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ETX125 (vintage 2000) star test (so how good/bad is it?)

collimation Maksutov optics
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#1 cytan299

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Posted 21 July 2019 - 02:42 PM

Hi guys,

   I've spent quite a bit of time (days upon days with both an indoor fake star, and outside feeding mosquitoes smile.gif ) collimating my US made, ETX125 (circa Jan 10, 2000) after modifying it with a Wegat backplate. See: https://www.cloudyni...-etc/?hl=+wegat

 

The setup is ETX125+2x Barlow + ASI120MC. This is the defocused diffraction ring which I took of Polaris:

 

Capture_00002_WithDisplayStretch_cropped.jpg

 

which doesn't look too bad. But it is well known that the quality of the collimation shouldn't be looked at so far out of focus, especially for Maksutov's. So I made a movie of going from inside-focus to outside-focus. From aberrator, I think the range of defocus is +/- 1.2 wavelengths, but I could be wrong. See below. This was taken with Vega which was overhead on 19 Jul 2019. Seeing is as good as I've seen around here. Here's the youtube movie:

 

https://youtu.be/Ie9j0Z9MNC8

 

Again, doesn't look too bad.

 

I've picked out a few frames from the movie just for illustration:

 

Frame 149 (inside focus)

C_22_39_30_F_00000149.jpg

 

Frame 226 (inside focus)

C_22_39_30_F_00000226.jpg

 

Frame 461 (at focus)

C_22_39_30_F_00000461.jpg

 

Frame 794 (outside focus)

C_22_39_30_F_00000794.jpg

 

Frame 911 (outside focus)

C_22_39_30_F_00000911.jpg

 

The collimation looks very close. Here's the tri-bahtinov example looking at Vega:

 

tribahtinov.jpg

 

 

I tried using aberrator to see how much spherical error there is, but I couldn't quite reproduce the frames above:

 

aberrator_settings.jpg

 

I think the settings are not quite right. Any help here is appreciated.

 

My question: From the presented data, 

  1. Do you think the collimation is good? 
  2. How is the quality of the optics of this vintage etx125?
  3. Can a Strehl number of P-V number be extracted from the data?

Any other observations and criticisms appreciated. I've always heard that the vintage etx125 optics are good, hopefully this will show how good/bad from this one example.

 

Thanks!

 

cytan


Edited by cytan299, 21 July 2019 - 02:44 PM.


#2 Richard O'Neill

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Posted 21 July 2019 - 03:31 PM

Collimation looks pretty good but the images do show a small amount of astigmatism as the image passes through focus..



#3 Eddgie

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Posted 21 July 2019 - 03:39 PM

At this amount of defocus, higher order spherical aberration (which is inherent in the design) will cause issues.

 

I recommend 10 waves inside and outside.   20 To 30 waves for zones but I have not seen zones much in small MCTs.



#4 cytan299

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Posted 21 July 2019 - 03:57 PM

At this amount of defocus, higher order spherical aberration (which is inherent in the design) will cause issues.

 

I recommend 10 waves inside and outside.   20 To 30 waves for zones but I have not seen zones much in small MCTs.

Can you please have a look at the aberrator settings above. Are they correct? I’d like to know how 10 waves look like.

 

Thanks!

 

cytan


Edited by cytan299, 21 July 2019 - 03:57 PM.


#5 cytan299

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Posted 21 July 2019 - 04:01 PM

Collimation looks pretty good but the images do show a small amount of astigmatism as the image passes through focus..

Yes, looks a little oval going through focus ...

 

cytan


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#6 Eddgie

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Posted 21 July 2019 - 04:19 PM

Well, the settings don't make sense to me. Your obstruction setting appears to be set up for percentage of secondary shading, but it should be set for the percentage of obstruction by diameter, which on the ETX 125 is probably something like 33% to 38%.

 

Next, use the slider in the top left of the window to move to 10 waves and click the Check Mark to refresh. 

 

10 waves.jpg

 

 

 

The thing though is that you can't go by the size of the pattern to find 10 waves, you have to go by the distance inside or outside of focus.

 

For an f/15 scope, as I recall, that will be about 6.7mm of defocus inside, and 6.7mm outside.  Now I am not positive that this is the number but it seems about right.

 

The most important thing is to be exactly the same distance inside as outside.

 

Here is the method I use on a scope like this, where you can't measure it at a focuser tube. 

 

Bring the scope to perfect focus.  Now, using your measuring device, slip the eyepiece out 6.7mm (or as close as you can get).  Look at your pattern or take your image.

 

Now, without moving the eyepiece, focus the telescope using the focuser for best focus.  Once you are in best focus, now loosen the eyepiece and and slide it in, and now take your second image or make your observation.  

 

What is most critical is not having exactly 6.7mm of defocus but rather ensuring that whatever distance you can use is exactly the same on either side.  If it is 6.5mm, then this method ensures that it is exactly 6.5mm on both sides.   That is the most important thing, not the exact amount, but the amount used is exactly the same in and out.

 

This is because in the presence of SA, the diameter of the pattern will be bigger on one side than on the other, so if you used different amounts of defocus, you are using different wavelengths of defocus and this will make the test give a false result. 

 

 


Edited by Eddgie, 21 July 2019 - 04:25 PM.

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#7 cytan299

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Posted 21 July 2019 - 05:23 PM

Hi Eddgie,

   You're right! The obstruction should be the ratio of diameters:

 

    diameter of telescope = 127 mm.

    diameters of obstruction = 39.4 mm

 

   Therefore obstruction is 39.4/127 = 31%

 

 

 

  I believe the obstruction of 9.6% is correct. Please see last line of the ETX125 specs:

 

track.jpg

 

Here's a check:

 

radius of telescope Rt = 127/2.0 = 63.5 mm

radius of obstruction Ro = 39.4/2.0 = 19.7 mm

 

Area of telescope = pi*(Rt)^2 = 12668 mm^2

Area of obstruction = pi*(Ro)^2 = 1219 mm^2

 

% of obstruction = Area of obstruction/Area of telescope = 1219/12668 = 0.096 = 9.6%

 

Is this not right?

 

 

Setting the focuser to the same distance both sides of focus is not that difficult because I'm using a Crayford focuser. So with calipers, I can measure how far the focuser moves.

 

cytan


Edited by cytan299, 21 July 2019 - 09:58 PM.


#8 Okcman

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Posted 22 July 2019 - 02:34 PM

great information and timely since I am working on ....wait for it ... a Mead ETX 125 from around the year 2000!

 

More importantly after the mod - how is the views? Can you see the bands across Jupiter? - which to me a is a true  definition of contrast - with the new back - do you have the same

cooling issues prior to the mod ? or any cooling issues?


Edited by Okcman, 22 July 2019 - 02:36 PM.

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#9 Okcman

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Posted 22 July 2019 - 02:43 PM

THe reason I know it is from 2000 - it was a time capsule from the lady I bought it from , and it actually had the catalog from the year 1999 and the stuff he wanted circled on a order sheet - it came with 2 - identical 18 mm eypieces

I assume he was going to do a binocular setup.


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#10 cytan299

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Posted 22 July 2019 - 03:48 PM

great information and timely since I am working on ....wait for it ... a Mead ETX 125 from around the year 2000!

 

More importantly after the mod - how is the views? Can you see the bands across Jupiter? - which to me a is a true  definition of contrast - with the new back - do you have the same

cooling issues prior to the mod ? or any cooling issues?

You can judge for yourself. Yes, I can see bands on Jupiter but that's looking at the raw image from my ASI120MC camera. 

 

See following links for Jupiter (search for cytan):

 

https://www.cloudyni...less/?p=9484104

 

https://www.cloudyni...less/?p=9498207

 

https://www.cloudyni...less/?p=9514551

 

and Saturn:

 

https://www.cloudyni...less/?p=9498192

 

I've wrapped my scope with Reflectix,  

 

IMG_0816.jpeg

 

but since it's summer, the temperature differential isn't too great. I don't know whether Reflectix helps or hurts. At least from what I've seen so far, the defocused Polaris shot above was taken pretty soon after I set up. It did not show any heat plume.

 

cytan


Edited by cytan299, 22 July 2019 - 03:51 PM.


#11 cytan299

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Posted 27 July 2019 - 11:35 PM

Hi guys,

   Here's my winroddier results. I'm sure I havent' done it right, so I'm taking the 0.95 Strehl number with a super large grain of salt smile.gif . It is obvious to me that that there is spherical aberration because the inside and outside focus diffraction patterns are not the same size or intensity. I made sure that the inside and outside distances are the "same": 12 mm. The technique that I set the 12 mm is from that described by Edggie above. However, this may not be the right thing to do because the winroddier manual wants the size of the inside and outside images to be as close as possible.

 

winroddier.jpg

 

 

Anyway, anyone who actually knows what they are doing can try their hand at analyzing the data which can be downloaded here:

 

https://www.dropbox....5_data.zip?dl=0

 

 

Any help appreciated.

 

cytan


Edited by cytan299, 27 July 2019 - 11:36 PM.


#12 Jon Isaacs

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Posted 28 July 2019 - 04:55 AM

The original ETX 125's had a 40% CO. I had one. 

 

The secondary mirror obstruction might have been 31% but the secondary baffle was 40%.

 

Jon


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#13 cytan299

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Posted 28 July 2019 - 04:10 PM

Hi guys,

 

I did a quick calculation of the spherical aberration by measuring the size of the central obstruction. 

 

obstruction.jpg

 

From the above, I calculate from the ratio of the obstruction:

 

obs = 1.09/1.46 = 0.75 and thus the spherical wave error is 1-0.75 = 0.25 waves.  (see note below about f number of the etx125)

 

My guess is that the spherical wave error is between 0.2 to 0.25 waves. 

Here's the aberrator simulation:

 

aberrator_f16.5.jpg

 

which looks pretty close to what I measured.

 

Note: I'm assuming that I have 10 waves of defocus because I know that the defocus distance is exactly 12 mm because I used calipers. And using winroddier which calculates the expected size of the diffraction rings on screen, it says that in this experiment the ETX125 is an f/16.5 scope rather than a f/15 scope. I think this is because my camera is now spaced a lot further than the usual camera position because of the JMI Crayford focuser.

 

winroddier_f16.5_settings.jpg

 

The defocus distance of 10 waves comes from:

 

defocus distance of 10 waves = 8*10*(16.5)^2*0.00055 = 12 mm

 

So, is 0.25 waves of spherical error typical of Maksutovs? Perhaps this is to be expected, see 

  1. https://www.telescop...k_spherical.htm
  2. https://www.cloudyni...-cas/?p=3906492
  3. https://www.cloudyni...-cas/?p=3906492
  4. https://www.cloudyni...test/?p=9098444

 

cytan


Edited by cytan299, 28 July 2019 - 09:28 PM.


#14 cytan299

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Posted 10 August 2019 - 01:11 PM

Hi guys,

  I redid the star test (with Polaris) of my ETX125 and this time I made sure that the in-focus and out-focus size of the diffraction pattern at 15 waves is within 4 pixels as required by the winroddier manual. And the Strehl ratio is 0.958 which is, IMO, very good smile.gif (unless I messed up somewhere)

 

winroddier_results_small.jpg

 

Comments?

 

cytan


Edited by cytan299, 10 August 2019 - 01:27 PM.


#15 Vla

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Posted 11 August 2019 - 07:51 AM

Something is not adding up, because diffraction images clearly show the presence of spherical aberration, while the reconstructed wavefront and corresponding Zernikes don't (Zernikes are given in nm, directly as the RMS error, which is unusual and not agreeing with the notation used, which is for Zernike terms). Zernikes add up as the square root of the sum of their values squared, and the individual value for primary spherical is 0.54nm, practically non-existent.



#16 cytan299

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Posted 11 August 2019 - 08:07 AM

Something is not adding up, because diffraction images clearly show the presence of spherical aberration, while the reconstructed wavefront and corresponding Zernikes don't (Zernikes are given in nm, directly as the RMS error, which is unusual and not agreeing with the notation used, which is for Zernike terms). Zernikes add up as the square root of the sum of their values squared, and the individual value for primary spherical is 0.54nm, practically non-existent.

I agree that I find it strange myself. Any comments from people who have used winroddier? I’ve posted to the yahoo roddier group, but I think it might be dead.

 

cytan



#17 cytan299

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Posted 11 August 2019 - 01:17 PM

Hi guys,

 

Here's more info from winroddier:

 

winroddier_results_small.jpg

 

cytan



#18 starman876

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Posted 13 August 2019 - 05:29 PM

Looks like nice optics



#19 azure1961p

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Posted 14 August 2019 - 07:43 PM

Collimation looks pretty good but the images do show a small amount of astigmatism as the image passes through focus..

I thought the astigmatism was horrendous and not worth any further testing but yo get another scope.

 

Pete



#20 cytan299

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Posted 14 August 2019 - 10:03 PM

I thought the astigmatism was horrendous and not worth any further testing but yo get another scope.

 

Pete

Hi Pete,

  I believe all MCTs ~f/15 or higher have lower order astigmatism due to its design. See:

 

https://www.telescop...htm#astigmatism

 

Quote: "Thus, MCT lower-order astigmatism ranges from practically non-existent at ƒ/10 to low, visually inconsequential at ƒ/15"

 

and this (search for astigmatism)

 

http://www.weasner.c...ollimating.html

 

So, is the astigmatism really that horrendous for my scope?

 

cytan


Edited by cytan299, 15 August 2019 - 08:46 AM.


#21 cytan299

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Posted 02 September 2019 - 12:00 AM

Hi guys,

   Here's my first attempt at a DPAC test of my ETX125. I used a 133 lpi ronchi screen. Here's my setup:

 

setup_small.jpg

 

I used an "oil" flat (actually I used Dawn, the washing detergent, rather than oil, to stop from making a mess). Here's the outside focus photo:

 

outside.png

 

inside focus photo:

inside.png

 

Taking the photos of the interference diagrams was a real pain in the a*s. The photos show that the ETX125 isn't perfect, but IMO, isn't too terrible either. Comments? Is this consistent with the winroddier results? Strehl 0.95 to 0.96?

 

cytan


Edited by cytan299, 02 September 2019 - 12:01 AM.

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#22 cytan299

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Posted 02 September 2019 - 02:35 PM

Hi guys,

 

Here's better photos of inside and outside focus. I stopped using Dawn and just used water. For whatever reason, Dawn seems to contain more and more bubbles as time went on. My guess is that it's from the humidity. I gave up and used water instead -- which seems to work as well. I just had to be patient and wait for it to calm down.

 

Outside focus from two different runs:

 

outside3.png

 

outside4.png


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#23 cytan299

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Posted 02 September 2019 - 02:37 PM

And two inside focus photos:

 

inside4a.png

 

inside4.png


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#24 cytan299

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Posted 02 September 2019 - 04:13 PM

More data smile.gif. I used autostakkert to average out between 6 to 7 frames.

 

Inside focus:

 

inside5.jpg

 

outside focus:

 

outside5.jpg

 

cytan


Edited by cytan299, 02 September 2019 - 04:13 PM.

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#25 Boom

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Posted 02 September 2019 - 05:10 PM

Not too bad, not perfect. A small center zone, slight astigmatism and SA. What's interesting is how the DPAC and star test correspond to each other.

Is this a purple tube or blue tube ETX125?


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