. I am considering swapping the scope with a smaller Tak FC100DC or DF. I will obviously loose aperture, but gain better correction...

 

In any way, thanks for the DPAC Jeff!

Sure Michael. 

 

There are two main forms of on axis "CA" in refractors.  One is longitudinal "CA" which is primarily caused by the differences in focus positions between the colors.  The other "CA" is spherochromatism which is the variation of spherical aberration with color.  Both will bloat the spot images of the colors but in different ways but they can be related. 

 

In this case spherochromatism is indeed observed here by the different bending of the lines, especially seen when comparing the red and blue, as they bend in opposite directions.  But there is also a little bit of a focus difference with this lens between the red/green (which are very close to each other in focus) and the blue.  You can see this best by comparing the green and blue images. 

 

Notice in the blue image there is another bar starting to creep in from the left edge that is not there in the green image.  But also notice there is more of a gap between the right bar and the edge of the lens than there is in the green image.  This tells me that the blue focus position is maybe a little different than that of the green/red.  As these images are taken inside of focus, this tells me that best focus for blue probably falls a little closer to the lens as the more inside of focus I go, the more bars I see.

 

One of these days I'll reassemble my 6" F10 Istar achromat and put it back in DPAC.  Last time I did that, the DPAC showed rather straight bars in each color, meaning very low spherochromatism, but there were obvious differences in focus positions between the colors.  

 

Regarding seeing lateral color in DPAC, I don't really know what, if any symptoms there would be in DPAC.  Maybe a subtle shifting of the bars from side to side?  Good question.

 

This why I now like using a so called "white" LED as all of the color images with bars were derived from the one image in "white".

 

I hope this helps.

 

Jeff   

[Reid W]

For the readers, *this tube* is the one that gave stunning binoviews of Mars about 4 years (?) ago at our swampy humid observatory just south of Shreveport. 

[Jeff B]

Thanks, Jeff.

 

I am just trying to learn about optical testing and from what little I know so far, your DPAC images look about perfect. I probably just should have asked if an even better DPAC result was realistically possible. I certainly wasn't trying to get in to a "which is better" type debate.

 

Steve

No problem Steve. 

 

But we sometimes do have to be careful around here less the pitchforks come out and the hounds are let loose.  Tak owners can be especially touchy sometimes. 

 

Jeff

[Tyson M]

No problem Steve. 

 

But we sometimes do have to be careful around here less the pitchforks come out and the hounds are let loose.  Tak owners can be especially touchy sometimes. 

 

Jeff

Takitis is an ravenous disease that takes no prisoners and highly contagious.  Once afflicted, I don't think you can ever be fully cured from it.  

[areyoukiddingme]

No problem Steve. 

 

But we sometimes do have to be careful around here less the pitchforks come out and the hounds are let loose.  Tak owners can be especially touchy sometimes. 

 

Jeff

Hmm, I was actually expecting it would be the "how dare you imply that your expensive scope is better than an inexpensive scope" crowd.

[fate187]

Sure Michael. 

 

There are two main forms of on axis "CA" in refractors.  One is longitudinal "CA" which is primarily caused by the differences in focus positions between the colors.  The other "CA" is spherochromatism which is the variation of spherical aberration with color.  Both will bloat the spot images of the colors but in different ways but they can be related. 

 

In this case spherochromatism is indeed observed here by the different bending of the lines, especially seen when comparing the red and blue, as they bend in opposite directions.  But there is also a little bit of a focus difference with this lens between the red/green (which are very close to each other in focus) and the blue.  You can see this best by comparing the green and blue images. 

 

Notice in the blue image there is another bar starting to creep in from the left edge that is not there in the green image.  But also notice there is more of a gap between the right bar and the edge of the lens than there is in the green image.  This tells me that the blue focus position is maybe a little different than that of the green/red.  As these images are taken inside of focus, this tells me that best focus for blue probably falls a little closer to the lens as the more inside of focus I go, the more bars I see.

 

One of these days I'll reassemble my 6" F10 Istar achromat and put it back in DPAC.  Last time I did that, the DPAC showed rather straight bars in each color, meaning very low spherochromatism, but there were obvious differences in focus positions between the colors.  

 

Regarding seeing lateral color in DPAC, I don't really know what, if any symptoms there would be in DPAC.  Maybe a subtle shifting of the bars from side to side?  Good question.

 

This why I now like using a so called "white" LED as all of the color images with bars were derived from the one image in "white".

 

I hope this helps.

 

Jeff   

Thank you very much Jeff! I did some reading in "Telescope optics" from Rutten. He explains fairly well the different forms of abberations (spherical (monochromatic) abberation, longitudinal chromatic abberation and lateral color). I failed to understand the connection to DPAC images. Thanks for the clarification!

 

Regarding lateral color: I was thinking about it again. It is an off-axis ray effect according to Rutten. In DPAC you only observe parallel (on-axis) light. Thus DPAC is not sensitive to lateral color?

 

edit: I wish I had a proper flat to test my fracs. I could at least use the days of bad weather.

Edited by fate187, 23 February 2020 - 05:39 AM.

[peleuba]

It is an off-axis ray effect according to Rutten. In DPAC you only observe parallel (on-axis) light. Thus DPAC is not sensitive to lateral color?

Correct.  Autocollimation tests by definition use on-axis, parallel light.   It’s quite insensitive to lateral color.  

[Jeff B]

Good, I'm glad you're doing the research.  It takes me a bunch of re-reads to start to get some of this information.   

 

For me, after reading some of it for a while, I've learned not to drive or operate heavy equipment for at least four hours and, OMG, don't read this stuff right before bedtime.

 

Vlad's most excellent site at telescope optics.net is excellent, he constantly adds to it and he posts here on CNs as well.

 

I've been thinking about lateral color as well and you're right, it is typically an off axis aberration.  However, a poorly put together lens system or one that is well out of collimation may show it on axis.  Trouble is, other aberrations like coma (difficult to see in DPAC) and astigmatism (detectable in DPAC if it's bad enough as a clocking of the the lines through focus) may be riding on top of the lateral color as well.  Still it, if it's bad enough, you might be able to catch it out maybe as extra intense color fringing of the lines with a white LED.  But that's speculation on my part.

 

This is where star testing really compliments DPAC.

 

Jeff

Edited by Jeff B, 23 February 2020 - 09:52 AM.

[peleuba]

This is where star testing really compliments DPAC.

  

[punk35]

Jeff, thanks for doing these DPAC threads. I find them very interesting. If I could find someone close to me that does DPAC I’d love to have my scope done. I think the views are quite good, and would be interested to see what the test would show. Someone would have to explain the results to me though.  
Thanks again.

[SteveGR]

Jeff, thanks for doing these DPAC threads. I find them very interesting. If I could find someone close to me that does DPAC I’d love to have my scope done. I think the views are quite good, and would be interested to see what the test would show. Someone would have to explain the results to me though.  
Thanks again.

It doesn't look that hard to do yourself, actually.  You don't even really need a flat, you can make one with a pan of oil and test the scope in a vertical position, although I think you can find decent deals on used flats if you are patient.  I'm going to make one myself eventually, but unfortunately it ranks below a lot of home related items on the "to do" list.

Edited by SteveGR, 24 February 2020 - 04:42 PM.

[punk35]

It doesn't look that hard to do yourself, actually.  You don't even really need a flat, you can make one with a pan of oil and test the scope in a vertical position, although I think you can find decent deals on used flats if you are patient.  I'm going to make one myself eventually, but unfortunately it ranks below a lot of home related items on the "to do" list.

I agree it doesn’t seem to difficult, but for me that’s a project for down the road.

[fate187]

Oh I am interested to do one on my own. Beside the pal of oil...

How to you make one? I even ordered a mirror from Thor labs. But the results are miserable. I do only get good dpac results using a ERF energy filter from baader. But that is only 110mm and has a coating...

Edited by fate187, 24 February 2020 - 05:41 PM.

[peleuba]

It doesn't look that hard to do yourself, actually.  You don't even really need a flat, you can make one with a pan of oil and test the scope in a vertical position, although I think you can find decent deals on used flats if you are patient.  I'm going to make one myself eventually, but unfortunately it ranks below a lot of home related items on the "to do" list.

 

I tried the oil flat method and it worked but was a pain.  The telescope needs to be suspended directly above the oil pointed downward.  The best way to accomplish this is to place the telescope on a mount and tripod like you do for observing but instead of pointing up, you point it down directly at the pan of oil on the floor.  For me, it was much easier to obtain a flat, build a test stand to support the flat and V-blocks to support the telescope.   

 

Like anything, Autocollimation testing takes practice.  And I would not rely on it as the sole means/single arbiter of the goodness of an optic.  Because, like any test method, there pluses and minuses.  I always feel better when conducting Autocollimation in conjunction with a star test.  However, a telescope that performs well in double pass will be a good performer outside.  But to me, you really do need an unrelated test (the star test) to corroborate what you see in DPAC.

 

Here is a thread that was started to help others learn how to test their refractors using Double Pass Autocllimation.

 

https://www.cloudyni...ac#entry9987401

Edited by peleuba, 25 February 2020 - 11:05 AM.