15 replies to this topic

[Scott in NC]

My son found a Meade ETX-60 for a bargain price at a McKay's used book/media/electronics store on July 4th, and since the weather wasn't good enough to use it outside that night, we decided to DPAC test it. This is a very fast 60mm f/5.8 doublet achromat on a go-to mount, and was probably made in the early 2000s. I know that my Meade LX-90 8" SCT from 2002 was built in Irvine, CA, but I'm not sure whether this scope was made in the U.S. or China.  

 

Here's the scope set up on the optical bench for testing:

 

[Scott in NC]

We had some challenges with the testing, and the first issue was that I didn't have a Ronchi eyepiece that would not only fit into the scope's 1.25" diagonal but also have a short enough barrel to make up for the scope's limited amount of back focus.  This scope has a built-in diagonal with a flip mirror, so that a 1.25" eyepiece can be placed in the diagonal, or alternatively a camera (with the correct adapter, which we didn't have) can be placed directly into the back of the scope for use in straight-through configuration. And we found that the threaded opening on the back of the scope was less than 1.25" in diameter, so my 2" to 1.25" step-down adapter didn't fit.  The best I could do was hold my 2" Ronchi eyepiece up to the visual back with one hand and use my other hand to hold a camera.  This may have led to some alignment error, but I think that the alignment error was likely quite minimal as I was able to obtain round images rather than oblong images after multiple attempts to take the pictures.

 

The other issue was that my son had to adjust the focuser while my two hands were otherwise occupied, so I can't guarantee that the images that I believe were intra-focal and extra-focal were exactly the way that I labeled them here.

 

We attempted to redo the images just to make sure that we had taken the images correctly but at that point the flip mirror became dislocated from its plastic backing, and that halted any further testing efforts.  This issue required some minor surgery to repair. My son performed that repair later in the night, regluing the mirror to its backing, and then we allowed the scope to sit for two days to make sure that the glue had adequate time to dry.  And then I got busy with work, so we didn't have time to redo the testing process. Hopefully we'll be able to try that again later in the coming week.

[Scott in NC]

Now on to the images.

 

Green inside / outside focus:

 

 

Red inside / outside focus:

 

 

Blue inside / outside focus:

 

 

White inside / at / outside focus:

 

[Scott in NC]

I can't fully explain why the Ronchi bands are much straighter near the top and more curved at the bottom, and as I said above, I can't completely rule out the possibility of some degree of alignment error when performing the test. I'm wondering if the straight upper portion reflects decent spherical correction, and the more curved lower portion reflects a large zone in that area. Can you have differing levels of spherical correction in different parts of a lens? To answer my own question, I guess that's what a zone does after all, changes the amount of spherical correction in a specific part of a lens. But maybe someone who knows more about optics than I do can comment on this.

 

The white light images show that this is definitely an achromat rather than an apochromat, but I was actually expecting the degree of chromatic aberration to be much worse, given its fast focal ratio of f/5.8. The scope's small aperture (60mm) likely explains why CA wasn't too awfully bad here.

 

That's all that I have time to post now, and maybe later in the week my son and I can retest this scope to see if we can get some better images, or at least to confirm our prior results. I'll definitely welcome any comments that anyone has to offer!

[SandyHouTex]

Actually it doesn't look all that bad except for the bowing.  If it can be collimated out I would say it's a pretty nice scope optically.

[jgraham]

I have always enjoyed the ETX refractors for their wide, binocular like field of view. Excellent for star-hopping the Milky Way. They pair nicely with the narrower field of the ETX Maks.

Neat stuff.

[Scott in NC]

After my son repaired the issue with the scope's internal flip-mirror and we let the scope sit for a few days (both to allow the glue to set properly as well as to allow me to finally get off work after a long weekend), we repeated the testing on the Meade ETX-60 today. Plus my son did some coding to create a program to help me automate the image processing and put it in a format that's a little easier to view here on CN.  Sorry the blue images are so dark. I'll try to rectify that with the next round of scope testing.

 

[Scott in NC]

I'm still a little puzzled as to the significance of the rightward curvature of the bottom part of the Ronchi bands in the outside focus images, especially since the bands all curve in the same direction. Perhaps that may be indicative of coma or astigmatism? I'll have to do a star test one night, as that may help to solve the mystery. 

[Scott in NC]

And as if that wasn't enough, my son found another sample of a Meade ETX-60 today at the same store. This one was even less expensive than the first one, but the flip mirror appears to be working smoothly without issues, plus the scope came with all the original eyepieces and paperwork.  And of course we had to test this one out too!

 

 

 

 

[Scott in NC]

I took these last images in a little different manner, slightly further inside and outside of focus to show 4 bands instead of the usual 3 bands that I'll typically show. This helped to illustrate the central zone better. I think that if it weren't for that zone, the spherical correction would look quite a bit better.  I was pleased to see that the Ronchi bands didn't exhibit the rightward curvature across the bottom that we saw with the last scope, so maybe this second sample of an ETX-60 isn't plagued by the same astigmatism (or coma, or whatever it is) that the first sample was. But hopefully I'll be able to sort all that out with an actual star test.

[davidc135]

I'm still a little puzzled as to the significance of the rightward curvature of the bottom part of the Ronchi bands in the outside focus images, especially since the bands all curve in the same direction. Perhaps that may be indicative of coma or astigmatism? I'll have to do a star test one night, as that may help to solve the mystery. 

Firstly, I thought it was coma which could be caused by uneven air spacing (is it air-spaced) but I would have thought that the direction of curves should flip either side of focus, so ?

 

David

[Thomas_M44]

My son found a Meade ETX-60 for a bargain price at a McKay's used book/media/electronics store on July 4th, and since the weather wasn't good enough to use it outside that night, we decided to DPAC test it. This is a very fast 60mm f/5.8 doublet achromat on a go-to mount, and was probably made in the early 2000s. I know that my Meade LX-90 8" SCT from 2002 was built in Irvine, CA, but I'm not sure whether this scope was made in the U.S. or China.  

 

Here's the scope set up on the optical bench for testing:

 

IMG_5918.jpeg

You are a patient man, and a good Dad to take such time to spend with your son on a learning project such as this.

 

Although this is a rather poor telescope, you are making the most of an opportunity to transform the situation into a sharing experience which builds understanding and nurtures the curiosity of a young mind.

 

Kudos 

Edited by Thomas_M44, 09 July 2024 - 02:30 PM.

[Scott in NC]

Firstly, I thought it was coma which could be caused by uneven air spacing (is it air-spaced) but I would have thought that the direction of curves should flip either side of focus, so ?

 

David

Yes, and that was what puzzled me.

[BRCoz]

After my son repaired the issue with the scope's internal flip-mirror and we let the scope sit for a few days (both to allow the glue to set properly as well as to allow me to finally get off work after a long weekend), we repeated the testing on the Meade ETX-60 today. Plus my son did some coding to create a program to help me automate the image processing and put it in a format that's a little easier to view here on CN.  Sorry the blue images are so dark. I'll try to rectify that with the next round of scope testing.

 

Meade ETX-60 #1.png

I have to hand it to your son.  If he likes coding great.  I tried it back in the early 80s and it was not for me.  I was able to program, I liked being a computer operator. I wish him the best in what ever he decides to do.  

[RichA]

I can't fully explain why the Ronchi bands are much straighter near the top and more curved at the bottom, and as I said above, I can't completely rule out the possibility of some degree of alignment error when performing the test. I'm wondering if the straight upper portion reflects decent spherical correction, and the more curved lower portion reflects a large zone in that area. Can you have differing levels of spherical correction in different parts of a lens? To answer my own question, I guess that's what a zone does after all, changes the amount of spherical correction in a specific part of a lens. But maybe someone who knows more about optics than I do can comment on this.

 

The white light images show that this is definitely an achromat rather than an apochromat, but I was actually expecting the degree of chromatic aberration to be much worse, given its fast focal ratio of f/5.8. The scope's small aperture (60mm) likely explains why CA wasn't too awfully bad here.

 

That's all that I have time to post now, and maybe later in the week my son and I can retest this scope to see if we can get some better images, or at least to confirm our prior results. I'll definitely welcome any comments that anyone has to offer!

Regarding the band differences.  I wonder what would happen if you rotated one element 90 degrees?

[Scott in NC]

Yes, that would be an interesting experiment. I haven’t attempted to disassemble the lens cell. I haven’t looked at it too closely either, but suspect that it may be difficult since the lens cell is part of the focusing mechanism. Unlike most refractors that move the eyepiece end to focus, on this scope the focus knob moves the objective in and out.