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Crayford focuser for Cats

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#26 mvas

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Posted 17 February 2018 - 10:58 AM

 

I think it is extremely important for us to discuss how we can get the best images possible from our SCT's.

We know that it is best to keep the image near the Back Focus as specified by Celestron.

The engineers at Celestron have performed the "hard" calculations for us.

 

The optimal Back Focus for the Best Image is simply:

  • 5.25" = EdgeHD 800
  • 5.75" = EdgeHD 925
  • 5.75" = EdgeHD 1100
  • 5.75" = EdgeHD 1400

 

Significant deviation from the above will degrade your image.

From: https://www.innovati...ocus-tolerance/

Conclusions
In a nutshell the tolerance (offset from the NBF distance) is tighter for good seeing and a large chip (diagonal), as we would expect.
The table below summarizes the maximum tolerances (rounded values) for a 10% spot diameter (encircled energy) increase v.s the diffraction and/or scope optical limits:

APS-C chip Full frame chip
1″ FWHM seeing +/-  20 mm +/- 20 mm*
2″ FWHM seeing +/- 50 mm +/- 20 mm

 

Those graphs clearly show there is one, and only one, Optimal Back Focus distance - at exactly "0.0 mm Offset from NBF" as shown in the top 2 graphs.

But let's be clear ... the graphs are the Worst Case location on each chip ( APS-C & FF) - in the extreme outer (4) corners only.

FF       =  21   mm off-axis from the optical center line ( upper curve )

APS-C = 13.5mm off-axis from the optical center line ( lower curve )

None of those graphs indicate how much better the spot size actually is near the center of the imaging chip.

Cropping can remove much of the outer edge error.

And if the "seeing" is random noise then stacking many images may reduce the bad effects due to the atmosphere.

 

It would be great to see a 3rd line in those top 2 graphs showing the the error at location 0.0 mm off-axis ( ie the center of the imaging chip ), too.

This 3rd line would certainly be below ( better than ) both blue curves.


Edited by mvas, 17 February 2018 - 11:12 AM.


#27 mvas

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Posted 17 February 2018 - 11:01 AM

 

 

Here's some reading for you:

 

"Strictly talking, commercial SCT using primary mirror focusing are misaligned every time the two mirrors are not at a single optimum separation."

 

"The relation implies that, for nominal primary mirror shift ∆, error induced by extending back focus is independent of aperture. It only depends on secondary magnification and primary's focal ratio. For F1=2, m=5, and K2=0, every mm of reduction in mirror separation (∆=1), or nearly for every inch of focus extension, induces ~1/23 wave P-V wavefront error of over-correction, and as much of under-correction for widening the separation."

 

http://www.telescope...cs.net/SCT2.htm

 

if the focuser moves the focal plane 3 inches from the optimal location,  it would add about 1/8th wave of overcorrection. 

 

Jon

 

I tried plugging in their example numbers ( as given in your text above ) into their P-V Wave-Front Error formula ...

 

W = (1 - m^2 ) x ∆ / ( 512 x m^3 x FRpri^4 )

 

But I did not get the same answer of ~1/23 P-V Wave-Front Error, as stated in their text.

 

W = ( 1 - 5^2 ) x 1 / ( 512 x 5^3 x 2^4 ) 

W = 24 / 1,024,000

W = 1 / 42,667

 

I thought, "W" represented the P-V Wave-Front Error.

So, how does ( W = 1/42,667 ) convert to ~1/23 P-V Wave-Front Error?

 

I added the parentheses around the denominator to indicate order of operation.

 

In the above, I made the assumptions ( which I think was their intentions ) ...

a) Everything to the left of the divide symbol was the numerator

b) Everything to the right of the divide symbol was the denominator

 

Or computing from left to right  ...

W = (1 - m^2 ) x ∆ /  512 x m^3 x FRpri^4

W = 24 x 1 / 512 x 125 x 16

W = 48,000 / 512

W = 93 Huh?

 

 

 

That would be something to discuss with Vlad.  I think its well established that there is one optimal back focus for an SCT and deviating from it causes spherical aberration.

 

My goal here was just to point this out so those adding a Crawford would be aware of it.  

 

https://www.cloudyni...primary-on-sct/

 

jon

 

I agree with you goal.

I have sent an email to vlad regarding his formula, his example values and his result of "1/23 Wave Error", that does not compute.

No reply ...



#28 oldtimer

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Posted 21 February 2018 - 08:01 AM

SOOOOOOOOOOOOOO   How does all of this apply to MAKs????



#29 mvas

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Posted 21 February 2018 - 08:47 AM

SOOOOOOOOOOOOOO   How does all of this apply to MAKs????

Schmidt Cassegrain   = Corrector + Concave Primary Mirror + Convex Secondary Mirror

Maksutov Cassegrain = Corrector + Concave Primary Mirror + Convex Secondary Mirror



#30 jwheel

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Posted 21 February 2018 - 10:09 AM

I have a moonlite crayford on my C-14 and it works great!



#31 Spikey131

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Posted 21 February 2018 - 10:38 AM

SOOOOOOOOOOOOOO How does all of this apply to MAKs????

It does, but it all depends upon the design of the individual optics, and how far behind the scope the optimal focal plane lies. The discussion above focused (pun : ) on standard SCTs and their specific design.

Edited by Spikey131, 21 February 2018 - 11:43 AM.


#32 oldtimer

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Posted 21 February 2018 - 11:46 AM

Since all 150 Maks (with the exception of Explore Scientific)  are F12 or 1800mm FL, is there any info on these with regard to optimum distance behind backplate?



#33 junomike

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Posted 21 February 2018 - 08:53 PM

 

SOOOOOOOOOOOOOO   How does all of this apply to MAKs????

Schmidt Cassegrain   = Corrector + Concave Primary Mirror + Convex Secondary Mirror

Maksutov Cassegrain = Corrector + Concave Primary Mirror + Convex Secondary Mirror

 

I've always referred to the Mak-Cass as having a Meniscus?


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#34 mvas

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Posted 22 February 2018 - 12:03 AM

Schmidt Cassegrains have a Schmidt type Corrector Plate

Maksutov Cassegrians have a Meniscus type Corrector Plate

 

What you call the Corrector Plate is irrelevant to this discussion.

 

BTW, you missed the point.

I was making the piont that both telescopes ( MAK and Schmidt-Cass ) have very similar designs.

And since the two telescopes have very similar designs,

then any deviation from the optimal back-focus point will cause aberrations in the MAK, too.



#35 dotnet

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Posted 22 February 2018 - 05:22 AM

I was making the piont that both telescopes ( MAK and Schmidt-Cass ) have very similar designs.

And since the two telescopes have very similar designs,

then any deviation from the optimal back-focus point will cause aberrations in the MAK, too.

Yes, but aren't we polishing asymptotes here? How are the poor Newtonian owners supposed to feel? Their scopes can't bring any off-axis rays to focus. But do they care? No. Because it doesn't matter most of the time. And when it does, they just turn their Newts into catadioptric systems ("Paracorr"), giving them similar problems of geometry to worry about that other catadioptric scope users have.


Edited by dotnet, 22 February 2018 - 05:22 AM.


#36 junomike

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Posted 22 February 2018 - 06:31 PM

Schmidt Cassegrains have a Schmidt type Corrector Plate

Maksutov Cassegrians have a Meniscus type Corrector Plate

 

What you call the Corrector Plate is irrelevant to this discussion.

 

BTW, you missed the point.

I was making the piont that both telescopes ( MAK and Schmidt-Cass ) have very similar designs.

And since the two telescopes have very similar designs,

then any deviation from the optimal back-focus point will cause aberrations in the MAK, too.

Agreed



#37 Volans

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Posted 23 February 2018 - 12:53 PM

Thanks Jon n others for discussing this issue. I was wondering though, is it better to get a slightly undercorrected scope so we can use the external focuser? (Apologize if this question sounds silly 😀)

#38 oldtimer

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Posted 24 February 2018 - 06:55 PM

Since I seem to spend a lot more time looking at my 150 Mak then through it, I went ahead and attached my Feathertouch focuser to the back. It sure is pretty.lol.gif 



#39 prio

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Posted 16 June 2018 - 10:35 PM

I tried plugging in their example numbers ( as given in your text above ) into their P-V Wave-Front Error formula ...

 

W = (1 - m^2 ) x ∆ / ( 512 x m^3 x FRpri^4 )

 

But I did not get the same answer of ~1/23 P-V Wave-Front Error, as stated in their text.

 

W = ( 1 - 5^2 ) x 1 / ( 512 x 5^3 x 2^4 ) 

W = 24 / 1,024,000

W = 1 / 42,667

 

I thought, "W" represented the P-V Wave-Front Error.

So, how does ( W = 1/42,667 ) convert to ~1/23 P-V Wave-Front Error?

 

I added the parentheses around the denominator to indicate order of operation.

 

In the above, I made the assumptions ( which I think was their intentions ) ...

a) Everything to the left of the divide symbol was the numerator

b) Everything to the right of the divide symbol was the denominator

 

Or computing from left to right  ...

W = (1 - m^2 ) x ∆ /  512 x m^3 x FRpri^4

W = 24 x 1 / 512 x 125 x 16

W = 48,000 / 512

W = 93 Huh?

I know this is an old thread but I think I know where the confusion with the equation is coming from...

 

It looks like the equation gives the error in millimeters.  If you assume a wavelength of 550 nm = 0.00055 mm you get (1/42667) / 0.00055 = 0.0426 or about 1/23.

 

Phil


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#40 Edd Weninger

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Posted 17 June 2018 - 03:09 PM

I've been using a Feathertouch focuser on my Nexstar 11 for a long time.  I can't really say it caused any notable degradation when I installed it.  However, due to this thread, I decided to see what effect this install had on the optimal back focus.  The primary to secondary distance is set to a number 16.5" which I found somewhere years ago.

 

Here's an accurate scale drawing in AutoCad. 

 

It shows the A-P Maxbright diagonal racked in all the way.  The F2 and F10 cones are accurate I believe.  I puts the focal plane at 7.2 inches and it is still short of the Nagler 22 field stop.  

 

It is also interesting if you consider moving the primary forward, it looks like there will be vignetting from the secondary baffle (I don't have accuratedimensions for the baffle as I loaned the scope to a friend).  

 

Anyway, interesting stuff.  If anyone knows for certain the optimal primary to secondary spacing, let me know.  Thx,

 

NX11 Feathertouch.JPG




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