44 replies to this topic

[Joe G]

I want to keep the testing of different scopes in separate threads.

 

So here is my WO FLT 132.  I used a fresh 100 LPI glass etched Ronchi screen.  The photos were taken with my Nikon D600 and a Sigma 50mm lens on a tripod with the mirror locked up before the shot was taken.  Exposure was 1/20s, F4, ISO 800.

 

My flat is an Edmund Optics 8" bought off Ebay.  It was owned by Litton Industries according to to a sticker on the side and it looks like it was re-tested over time.  Supposedly 1/10th wave which as Dave says is more than adequate.  The flat does have man micro scratches which are hard to see depending upon orientation.

 

So here are the results starting with green.

 

 

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[Joe G]

Here is Red:

 

 

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[Joe G]

Blue:

 

 

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[Joe G]

Finally White:

 

 

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[Joe G]

So the bands are fuzzy.  Maybe worse than my SW 150 from the other thread.  The surface seems rougher as well.

 

If that is correct, it makes sense as visually this scope does not seem to be to the standard of my SW 150.  The star test is not nearly as good, althoug it puts up fine views up to 200x.

 

Hopefully my test isn't too flawed.

 

So how would your rate this scope?  The bands are reasonably straight in all three colors.  It seems to me that there is no dramatic edge issues.  How does the surface roughness affect the quantitative measurements/guesstimates?

 

Does it seem this testing is in-line within reason to the price-point of this scope?

 

Thanks once again!

 

Joe

 

Edit:  I have a Tak FSQ 106 fluorite version.  I have a 2" diagonal for it.  I don't think I can test it without the diagonal?  Any suggestions welcome.

Edited by Joe G, 08 April 2023 - 02:29 PM.

[Scott in NC]

So the bands are fuzzy.  Maybe worse than my SW 150 from the other thread.  The surface seems rougher as well.

Since the bands look just as fuzzy with this scope as with your other one, I’m inclined to believe that there’s something about your equipment or technique that’s causing that.  Odds are against both of your scopes having he same degree of surface roughness.  There could be an issue with your flat, but I’m betting on a camera issue.

[Scott in NC]

I think your scope’s spherical correction is pretty good in green, but it’s hard to say with 100% certainty.  I suspect the curvature at the ends is likely an artifact.  If it was indicative of an edge issue, I’d expect to see curvature in opposite directions when testing on both sides of focus.  There appears to be very slight under/overcorrection in opposite signs for red/blue.  I don’t think we can really say much more with certainty without clearer pictures, and I’m sorry but I’m completely stumped as to why your images aren’t clearer.  But as I said, I don’t really think that that’s a reflection of any of the properties of your scope’s optics.

[Scott in NC]

I have a Tak FSQ 106 fluorite version.  I have a 2" diagonal for it.  I don't think I can test it without the diagonal?  Any suggestions welcome.

Why can’t you test it without a diagonal?  If it’s because you can’t rack the focuser out far enough to reach the focal plane, then you can use an extension tube in place of the diagonal.

[CHASLX200]

Pics don't seem sharp enough for me to tell what is up. Seems they are blown up too much.

[Joe G]

Yeah, I am stumped.  I have tried 2 different 2" square etched glass Ronchi screens.  I have tried the high resolution transparencies. I have tried my cobbled together Ronchi eyepiece and Moshen's eyepieces.

 

Now two different scopes.  Two different cameras.  Two different lenses.

 

Kind of leads to the flat.  It seems to test a flat you need another flat.  I am somewhat surprised that the flat could be that NOT flat given the initial Edmund rating. It seems the scratches probably could cause that.

 

I dunno.

 

I have my lens without the dew shield pressed against the flat.  The flat is just being supported by my kitchen counter, some masking tape on top and some dish towels on the side. The scopes are in their rings on dovetails.

 

If I understand things correctly I wouldn't get any bands if things weren't aligned. If it was test band astigmatism that would show differently than fuzzy bands,I assume.

 

If the flat was too un-flat would that cause the fuzziness?

 

I spent an evening with a friend comparing this scope to his Tak TOA 130.  Excellent seeing.  They were so close that I wanted to call it a draw and he thought the Tak just a tad bit better.  Likely we just used a 5mm Nagler which gives 185x.

 

Doh.

[Joe G]

Why can’t you test it without a diagonal?  If it’s because you can’t rack the focuser out far enough to reach the focal plane, then you can use an extension tube in place of the diagonal.

Well because the FSQ doesn't really have an attachment for two inch accessories.  You can view it straight through with 1.25" eyepieces, but not 2". 

 

I am not sure if Takahashi even made a diagonal for it.  I did buy one from Texas Nautical years ago.  I am not sure if they had some made or if it is indeed a Tak.

 

I also have the Extender Q for  it which increases the focal length.

 

I understand I could use an extension tube, but I think I might need to get some kind of adapter made to attach to the back of the scope.

[Scott in NC]

Well because the FSQ doesn't really have an attachment for two inch accessories.  You can view it straight through with 1.25" eyepieces, but not 2". 

 

I am not sure if Takahashi even made a diagonal for it.  I did buy one from Texas Nautical years ago.  I am not sure if they had some made or if it is indeed a Tak.

 

I also have the Extender Q for  it which increases the focal length.

 

I understand I could use an extension tube, but I think I might need to get some kind of adapter made to attach to the back of the scope.

How do you attach your 2" diagonal to it then?

[Scott in NC]

 

If I understand things correctly I wouldn't get any bands if things weren't aligned. If it was test band astigmatism that would show differently than fuzzy bands,I assume.

 

Correct on both counts.

[Scott in NC]

 

If the flat was too un-flat would that cause the fuzziness?

 

It's not so much the flatness per se, but more the smoothness (or lack thereof) of the surface of the flat.  I imagine if the surface were extremely scratched then that could lead to light scattering, which could account for your findings.

 

Would it be too much trouble for you to take a picture of your flat and post it here?

[Joe G]

It's not so much the flatness per se, but more the smoothness (or lack thereof) of the surface of the flat.  I imagine if the surface were extremely scratched then that could lead to light scattering, which could account for your findings.

 

Would it be too much trouble for you to take a picture of your flat and post it here?

This help.  Lots of these scratches hard to photograph.

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[Joe G]

How do you attach your 2" diagonal to it then?

It has these threads.  Might be 72mm.

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[Scott in NC]

This help.  Lots of these scratches hard to photograph.

Those scratches do look suspiciously like the fuzziness in your images. I’ll be interested to see what people who are more knowledgeable than I think, but I suspect we’ve found our culprit.

 

But don’t fret. Even if I’m correct, your flat is still very useful for detecting spherical aberration, even if not a helpful indicator of surface smoothness vs. roughness.

[Scott in NC]

It has these threads.  Might be 72mm.

Ah…now I understand.  So just use the diagonal in your testing and see what you get.  Jeff B. uses a diagonal in his testing, but has confirmed by testing both with and without that particular diagonal that it doesn’t contribute any significant visible aberrations to the test results.

 

If you notice aberrations in your testing with the diagonal in place, you won’t be able to say for sure whether those aberrations come from the diagonal or the scope.  If you notice no significant aberrations then you can either conclude that your scope is sensibly perfect or else that the diagonal has aberrations in the opposite direction, counteracting those of the scope.  But I don’t think that a mirror diagonal would be expected to contribute any spherical aberration to the image.  A prism diagonal might though.

[Joe G]

It seems logical what you say is correct.  A bummer if so, but I can't find any other explanation.  I emailed the seller who says he doesn't accept returns.

 

I wonder how much it costs to polish out the scratches.  But that likely must be as much as a brand new flat.

[Scott in NC]

It seems logical what you say is correct.  A bummer if so, but I can't find any other explanation.  I emailed the seller who says he doesn't accept returns.

 

I wonder how much it costs to polish out the scratches.  But that likely must be as much as a brand new flat.

Whether or not he “accepts” returns, if you paid with PayPal or a credit card, you might be able to contest this, especially if the flat turns out to be unacceptable for the purpose for which it was sold.  I guess it would matter how it was advertised.  If he said “as-is, can’t guarantee it will work, and no returns,” then you might not have a case.  But if something sold as a certified optical flat is totally unusable for the purpose for which one would buy such a flat (i.e., optical testing), then you might have a good chance of getting a refund.

[Joe G]

I did pay with Paypal on Ebay.  Have to see what other's here think about the scratches causing a significant amount of scatter to explain this.

[Scott in NC]

Yeah, definitely don’t just take my word for it. I think I’m correct, but really want to see what others think.  I’m just glad we’ve got a community here to bounce ideas off of each other.  I don’t know what I’d do without you guys!

[Scott in NC]

Well I've been advised that I'm apparently wrong, and the scratches on the flat might not be bad enough to cause the fuzziness in Joe's Ronchigrams.  So now I'm positively stumped. 

[Joe G]

Yep.  I think I will try to test my Tak FSQ106 soon.  Tak optics.  Can't get much better.

 

And I have two.  I almost 99% use them for AP.  But if the fringe clarity doesn't improve, well more trial and error testing.

 

Hopefully this process will make it easier to trouble shoot future DPACers.

 

[Kitfox]

Do yourself a favor, get a 2” diagonal in one of them and use it visually a little more than 1% of the time.  My EDX4 has become my grab and go on an SV M002C. A flat, well-corrected (understatement) flat field at f/5?  It is a truly amazing visual scope.  You get the feeling you are falling into a star field in the eyepiece. Immersive is a good description. 

 

And using it visually sadly puts the amazing f/3 reducer and a Nitecrawler into reserve status-and each of those is worth more than an average 4” scope 

 

 

Yep.  I think I will try to test my Tak FSQ106 soon.  Tak optics.  Can't get much better.

 

And I have two.  I almost 99% use them for AP.  But if the fringe clarity doesn't improve, well more trial and error testing.

 

Hopefully this process will make it easier to trouble shoot future DPACers.