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Best way for a novice to "test a mirror"

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#1 sparksinspace

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Posted 17 August 2019 - 05:18 AM

Hi,

I'm fairly  new to everything and have lurked around the ATM forums and am fairly "handy" with mechanical things but have completely no idea on optical systems.

 

I'm a tinkerer at heart and love to take things apart and back together again. 

 

I have a 10Inch "saxon" Read Synta/ commercial Chinese  dob.

 

Being curious I would love to know what my primary and secondary mirror look like.

Also being "pinched in the Bottom end. Read  "tight arse". I don't want to send it away some were to get it tested. Also I don't want to buy expensive equipment just to scratch my curiosity itch.

I just want some sort of reference to what aberrations I see to what it is, and what it could be if it is figured better or mechanically set up differently. Does that make sense?  

 

What does one need to build, Study and learn to be able to test both the shape of the primary and also the flatness of the secondary?

 

I've seen some posts of fairly simple sort of rigs (knife edge) and I assume computer programs that look at a image taken.

 

If someone wants to dumb it down so your average "aussie bogan". Read village idiot. can lean a little more or direct me to a place to lean that would be great.

 

Cheers


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#2 Pinbout

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Posted 17 August 2019 - 05:37 AM

1st star test it

 

https://youtu.be/QxUQJjjsdW4

 

 

use an eyepiece same fl as the scope f- ratio

 

6mm for a f6 scope that gives a 1mm exit pupil and is good for star testing

 

no Barlows allowed.

 

Here’s a not good but no too bad mirror 

 

https://youtu.be/MFmFpuST67M

 

heres a horrible one 

 

https://youtu.be/tAD-2r6gW9A


Edited by Pinbout, 17 August 2019 - 05:38 AM.

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#3 TOMDEY

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Posted 17 August 2019 - 07:25 AM

1st star test it

https://youtu.be/QxUQJjjsdW4

use an eyepiece same fl as the scope f- ratio

6mm for a f6 scope that gives a 1mm exit pupil and is good for star testing

no Barlows allowed.

Here’s a not good but no too bad mirror 

https://youtu.be/MFmFpuST67M

heres a horrible one 

https://youtu.be/tAD-2r6gW9A

Nice! Clearly shows what otherwise takes forever to try to put into words!  I also like your eyep efl mm ~scope F#, for visual scrutiny properly scales the impulse-response. [for very large scopes, one can/should back off on that. because atmospheric seeing precludes ever resolving that]    Tom


Edited by TOMDEY, 17 August 2019 - 07:30 AM.

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#4 sparksinspace

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Posted 17 August 2019 - 07:27 AM

1st star test it

 

https://youtu.be/QxUQJjjsdW4

 

 

use an eyepiece same fl as the scope f- ratio

 

6mm for a f6 scope that gives a 1mm exit pupil and is good for star testing

 

no Barlows allowed.

 

Here’s a not good but no too bad mirror 

 

https://youtu.be/MFmFpuST67M

 

heres a horrible one 

 

https://youtu.be/tAD-2r6gW9A

Quick and dirty before the clouds rolled in. I just pulled it out of the shed foud a star (no idea of what it was) focused with one shim  then tested as per your video

 

Ive got a 254/1200

so F/ 4.7

 

I've got a cheap cheap 4mm aspheric that I don't use as its unusable as far as I'm concerned but hopefully will be ok for the star test.

 

So here they are.

 

Inside

 

inside1.jpg

 

 

Outside

outside1.jpg

 

Looks like its slightly over corrected if I understand your comments in you video post. Does this mean the curve of the mirror is steeper than it should be?

 

not sure what is going on with the lines on the photo of the outside shot? I think it was a fingerprint on the camera lens? unless something bad is happening to my mirror.

 

Thoughts? and what's next? 


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#5 Pinbout

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Posted 17 August 2019 - 07:54 AM

Very nice pics btw

 

It’s alittle too far from focus but yes overcorrection, but you cheap arse eyepiece maybe adding to it. Also cooling will add correction also.

 

you can get a inexpensive screen from ronchiscreens.com and after you position a star in the eyepiece 20mm, take out the eyepiece and lay the screen over the empty focuser 

 

https://youtu.be/6W3-rJpGAuY

 

not too bad tho.

 

if you like that’s a nice speed of mirror to turn into an ultra light

 

https://youtu.be/G3xu8CNBe0k


Edited by Pinbout, 17 August 2019 - 07:57 AM.

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#6 Pinbout

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Posted 17 August 2019 - 07:56 AM

Nice! Clearly shows what otherwise takes forever to try to put into words!  I also like your eyep efl mm ~scope F#, for visual scrutiny properly scales the impulse-response. [for very large scopes, one can/should back off on that. because atmospheric seeing precludes ever resolving that]    Tom

Got that scale thing from groski - attending Delmarva mirror making seminar several years. Such a shame they stopped. crazy.gif



#7 sparksinspace

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Posted 17 August 2019 - 08:14 AM

Very nice pics btw

 

It’s alittle too far from focus but yes overcorrection, but you cheap arse eyepiece maybe adding to it. Also cooling will add correction also.

 

you can get a inexpensive screen from ronchiscreens.com and after you position a star in the eyepiece 20mm, take out the eyepiece and lay the screen over the empty focuser 

 

https://youtu.be/6W3-rJpGAuY

 

not too bad tho.

 

if you like that’s a nice speed of mirror to turn into an ultra light

 

https://youtu.be/G3xu8CNBe0k

Thanks.

I'll Let you know when I get the screen and take a pic 

 

If your ever looking for the wicked witch of the west, Before you tap your ruby slippers for home, look us up. I might just have a head full of straw but I'll shout you a beer!  waytogo.gif 


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#8 Jimmy462

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Posted 17 August 2019 - 10:18 AM

Quick and dirty before the clouds rolled in. I just pulled it out of the shed foud a star (no idea of what it was) focused with one shim  then tested as per your video

 

Ive got a 254/1200

so F/ 4.7

 

I've got a cheap cheap 4mm aspheric that I don't use as its unusable as far as I'm concerned but hopefully will be ok for the star test.

 

So here they are.

 

Inside

 

attachicon.gif inside1.jpg

 

 

Outside

attachicon.gif outside1.jpg

 

Looks like its slightly over corrected if I understand your comments in you video post. Does this mean the curve of the mirror is steeper than it should be?

 

not sure what is going on with the lines on the photo of the outside shot? I think it was a fingerprint on the camera lens? unless something bad is happening to my mirror.

 

Thoughts? and what's next? 

 

My preliminary assessment is that there's nothing too egregious going on here with your optics, though a more fine-grained series of images going through focus (both in and out) would yeid a much better assessment...

 

Star testing telescope optical quality:

https://www.telescop...g_telescope.htm

 

...but, for what you've provided, your optics look to be providing a fairly balanced look both inside and outside of focus. Personally, I'd not sweat the slight over-correction that's showing up, but, again, I'd be curious to see 1/16th-, 1/32nd-inch and in-focus images to be better able to make a more accurate and informative assessment. Beyond that, collimation seems a bit off, but that might be an artifact of the test star not being centered in the scope for those images.

 

I wholeheartedly agree that a series of tests with ambient optics and a better quality ocular would help with attaining better data and provide more informative results. (Any chance you can provide/post a short video going back-and-forth through focus ±1/8th-inch? It'd be helpful here.)

 

Where to next? ...Mel Bartels recommends performing the test with a "high power eyepiece giving 1-2mm exit pupil back and forth about an eight of an inch [3mm]" (Danny's shim method), which, for your f/4.5 Newtonian would be in the 4.5-to-9mm range respectively (no need to sweat exactitude here). He also provides an informative guide for conducting this test while also providing a good primer on what to "look out for" in both the testing and assessing one's results...

 

Star Testing a Telescope:

http://www.bbastrode...tarTesting.html

 

...and then there's this easy-to-digest and informative PDF with lots of diagnostic computer-generated patterns from the kind folks at BackyardAstronomy...

 

Appendix A-Testing - Appendix A-Testing.pdf:

https://www.backyard...x A-Testing.pdf

 

...knowing "what one shouldn't be seeing" is as important as knowing "what one should be seeing". :)


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#9 gr5org

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Posted 17 August 2019 - 12:24 PM

You need to let your mirror adjust to air temp a long time (potentially hours depending how thick it is and temp difference).

 

A star test is a fantastic test and I'd just stick with that but if you want to test the primary mirror itself (where most of the error is likely to come from) then you could get a bath interferometer for under $150.  There are instructional videos and such.



#10 MKV

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Posted 17 August 2019 - 04:08 PM

A star test is a fantastic test and I'd just stick with that but if you want to test the primary mirror itself (where most of the error is likely to come from) then you could get a bath interferometer for under $150.  

For a novice, a star test with a Ronchi eyepiece  -- nothing to interpret or measure other than how straight the bands are -- is more like it. Starting a novice with an interferometer may be just a tad bit too ambitious.


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#11 BGRE

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Posted 17 August 2019 - 05:57 PM

A star test isn't devoid of interpretation.



#12 dave brock

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Posted 17 August 2019 - 09:00 PM

It's clear when he says  "a star test with a Ronchi eyepiece" he means doing a Ronchi test in the scope

on a star as opposed to an actual star test. 


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#13 Asbytec

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Posted 17 August 2019 - 10:44 PM

Did you use a cell phone camera to take the images? If so, congratulations for actually getting an image. That's not easy. In my experience, the simple camera lens can alter what is actually seen. How close are your images to what you actually saw? How does the image appear in focus, relatively sharp and high contrast? Is there something that gave you cause or simply curiosity? The reason I ask is because in focus images matter, and if you're not happy with it. If you are, great, it's just an exercise in curiosity. 

 

I trust your at the same "shim" distance on either side of focus. For over correction, the shadow outside will be a bit larger than inside. You seem to show this, but only if you are very nearly the same distance on both sides of focus. However, the geometric size of the outside image is larger suggesting under correction. This happens because the edge comes to focus well before best focus and diverges more as you move outside focus providing an overall larger image. The edge defines the size of the pattern we see. 

 

So, a few things can be happening in terms of spherical aberration. First, you could be a bit further outside focus in which case both the secondary shadow and the geometric size of the pattern will be larger as shown. If the outside image is a little smaller being closer to focus, both the shadow and the size will be more inline with your inside image showing better correction. So, ensuring both images are close to the same defocus is important.

 

Secondly, I'd like to see the ring patterns more clearly looking for a brighter central ring or edge ring on either side of focus. Use both the shadow and the ring brightness to confirm over or under correction. If the inner diffraction ring emerges from the shadow and it is brighter inside focus, that would confirm over correction generally. With pure over correction, the edge ring will be brighter outside focus and the geometric size of the pattern will be a little smaller with a larger shadow. That presumes a smooth correction error over the entire mirror surface, which may or may not be the case.

 

So, third, you could have a mix of correction error where the edge is steeper (under corrected) producing a larger geometric pattern outside focus and the center is over corrected showing a larger shadow outside focus. If your images are at the same defocus, this is what is indicated. This larger shadow happens because, as Suiter says, the energy at the center evacuates allowing the shadow to show up a little earlier and grow as we defocus outward. This is because the center comes to focus before best focus and diverges more outside focus allowing the shadow to emerge. So, it's possible to have a larger outside image and a larger shadow as you show. The star test becomes more difficult to interpret because we may not have a smooth correction error. 

 

I'd like to see the edge and central ring brightness more clearly in conjunction with the shadow size to get a better feel for what's going on. In your images above, it does appear as if the central ring inside focus is a little brighter suggesting over correction (center focusing short and edge focusing long) as well as a hint of an edge ring outside focus possibly being brighter. The brightness of the ring is suggestive of how far you are from it's focal point. You can use relative ring brightness to kind of estimate where your edge and center are coming to focus. The brighter the ring, the closer you are to it's focal point. For example, if you're inside focus and the inner ring bordering the shadow is brighter, you are closer to the central focus point suggesting it is focusing short of best focus. 

 

So, with over correction, inside focus you are closer to the central zone focus (focusing short) thus the central ring will be a little brighter. When outside focus, you are closer to the edge focal point (as it is focusing long) and the edge ring will be brighter (and the central ring relatively dimmer). But, at the same defocus distance, geometry matters, too. Also with over correction, inside focus and further from the edge focal point, the size of the pattern will be larger than outside focus because of convergence and divergence of the edge. You show just the opposite suggesting some under correction, at least at the edge (and hopefully no turned edge, which is a form of over correction at the edge). So, I see conflicting signals above. 

 

I'm taken aback a little by the amount of space between the inner shadow and the edge of the pattern. There is room for more than one diffraction ring between the shadow and the edge suggesting you may be several waves further from focus than what we might expect. (Either that or the simple camera lens is "correcting" your image into something else, something different than what you see visually with the shim). Ideally, we want to see a bight inner ring bordering the shadow and a bright outer ring at the edge with one diffraction ring between them. If you see anything close to that, you're fine. That's pretty much perfect if the shadows are about the same size, too.

 

In my opinion, if the inner ring is broken out on both sides not too far from focus, that's a good sign. You want to see this inner ring not too far from focus on both sides, and probably by the amount of defocus provided by the shim. This suggests a more equal distribution of energy across the defocused pattern (the edge and the center have the roughly the same energy) because all zones are focusing close enough together and likely to offer better than 1/4 PV SA (as a reference point). The longer it takes the central ring to emerge, the less the energy distribution is equally distributed across the defocused pattern suggesting greater correction error. So, IMO, if it emerges early on both sides of focus, you're probably fine. How fine is anyone's guess. (Obstruction size matters, too, how large is your obstruction)?

 

If you can get cleaner images, that'd be great. To me, the images provided are a bit difficult to interpret other than the shadow size. As mentioned above, on the shadow alone, I see nothing egregious either. At 1/4 PV SA, the smaller shadow should be about 0.7x the larger shadow (according to Suiter's graphic). You might be near that eyeballing it. But, don't forget to check for astigmatism in focus and slightly out of focus. Look closely for a slight eccentricity of the defocused image being perpendicular on either side of focus and a + pattern in focus (if seeing allows). 

 

Classic over correction: http://www.astrogene..._45ft_f10_c.jpg

 

Sorry for the length...just having morning coffee. smile.gif


Edited by Asbytec, 18 August 2019 - 06:47 AM.

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#14 BGRE

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Posted 18 August 2019 - 12:44 AM

It's clear when he says  "a star test with a Ronchi eyepiece" he means doing a Ronchi test in the scope

on a star as opposed to an actual star test. 

The last time I did a Ronchi grating interferometer measurement with a real star the fringes looked like moving wavy lines, and an exposure of 30 seconds or more was required to average out most of the turbulence. Thus either a smallish aperture combined with good seeing are required for naked eye use or a camera to allow averaging is required.



#15 Asbytec

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Posted 18 August 2019 - 12:56 AM

For the Ronchi test on a  star, how many lines per mm on the screen should be sufficient to show spherical aberration down to, say, 1/8th wave? And how many lines should be visible across the star's image, 3 to 5 lines? On a star at infinity, the lines should be pretty much straight. Yes? 


Edited by Asbytec, 18 August 2019 - 12:58 AM.

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#16 BGRE

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Posted 18 August 2019 - 02:50 AM

For a novice, a star test with a Ronchi eyepiece  -- nothing to interpret or measure other than how straight the bands are -- is more like it. Starting a novice with an interferometer may be just a tad bit too ambitious.

Like it or not, the Physics is inescapable, a Ronchi test is an interferometric measurement.

Its an achromatic lateral shearing grating interferometer which is sensitive to the slope of the wavefront error (in the low shear approximation typically applicable to the gratings used by amateurs). The lateral shear is a result of diffraction by the grating. The effective wavelength is d/2*F# where d is the distance between rulings.

The geometric description is merely a (usually) convenient fiction that fails to predict some of the effects seen (such as Talbot imaging when the grating also covers the source) and to some extent conceals how the wavefront error can be calculated from a suitable set of Ronchigrams (actually interferograms). As interferometers go its relatively insensitive for the typical gratings used by amateurs and combined with its common path nature makes it very stable.



#17 BGRE

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Posted 18 August 2019 - 03:19 AM

For the Ronchi test on a  star, how many lines per mm on the screen should be sufficient to show spherical aberration down to, say, 1/8th wave? And how many lines should be visible across the star's image, 3 to 5 lines? On a star at infinity, the lines should be pretty much straight. Yes? 

It depends on the F# of the telescope.

Its actually much easier to detect for the same grating, 1/8 wave of 4th order spherical aberration with a large F# than a small F#.

Ronchi_1.GIF

Ronchi_2.GIF

NB alpha (the wavefront error slope) is measured per semidiameter (D/2)

 

IIRC 0.125 wave of 4th order SA corresponds to a maximum slope of 0.5 waves/semidiameter (or 0.25 micron/semidiameter  for a wavelength of 0.5 microns) at the edge

with F# =8 and d = 191 microns (133 lpi grating) the  effective wavelength is ~ 11.9 microns and the corresponding phase shift is ~0.132 radians or 2% of band spacing (6.283.... radians).


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#18 MKV

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Posted 18 August 2019 - 03:37 AM

For the Ronchi test on a  star, how many lines per mm on the screen should be sufficient to show spherical aberration down to, say, 1/8th wave? And how many lines should be visible across the star's image, 3 to 5 lines? On a star at infinity, the lines should be pretty much straight. Yes? 

Norme, you'll at least need 10 lines/mm (250 lpi) grating, 12 l/mm (300 lpi) is better. The usual 5.24 l/mm (133 lpi) is not sensitive enough.

 

Here are two simulations (based on raytracing) at prime focus, with 1/8 wave minimum rms OPD residual (you can call it  the "best focus") in green (0.55 μm wavelength wavelength. One is a 6-inch f/8 with a conic of -0.540 and the other is a 10-inch f/4 with a conic of -0.966.  

 

ronchi star test 0_125w.jpg

 

I am not sure how much the diffraction effects will play a role. I have never used a 300 lpi screen on stars, even though it's a very forgiving test. Theoretically 10 l/mm or higher should detect 1/8 wave. I may look into Dale Eason's DFTFRinge Ronchi simulation and see what his sim results give. They usually correlate well with what's observed.

 

Good luck guys!

 

Mladen


Edited by MKV, 18 August 2019 - 05:59 AM.

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#19 dave brock

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Posted 18 August 2019 - 04:38 AM

The last time I did a Ronchi grating interferometer measurement with a real star the fringes looked like moving wavy lines, and an exposure of 30 seconds or more was required to average out most of the turbulence. Thus either a smallish aperture combined with good seeing are required for naked eye use or a camera to allow averaging is required.


How many times have you actually tried? If you got out with a telescope occasionally then you would know that seeing conditions good enough for testing with a Ronchi screen are not rare at all. In Hamilton at least. In fact I don't recall ever seeing moving wavy lines and I've done a lot of testing.

Edited by dave brock, 18 August 2019 - 04:59 AM.

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#20 Pinbout

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Posted 18 August 2019 - 06:45 AM

For the Ronchi test on a  star, how many lines per mm on the screen should be sufficient to show spherical aberration down to, say, 1/8th wave? And how many lines should be visible across the star's image, 3 to 5 lines? On a star at infinity, the lines should be pretty much straight. Yes? 

You can get to 1 line but 4 is ok 

 

if it’s worse than 1/4~ you’ll see it with 133 lpi screen

 

im not saying it’s a 1/4~ test just sayin if it’s worse it will be obvious


Edited by Pinbout, 18 August 2019 - 06:46 AM.

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#21 BGRE

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Posted 18 August 2019 - 06:45 AM

If one has a drive that tracks well then an exposure of a minute or so works well. Drift scanning along the direction of the rulings should also be effective. 

 

If the aperture of the telescope exceeds the Kolmogorov coherence length ( also known as Fried parameter) (the coherence length depends on the seeing , 10cm corresponds to seeing of 1 arc sec) then the image of a star breaks up into a speckle like pattern that moves about randomly with each speckle moving respect to each other the corresponding result for the Ronchi test is a writhing series of snake like Ronchi bands. The speckle like image for a bright star is obvious if viewed with sufficient magnification.

 

24InchRonchigram1.jpg

 

Exposure time is several seconds with 150 lpi grating at ~F13 or thereabouts, aperture was 61cm

Star was Sirius

 

Should have some Ronchigrams taken with higher spatial frequency grating on an older machine.

The writhing of the Ronchi bands was much more pronounced for the higher spatial frequency gratings.

 

the seeing was likely somewhat worse than 1 arcsec (the corresponding coherence length smaller than 10cm. ) 


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#22 Pinbout

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Posted 18 August 2019 - 06:48 AM

Way too many lines and zoney as all $&@&


Edited by Pinbout, 18 August 2019 - 06:49 AM.


#23 Asbytec

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Posted 18 August 2019 - 07:09 AM

Way too many lines and zoney as all $&@&

 

(...10cm corresponds to seeing of 1 arc sec) then the image of a star breaks up into a speckle like pattern that moves about randomly with each speckle moving respect to each other the corresponding result for the Ronchi test is a writhing series of snake like Ronchi bands. The speckle like image for a bright star is obvious if viewed with sufficient magnification.

 

poke.gif


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#24 dave brock

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Posted 18 August 2019 - 07:20 AM

Way too many lines and zoney as all $&@&


Yep. That's the local Society's 24" Cass.
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#25 BGRE

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Posted 18 August 2019 - 08:04 AM

Way too many lines and zoney as all $&@&

Nonsense.

Fewer lines lose the detail I was looking for to confirm other measures using an interferometric Hartmann test which also shows the effect of air currents hugging the metal baffle tubes.

 

The telescope in question was known to have a poor wavefront (only to be expected given some descriptions of how the primary was figured).




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