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Crayford focuser for Cats

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#1 oldtimer

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Posted 11 February 2018 - 02:33 PM

I lot of folks (including myself) hate the mirror slop when focusing their Cat.  For many (again including me) the solution is to add a Crayford focuser. However recently I have been told that 'dragging' the focus point so far back to accommodate the focuser compromise  image quality. Is this true? If so how bad is it?



#2 StarsAbove

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Posted 11 February 2018 - 03:04 PM

You don't have to "drag the focus point back" much. You still achieve main focus with the focusing knob on the Cat, which is of course farther back some. But that is not going to cause a reduction in image quality. A deep focus like that is one of the strengths of SCTs. Then you simply use the coarse and/or fine focus on the Crayford to bring things to a final focus. After that you only have to use the Crayford. The only downside I found is that, depending on the length of the Crayford, you may run into the base of the SCT when pointed at the zenith. Some models may be shorter than others so you may want to remember that when shopping for one!


Edited by StarsAbove, 11 February 2018 - 03:09 PM.


#3 carolinaskies

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Posted 11 February 2018 - 10:51 PM

Because the primary mirror of an SCT is what moves to achieve Focus the use of an external focuser allows you to maintain a specific focal length. Some people get very anal about having a specific focal length match to a specific pixel array and that is likely where such erroneous comments arise. Weather the focal point is at 2000 mm or 2250 mm the only thing that changes is the image scale not the image quality. If you always shoot at the same image scale regardless of where you're pointing in the sky it really doesn't matter. On the other hand if you want everything was shot at such a spec in doing a mosaic then having a variable focal length would be a problem. It would not be a matter of quality but rather a matter of mismatched scale if you are trying to stitch together multiple images

#4 oldtimer

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Posted 12 February 2018 - 06:49 AM

It now dawns on me that the movement of the focal point a few inches involves a very little movement of the primary mirror due to the ampilfication factor of the secondary mirror.



#5 Jon Isaacs

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Posted 12 February 2018 - 06:59 AM

SCTs do have an optimal backfocus for minimal spherical aberration.  

 

Jon


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#6 carolinaskies

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Posted 12 February 2018 - 10:35 AM

SCTs do have an optimal backfocus for minimal spherical aberration.  

 

Jon

Spherical aberration (non-convergence of light paths) is corrected via the corrector plate on SCTs.  Even though the primary mirror can move toward and away from that corrector as it focuses the convergence of light path still remains consistent, what is changing is the focal ratio as the primary moves closer to the secondary.   

The point where convergence occurs, the actual finite point of focus, shifts toward or away from the back of the telescope.    So what's all this 'optimal backfocus' talk?  Well it goes to the fact that telescopes are sold with a certain 'specifications'.  In the case of an SCT or Mak you don't say variable focal ratio, you pick a number and publish the backfocus figure for that focal ratio.  So is this number a magical and optimal backfocus point to reduce spherical aberration? 

Nope,  it is realistically the point which allows the telescope equipped with standard accessories to have focus travel!   So if you've screwed on the SCT visual back, put the 1.25" diagonal in there, and placed the 25mm Plossl supplied with the telescope you can find focus pretty easy somewhere around the F10 f/ratio.  And if you happen to put a longer or shorter 1.25" 50*FOV Plossl in it's place you'll find focus within a couple turns. 

Now if you decide to use some exotic combination will it come in to focus?  Not necessarily.  That's why we have extension tubes and barlows.   



#7 Napp

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Posted 12 February 2018 - 10:50 AM

I have an 8” Meade LX200-ACF.  I hated focusing with the object image moving around.  I added a MoonLite focuser and the world changed.  I do an initial focus with the scope’s focuser and lock the mirror.  Then I use the MoonLite.  Now I can easily achieve a fine focus with no image movement.  IMO image quality is better because I achieve a much crisper focus.    One night at an outreach session I had forgotten my diagonal.  I borrowed one but it screwed directly to the scope so I couldn’t use the MoonLite.  I realized that night just how much the MoonLite had improved my observing sessions.  FYI, I have an equatorial mount so no obstruction issues.


Edited by Napp, 12 February 2018 - 10:59 AM.

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#8 Jon Isaacs

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Posted 13 February 2018 - 06:52 AM

Spherical aberration (non-convergence of light paths) is corrected via the corrector plate on SCTs.  Even though the primary mirror can move toward and away from that corrector as it focuses the convergence of light path still remains consistent, what is changing is the focal ratio as the primary moves closer to the secondary.

 

 

There is only one position of the primary mirror where an SCT is fully corrected for spherical aberration.  I believe for a C-8, this when its focused 100 mm behind the rear port. 

 

Jon


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#9 carolinaskies

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Posted 13 February 2018 - 11:51 AM

 

Spherical aberration (non-convergence of light paths) is corrected via the corrector plate on SCTs.  Even though the primary mirror can move toward and away from that corrector as it focuses the convergence of light path still remains consistent, what is changing is the focal ratio as the primary moves closer to the secondary.

 

 

There is only one position of the primary mirror where an SCT is fully corrected for spherical aberration.  I believe for a C-8, this when its focused 100 mm behind the rear port. 

 

Jon

 

Do you understand how absurd that statement is? 

If that statement were true any SCT without a fixed primary mirror would be useless without an external focuser and they would not have the long storied history of success for visual and photographic use.   Thousands of observatories, private and professional would never choose such an instrument.   Thousands of users of Celestron Fastar/Hyperstar equipped telescopes would have to use only very specific cameras to get successful images that didn't have such aberration.   

Recall that the primary is SPHERICAL?  That means it has a defined curve.  A corrector is shaped to that curvature and spaced to be effective at the central point of the curve.  Effectively this give the telescope correction through a specific range along that curve from that central point of the curve towards the outer edge and from that center point towards the hole in the center of the mirror itself.   This design allows the mirror to be moved toward and away from the corrector as it achieves focus without inducing spherical aberration. 
  





    

 



#10 Rob McKenna

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Posted 13 February 2018 - 02:07 PM

 

 

Spherical aberration (non-convergence of light paths) is corrected via the corrector plate on SCTs.  Even though the primary mirror can move toward and away from that corrector as it focuses the convergence of light path still remains consistent, what is changing is the focal ratio as the primary moves closer to the secondary.

 

 

There is only one position of the primary mirror where an SCT is fully corrected for spherical aberration.  I believe for a C-8, this when its focused 100 mm behind the rear port. 

 

Jon

 

Do you understand how absurd that statement is? 

If that statement were true any SCT without a fixed primary mirror would be useless without an external focuser and they would not have the long storied history of success for visual and photographic use.   Thousands of observatories, private and professional would never choose such an instrument.   Thousands of users of Celestron Fastar/Hyperstar equipped telescopes would have to use only very specific cameras to get successful images that didn't have such aberration.   

Recall that the primary is SPHERICAL?  That means it has a defined curve.  A corrector is shaped to that curvature and spaced to be effective at the central point of the curve.  Effectively this give the telescope correction through a specific range along that curve from that central point of the curve towards the outer edge and from that center point towards the hole in the center of the mirror itself.   This design allows the mirror to be moved toward and away from the corrector as it achieves focus without inducing spherical aberration. 
  



Hi.

 

A complete non expert here. 

Just curious here but why does Celestron go to so much trouble to point out, to within

0.5mm, the ideal back focus distance for their Edge HD range?

    

 

 


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#11 Jon Isaacs

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Posted 13 February 2018 - 02:39 PM

Do you understand how absurd that statement is?

 

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


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#12 George Methvin

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Posted 13 February 2018 - 05:00 PM

Wow all those year I used a crayford focuser on my SCT I never noticed any degrade in the views or images I think you all might be over thinking this or I may not know what a good view or image really is or lol as usually I am talking about something I know nothing about which is what I seem to do best so I need to just shut up lol.  Where is my bottle of Jack Daniels it has all the answers. 


Edited by George Methvin, 13 February 2018 - 05:13 PM.


#13 Cyberpunk

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Posted 13 February 2018 - 05:36 PM

Where is my bottle of Jack Daniels it has all the answers.


Lol. Double lol. So true though. I've learned much about life in general out of a bottle. Triple lol.

#14 mvas

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Posted 13 February 2018 - 06:18 PM

Example

=======

 

FTF Low Profile Crayford Focuser - adds 1.4" back focus

Extend Tube 50% = adds 0.4" back focus

FTF Adapter - adds 0.45" back focus

Total Back Focus 2.25" = 1.4" + 0.4" + 0.45"

 

Use lowest profile Refractor Style Prism Diagonal and slide it in all the way.

 

Your Primary will be approx 2mm Forward of the "theoretical optimal focus" position.

 

The bottom line is ...

Can you actually see a difference? 

Is the ease of focus worth the additional wavefront error?

 

How many millimeters do you push the SCT Primary Mirror forward when you add a Focal Reducer for astrophotography?

The image is still acceptable, right?

 

How many millimeters do you move the SCT Primary Mirror between the lowest power 2" eyepiece to the highest power 1.25" eyepiece?

Image is still acceptable, right?

 

Few can say, "My Primary Mirror is always exactly at the theoretical optimal focal point position".



#15 mvas

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Posted 13 February 2018 - 06:20 PM

 

 

Hi.

 

A complete non expert here. 

Just curious here but why does Celestron go to so much trouble to point out, to within

0.5mm, the ideal back focus distance for their Edge HD range?    

 

 

Because the Celestron EDGE has a set of lenses inside the main baffle that work best when the image is focused at a specific back-focus.

 

Some focal reducers have a very specific back-focus specification for optimal image.


Edited by mvas, 13 February 2018 - 06:23 PM.


#16 carolinaskies

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Posted 13 February 2018 - 09:44 PM

 

 

 

Hi.

 

A complete non expert here. 

Just curious here but why does Celestron go to so much trouble to point out, to within

0.5mm, the ideal back focus distance for their Edge HD range?    

 

 

Because the Celestron EDGE has a set of lenses inside the main baffle that work best when the image is focused at a specific back-focus.

 

Some focal reducers have a very specific back-focus specification for optimal image.

 

 

 

Do you understand how absurd that statement is?

 

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

 

The link is heavy reading and quite a bit of formulas.  However if you read the entire section on SCTs you'll find the author dictates the commercial SCT designs essentially cancel out the under/over-correctness issues.  Further, SCTs of the traditional sizes we tend to use from 8"-16" are optimized more towards focus at infinity.  And if you've never taken the focuser out of an SCT you should.  The actual travel distance of refocusing the primary at infinity falls at a very short range indeed.  

While you say 'if the focuser moves the focal plane 3 inches from the optimal location,  it would add about 1/8th wave of overcorrection"  in actuality the movement would induce something like 1/16th wave overcorrection in one direction and 1/16th wave undercorrection in the other.  Yet that prime focus point is much nearer infinity with a standard 1.25" accessory setup(visualback/diagonal/Plossl) than away from it for SCTs from 8"-11"(And 2" accessories for 12-16"), Therefore the actual supposed spherical aberration you suppose would be negligible either side of the magic number.  Further, due to manufacturing tolerances the actual perfect focal point will vary due from telescope to telescope.   I would suggest that both Meade and Celestron likely test for an optimal range when matching corrector/primary and why correctors are indexed to the primary from the factory within a tolerance to achieve focus.  

And finally, there is one consideration not mentioned in the esoteric information on telescopes in the link.... the effect of seeing condition on focus point.  All the equations are great for a mathematical model, but the reality is in practical use seeing dictates the point of best focus.  Even for the Edge telescopes there is a range set for what they call the "Nominal back focus" based on seeing.  That nominal back focus in fact it different based on which type of chip is being used for imaging (APS-C or Full frame).   

In conclusion, as I stated earlier, the idea that there is a single point of perfect correction is absurd to the real world.  Whether visually or from the standpoint of use for imaging, there is no magic point.  The relative optical considerations of error are so small in the short range of focus at infinity.  And seeing is going to have a greater effect on result than a defined point of backfocus.  
 



#17 mvas

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Posted 14 February 2018 - 01:09 AM

The Celestron documentation states,

"...

In a telescope designed for imaging, users expect to attach heavy filter wheels, digital SLRs, and astronomical CCD cameras.

We designed the rear threads of the EdgeHD 925, 1100, and 1400 telescopes with a heavy-duty 3.290 × 16 tpi thread,

and we set the back focus distance to a generous 5.75 inches from the flat rear surface of the baffle tube locking nut.

The rear thread on the EdgeHD 800 remains the standard 2.00 × 24 tpi, and the back-focus distance is 5.25 inches.

..."

 

Best Focus - There is some specific distance between the Primary Mirror & Secondary Mirror that minimizes the error at the image plane

and therefore there is one Best Back-Focus distance.

 

 

Given that the Imaging Plane is essentially flat, I do not understand why only you, but not Celestron make this claim:

"... The optimal back-focus point changes based upon whether you attach an APC-C or Full Frame Sensor? ..."

HOW ?

 

 

Which Celestron document supports your claim "... what they call the "Nominal back focus" based on seeing ..." ?

WHERE ?

 

 

Regarding improper spacing between the Primary Mirror and the Secondary Mirror to increase back focus, the author actual states:

"...

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

Depending on the accessories used, required shift from the optimum focus can be more or less than that.

Since moving the primary only changes the aberration contribution of the secondary,

it is the error differential at the secondary determining the resulting system error.

The resulting system error is, in effect, the despace error for two mirror system.

For spherical secondary (K2=0) it is from Eq. 91.3 given as:

W = ( 1 - m^2 ) ∆ / 512 x m^3 x F^4

as the P-V Wavefront error at the best focus, where m is the secondary magnification and F the primary focal ratio ƒl/D.

.."

which is very different than your interpretation.

 

The additional P-W Wavefront Error is dependent upon "∆", which is the additional distance the Primary is shifted away the from the Optimal / Best Focus Point.

 

Again, my question is at what "∆" ( change from optimal Pri-Sec separation ) will this additional wavefront error be visible?


Edited by mvas, 14 February 2018 - 01:18 AM.

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#18 Jon Isaacs

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Posted 14 February 2018 - 01:15 AM

However if you read the entire section on SCTs you'll find the author dictates the commercial SCT designs essentially cancel out the under/over-correctness issues.  Further, SCTs of the traditional sizes we tend to use from 8"-16" are optimized more towards focus at infinity.

 

This is only the case if the mirrors are at the proper spacing.  If the mirror separation is shifted by 1mm, it amounts to 1/23rd wave added over correction.

 

While you say 'if the focuser moves the focal plane 3 inches from the optimal location,  it would add about 1/8th wave of overcorrection"  in actuality the movement would induce something like 1/16th wave overcorrection in one direction and 1/16th wave undercorrection in the other.

 

1/23rd wave per inch of shift from the optimal backfocus.  3inches x 1/23 x wave/inch = 3/23 wave ~ 1//8 wave over correction.

 

And finally, there is one consideration not mentioned in the esoteric information on telescopes in the link.... the effect of seeing condition on focus point.

 

The seeing does not affect the focal point.  The image is at infinity.  It affects the sharpness of the image.

 

In conclusion, as I stated earlier, the idea that there is a single point of perfect correction is absurd to the real world.  Whether visually or from the standpoint of use for imaging, there is no magic point.  The relative optical considerations of error are so small in the short range of focus at infinity.  And seeing is going to have a greater effect on result than a defined point of backfocus.

 

 

I think you have not understood the optical realities.  There is an optimal location of the focal plane.  If it is shifted sufficiently, which it can be if one adds significantly to the optical train with focusers and other devices, it does affect the spherical correction of the mirror.

 

I only bring this subject up so that Old Timer and others considering adding a focuser, understand the possible issues.  

 

Jon



#19 Axunator

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Posted 14 February 2018 - 01:16 AM

There is only one position of the primary mirror where an SCT is fully corrected for spherical aberration.  I believe for a C-8, this when its focused 100 mm behind the rear port. 

Jon is correct of course. There are three reasons why SCTs with their moving primaries still work so well in the real world: 1) Despite theoretical (i.e. based on ray tracing) worsening of image quality with non-optimal back focus distances, the on-axis image remains diffraction-limited (i.e. with spot sizes smaller than Airy disc) over quite a range of back focus distances, 2) our imperfect seeing, and 3) our imperfect eyes.

 

Having said that, if one wants to tease out the maximal available performance out of his/her SCT, it is worthwhile to realize that too much back focus achieved by focusing with the primary does indeed worsen the performance in a measurable manner. Now whether you or I can see it in real-life situation is another matter, and I'm not gonna start to argue on that. However, to give an idea how much back focus is too much, here are some figures from Ken Hutchinson's ray tracing analysis (originally published in 2007, unfortunately I don't know if it's available online anymore - I got a copy from Eddgie, thanks waytogo.gif):

 

In a C8:

- increasing back focus to more than 150 mm (i.e. >50 mm more than the optimal 100 mm) starts to reduce the effective aperture. It does this by vignetting the peripheral rays, which means that not only the aperture progressively drops to less than 8 inches, but the relative central obstruction increases as well (because vignetting only affects outer rim of entrance pupil, leaving secondary shadow as large as it is, thereby increasing the relative diameter of the secondary obscuration in relation to entrance pupil).

- increasing back focus to more than 300 mm (i.e. >200 mm more than the optimal 100 mm) means that field is no longer diffraction limited anywhere (including on axis) because of spherical aberration.This assumes that the optical figure of the scope is "perfect" - if the real scope specimen has optical imperfections (they all do, as we know), the tolerable additional back focus is obviously less than this, because of cumulative damage to the wavefront.

 

Using a focal reducer diminishes these design tolerances dramatically, and the sensitivity for back focus increase is much more severe.

 

I am not suggesting that you cannot use whatever accessories with your SCT you like. But at least you should be aware that there may be consequences to image quality - that you may or may not notice in real life. Would you buy a scope for high magnification use that you know is not diffraction limited, even on axis? Would you buy a scope that is advertised to be an 8 inch scope (with dimensions and weight of an 8 inch scope) that is actually a 7 inch scope. I wouldn't...

 

I like my C9.25, and use it with accessories that put my typical eyepiece focal planes slightly further back than optimal back focus distance (i.e. Baader ClickLock visual back and 2" mirror diagonal). But before doing that I made sure that the back focus does not creep so far that the scope no longer performs as diffraction limited or starts to vignette. The only "penalty" is somewhat increased focal length (compared to nominal).

 

(EDIT: Edited the reason for increased relative central obstruction associated with vignetting caused by excessive back focus - I realized that focal length increase is not needed as part of the explanation) 


Edited by Axunator, 15 February 2018 - 01:12 AM.

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#20 George Methvin

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Posted 14 February 2018 - 03:53 AM

Man my head hurts now I am not sure if I should use a two inch diagonal or a 1.25 diagonal for sure a crayford focusers is out of the question or just not use a diagonal at all and just look straight thought the scope though that would be hard on ones neck lol. Ok I am just going to stop using SCT they are just to much dam trouble and go with a good refractor,wait do I use a diagonal with the refractor or not OK forget that I am going with a reflector or binoculars yea that the ticket, I need to stop reading these forums lol they get me all confused I was so happy being ignorant wait I still am LOL, don't over think things it will just make your head hurt. Its 2:30 I am still drinking Jack and coke and watching Dr Who, funny sound like a show about owls that time travel lol ok time for bed. Clear skies to all now If I can just find my bed LOL.


Edited by George Methvin, 14 February 2018 - 04:07 AM.


#21 mvas

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Posted 14 February 2018 - 08:37 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.


Edited by mvas, 14 February 2018 - 08:38 AM.


#22 mvas

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Posted 14 February 2018 - 08:53 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?


Edited by mvas, 14 February 2018 - 10:29 AM.


#23 Paul J

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Posted 15 February 2018 - 01:40 PM

I added a Baader Steeltrack and Baader 2” clicklock diagonal to my 8” Edge. Did all the research on how the additional backfocus would affect the Edge optics. In the end I went ahead with adding the accessories. I believe after carrying out some back of a fag packet calculations that the scope is now closer to F12 than its nominal F10 rating. For what it’s worth, the views are still fantastic, and having the convenience of the Crayford focuser just makes life so much easier:-)

Edited by Paul J, 15 February 2018 - 01:40 PM.

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#24 carolinaskies

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Posted 16 February 2018 - 02:53 PM

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


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#25 Jon Isaacs

Jon Isaacs

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Posted 17 February 2018 - 10:51 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




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