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SCT Diagonal Used with Less Than Optimal Back Focus...

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#1 Dave Bush

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Posted 20 June 2019 - 01:28 PM

There are often questions asked (I've asked them) about the optical effects of exceeding at SCT's designed back focus. 

 

But, what about a setup that places the focal plane inside that ideal point?  Other than the obvious focal length changes are there any optical effects of doing so, any aberrations that are introduced or made worse?

 

This is asked in the context of an EdgeHD 8 used for visual purposes only with the f/7 reducer.  



#2 photoracer18

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Posted 20 June 2019 - 02:58 PM

As long as the eyepieces can reach the correct focal plane it should be OK. I have used 0.63x units in regular SCT visually for years off and on. Depending on the FL of the eyepiece you may or may not get a large secondary shadow so eye position is everything.



#3 Eddgie

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Posted 21 June 2019 - 11:23 AM

As long as the eyepieces can reach the correct focal plane it should be OK. I have used 0.63x units in regular SCT visually for years off and on. Depending on the FL of the eyepiece you may or may not get a large secondary shadow so eye position is everything.

That is not exactly true but from a "Practical" standpoint is roughly correct.

 

In the SCT design, even though the power of the corrector is very weak, there is only one spacing (out of an infinite number of spacings) that will produce the best spherical aberration (SA) correction.  If you move the primary toward or away from the corrector, the SA changes.  The formula is that for every 25mm of mirror movement from nominal, the spherical aberration will change by 1/23rd of a wave. 

 

 

Now if you just move the focal plane 50mm forward or backwards, the change will be too small to be detected by the human eye on and in-focus view.  Moving the focal plan 50mm would induce 2/23rd or about 1/12th wave of SA and I doubt that any observer alive could detect the change in contrast this makes.   

 

If though, you change the spacing by an excessive amount, as in the case with Binoviewers, the amount of error can be quite substantial.   For example, if the scope is designed with an optimal back focus of 100mm (pretty much the case for all standard Celestron SCTs other than the C14, which is f/11 and uses a bit different optical perscription) and you add a 2" visual back (typically a light path of 30mm) a 2" diagonal (Light path of 100mm) and a binoviewer with a long light path (Televue Bino View, about 130mm) then the variaton from optimal is (total light path of 260mm minus the optimal back path of 100mm means a 160mm delta), then the SA can be degraded enough to easily detect.  At 260mm of back focus, assuming the scope had perfect SA correction at 100mm of back focus, the reduced distance from the primarly to the corrector of 160mm will induce 6/23rds + of SA, and if we do the math, that is a quarter of a wave, and now the scope is right at the diffraction limit.

 

I do think though that you are roughly correct in that the vast majority of non-binoviewer configurations will not induce enough SA to be seen by the observer. 


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#4 Eddgie

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Posted 21 June 2019 - 11:51 AM

Dave,

With my above post in mind, here is some information that you might find useful.

 

The "Optiamal" back focus of Celestron SCTs other than the C14 appears to be about 96mm.  If the scope were perfectly made, this would give a focal lenght of about 2032 (f/10) and assuming again that the spacing were perfect, this would give perfect SA correction.

 

Now, let's get down to reality.. The reality is that it is rare to have an SCT that has perfect spherical aberration correction.  Now just because time is money, the sooner the worker gets the mirror off of the grinder, the sooner they can start production on another mirror, and for this reason, the predominant spherical aberration correction in most telescopes (if there is any) is undercorrection.

 

Now the interesting thing about the SCT design is that it is actually possible to improve SA correction by increasing back focus. 

 

And this gets to your question.   While the optimal back focus by design is 96mm, most SCTs will not have perfect correction at this amount of back focus.

 

To know the perfect back focus for your scope, it would be necessary to see how much spherical aberration is present when the scope is set in the configuration you normally use.

 

The easiest way to do this is to use the star test with 4.4mm of inward focus (as measured at the eyepiece) and 4.4mm of outward focus.  If you do this test and the secondary shadow ratio (size of shadow vs the diameter of the rings where perfect would be exactly 1/3rd of that pattern size if the obstrution was 33%) then the SA correction is perfect.

 

If it were not perfect, in theory, you could shorten or lengthen the light path to change the corrector/primary spacing to make the SA perfect.   The amount you would have to move it is given in the post above (25mm to achieve a 1/23rd change in SA correction).  So, if you determined that you had 1/8th wave of undercorrection, you could move the back focus about 75mmmm further to the rear (3/23rds or about 1/8th wave) and get the correction perfect. 

 

If though, you actually had an SCT with only 1/8th of a wave of correction, moving the focal plane to the rear by 75mm would not make a change in contrast you could see (though a camera can see contrast change to very tiny levels.)  If you had larger amounts, so if you had larger amounts, the physical limits to the configuration might become undesirable, and the baffle design usually makes between 170mm and 230mm the practical limit because past this and the scope may start to loose aperture.  And of course if you had an SCT with 1/8th wave of SA, this is about as good as they typically get and since it would be impossible to improve the visual contrast change past this, there would be little reason to do so.  

 

As photoracer has kind of indicated, a small amount of change (50mm) is going to barely make a difference that you can see, so unless your SA was very strong, there would be little merit in attempting to change it because it could take more back space than in piratical to do so and if you SA is very bad, the physical configuration would put top end limit of the amount of back focus you could add. 

 

Bottom line: As long as you stay reasonably close to the nominal back focus (96mm for most SCTs) there is no point in worrying about the change in SA and photoracer's advice is pretty valid. 

 

The primary time this is important is in the specialized cases of attempting to use excessive focal reduction and/or Binoviewers, where SA can push the correction below the diffraction limit.   I have tested configurations that have shown as much as 2/3rd wave of SA. It is easy to see the amount of contrast 2/3rds of a wave makes.  It is virtually impossible to see the difference 1/8th wave makes.   

 

But putting a 2" diagonal or reverting to the factory supplied components and their 96mm back spacing won't budge the ball enough to see.   If it is really bad, you can't really fix it, and if it is not that bad, making small changes is not going to make enough difference to see. 

 

And to re-iterate, unless you test, you won't know.  If you do the above test with your focal reducer in place (though you have to reduce the amount of in and out focus to about 3.1mm) and the secondary shadow is roughly the same size, then you are good to go.  If you observe a lot of SA, then remove it for planetary observing.


Edited by Eddgie, 21 June 2019 - 12:05 PM.


#5 Dave Bush

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Posted 21 June 2019 - 12:47 PM

Dave,

With my above post in mind, here is some information that you might find useful.

 

The "Optiamal" back focus of Celestron SCTs other than the C14 appears to be about 96mm.  If the scope were perfectly made, this would give a focal lenght of about 2032 (f/10) and assuming again that the spacing were perfect, this would give perfect SA correction.

 

Now, let's get down to reality.. The reality is that it is rare to have an SCT that has perfect spherical aberration correction.  Now just because time is money, the sooner the worker gets the mirror off of the grinder, the sooner they can start production on another mirror, and for this reason, the predominant spherical aberration correction in most telescopes (if there is any) is undercorrection.

 

Now the interesting thing about the SCT design is that it is actually possible to improve SA correction by increasing back focus. 

 

And this gets to your question.   While the optimal back focus by design is 96mm, most SCTs will not have perfect correction at this amount of back focus.

 

To know the perfect back focus for your scope, it would be necessary to see how much spherical aberration is present when the scope is set in the configuration you normally use.

 

The easiest way to do this is to use the star test with 4.4mm of inward focus (as measured at the eyepiece) and 4.4mm of outward focus.  If you do this test and the secondary shadow ratio (size of shadow vs the diameter of the rings where perfect would be exactly 1/3rd of that pattern size if the obstrution was 33%) then the SA correction is perfect.

 

If it were not perfect, in theory, you could shorten or lengthen the light path to change the corrector/primary spacing to make the SA perfect.   The amount you would have to move it is given in the post above (25mm to achieve a 1/23rd change in SA correction).  So, if you determined that you had 1/8th wave of undercorrection, you could move the back focus about 75mmmm further to the rear (3/23rds or about 1/8th wave) and get the correction perfect. 

 

If though, you actually had an SCT with only 1/8th of a wave of correction, moving the focal plane to the rear by 75mm would not make a change in contrast you could see (though a camera can see contrast change to very tiny levels.)  If you had larger amounts, so if you had larger amounts, the physical limits to the configuration might become undesirable, and the baffle design usually makes between 170mm and 230mm the practical limit because past this and the scope may start to loose aperture.  And of course if you had an SCT with 1/8th wave of SA, this is about as good as they typically get and since it would be impossible to improve the visual contrast change past this, there would be little reason to do so.  

 

As photoracer has kind of indicated, a small amount of change (50mm) is going to barely make a difference that you can see, so unless your SA was very strong, there would be little merit in attempting to change it because it could take more back space than in piratical to do so and if you SA is very bad, the physical configuration would put top end limit of the amount of back focus you could add. 

 

Bottom line: As long as you stay reasonably close to the nominal back focus (96mm for most SCTs) there is no point in worrying about the change in SA and photoracer's advice is pretty valid. 

 

The primary time this is important is in the specialized cases of attempting to use excessive focal reduction and/or Binoviewers, where SA can push the correction below the diffraction limit.   I have tested configurations that have shown as much as 2/3rd wave of SA. It is easy to see the amount of contrast 2/3rds of a wave makes.  It is virtually impossible to see the difference 1/8th wave makes.   

 

But putting a 2" diagonal or reverting to the factory supplied components and their 96mm back spacing won't budge the ball enough to see.   If it is really bad, you can't really fix it, and if it is not that bad, making small changes is not going to make enough difference to see. 

 

And to re-iterate, unless you test, you won't know.  If you do the above test with your focal reducer in place (though you have to reduce the amount of in and out focus to about 3.1mm) and the secondary shadow is roughly the same size, then you are good to go.  If you observe a lot of SA, then remove it for planetary observing.

Eddgie,

 

Wow, lots of good info as usual.

 

My scope, and EdgeHD 8 has per Celestron a 133mm optimal backfocus natively and 105mm with the f/7 reducer.

 

When using the reducer and a 2" SCT style diagonal I was always wondering if I had exceeded that 105mm but didn't know how to accurately measure that.

 

Now, with a new diagonal on its way which is not an SCT style, I will be removing it's nosepiece and attaching it directly to the reducer.  I know it will be shorter but again I'm not sure how to accurately measure that.

 

Now, if I understand you correctly, you're saying that as far as SA is concerned, if the central obstruction of the secondary on an out of focus star is the same size in proportion to the whole image on both sides of focus, than any SA is not really going to be noticeable.   Did I get that right?



#6 MikeMiller

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Posted 21 June 2019 - 12:58 PM

Dave, I think you have it right.

 

I am pretty sure I exceed the 133mm on my Edge8 when using a 2" Baader Clicklock SCT connector and diagonal. Because of the extra flattener, the backspacing is supposed to be more important.

 

I have noticed this when the backspace was incorrect when using a camera. But with an eyepiece, I don't notice any difference even with the big wide eyepieces.

 

It is possible to use a shorter SCT connector for Baader diagonals instead of the Clicklock; but I find it very easy to use, especially when rotating the diagonal.

 

While I have the .7x reducer, I only use it for imaging, not visual. I think this is where attaching the diagonal directly to the reducer is most useful.



#7 Dave Bush

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Posted 21 June 2019 - 02:03 PM

Dave, I think you have it right.

 

I am pretty sure I exceed the 133mm on my Edge8 when using a 2" Baader Clicklock SCT connector and diagonal. Because of the extra flattener, the backspacing is supposed to be more important.

 

I have noticed this when the backspace was incorrect when using a camera. But with an eyepiece, I don't notice any difference even with the big wide eyepieces.

 

It is possible to use a shorter SCT connector for Baader diagonals instead of the Clicklock; but I find it very easy to use, especially when rotating the diagonal.

 

While I have the .7x reducer, I only use it for imaging, not visual. I think this is where attaching the diagonal directly to the reducer is most useful.

Well, in my case I will be connecting the diagonal directly to the reducer.  That is, I'll remove the nose piece and screw it directly on it.    I don't do photography so I am altazimuthly mounted.  Never need to rotate the diagonal.  :-)


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#8 MikeMiller

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Posted 21 June 2019 - 02:18 PM

Well, in my case I will be connecting the diagonal directly to the reducer.  That is, I'll remove the nose piece and screw it directly on it.    I don't do photography so I am altazimuthly mounted.  Never need to rotate the diagonal.  :-)

Looks like you are in good shape. I'd like to know how you enjoy having the eyepeices through the .7x this way. I might decide to do the same thing on mine using something like this:

https://agenaastro.c...t-diagonal.html



#9 Dave Bush

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Posted 21 June 2019 - 02:31 PM

Looks like you are in good shape. I'd like to know how you enjoy having the eyepeices through the .7x this way. I might decide to do the same thing on mine using something like this:

https://agenaastro.c...t-diagonal.html

I've been using the reducer visually and it works perfectly.  I prefer 100º eyepieces and with the 21mm Ethos being the lowest made, I needed the reducer to get wide enough low power views.

 

As for the locking ring, get this one and save some $$.  I did...

 

https://agenaastro.c...-ring-l-02.html


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#10 Redbetter

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Posted 22 June 2019 - 01:14 AM

 

In the SCT design, even though the power of the corrector is very weak, there is only one spacing (out of an infinite number of spacings) that will produce the best spherical aberration (SA) correction.  If you move the primary toward or away from the corrector, the SA changes.  The formula is that for every 25mm of mirror movement from nominal, the spherical aberration will change by 1/23rd of a wave. 

 

Eddgie,  did you mean focal plane movement for the part I italicized rather than mirror movement?  I assumed that is what you meant because moving the primary mirror that much would be something like a 3x multiplier for the change in the focal plane, if I understand correctly.



#11 Eddgie

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Posted 22 June 2019 - 03:33 PM

 

 

Now, if I understand you correctly, you're saying that as far as SA is concerned, if the central obstruction of the secondary on an out of focus star is the same size in proportion to the whole image on both sides of focus, than any SA is not really going to be noticeable.   Did I get that right?

It is important to remember that the EdgeHD 8" is actually not f/10. It is f/10.45 at 133mm of back focus.

 

To your question though, I would only modify your statement to say that if the size of the secondary shadow is the same size in proportion when compared at the exact same distance inside and outside of focus (about 3.1mm at f/7) then this would suggest that the SA is very close to perfect.  If it is much larger or much smaller on one side than the other (when measured with 3.1mm of defocus in and out), then that suggests the presence of spherical aberration.  You can get an estimate by the difference in size.   For example, if the shadow is 25% larger on one side than the other, this would be equal to about 1/4th wave of SA.  If it differed by 20%, this would be 1/5th wave of SA. 

 

As long as it looks reasonably balanced, it is hard to worry about, and frankly, if you are using a focal reducer, there is nothing you can really do.   If you change the back space from the nominal space for the reducer, you change the power of the reduction.  To shorten it would make the focal ratio slower, and to increase the distance would make for a faster focal ratio, and at some point you would have vignetting.   So, if this is the goal (f/7) then the back space has to be what the Celestron says and you are stuck with the SA that you measure. 

 

This is why I said that if you see significant SA, you would probably want to remove the reducer for planetary work. 



#12 Eddgie

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Posted 22 June 2019 - 03:35 PM

Eddgie,  did you mean focal plane movement for the part I italicized rather than mirror movement?  I assumed that is what you meant because moving the primary mirror that much would be something like a 3x multiplier for the change in the focal plane, if I understand correctly.

Thank you for catching this. Yes, I meant for every 25mm that you displace the focal plane (not the mirror spacing), you change the SA by about 1/23rd of a wave.

 

Good catch, and thank you for pointing it out. 


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#13 Dave Bush

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Posted 22 June 2019 - 03:52 PM

It is important to remember that the EdgeHD 8" is actually not f/10. It is f/10.45 at 133mm of back focus.

 

To your question though, I would only modify your statement to say that if the size of the secondary shadow is the same size in proportion when compared at the exact same distance inside and outside of focus (about 3.1mm at f/7) then this would suggest that the SA is very close to perfect.  If it is much larger or much smaller on one side than the other (when measured with 3.1mm of defocus in and out), then that suggests the presence of spherical aberration.  You can get an estimate by the difference in size.   For example, if the shadow is 25% larger on one side than the other, this would be equal to about 1/4th wave of SA.  If it differed by 20%, this would be 1/5th wave of SA. 

 

As long as it looks reasonably balanced, it is hard to worry about, and frankly, if you are using a focal reducer, there is nothing you can really do.   If you change the back space from the nominal space for the reducer, you change the power of the reduction.  To shorten it would make the focal ratio slower, and to increase the distance would make for a faster focal ratio, and at some point you would have vignetting.   So, if this is the goal (f/7) then the back space has to be what the Celestron says and you are stuck with the SA that you measure. 

 

This is why I said that if you see significant SA, you would probably want to remove the reducer for planetary work. 

Is there any easy way to know as I rack in and out of focus that I'm 3.1mm on either side?

 

And I'm not sure what you meant by...  " As long as it looks reasonably balanced, it is hard to worry about, and frankly, if you are using a focal reducer, there is nothing you can really do."  

 

Another question.  Any idea how I can measure the light path through the diagonal to where the eyepiece focal plane is?
I've heard the 100mm figure used often with respect to 2" diagonals but I'm not clear on where that's all measured from/to.



#14 Eddgie

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Posted 22 June 2019 - 04:51 PM

Easy answer first.   The light path of the diagonal is usually measured from the front of the mirror box to the top of the eyepiece holder.  Since the nose of the diagonal is normally inside of a visual back, if you measured the lenght of the visual back, the nose of the diagonal would be covered by that figure.

 

While you can measure the light path using a refractor, this can often involve using spacers and an object that can be focused with and without the diagonal (and spacers, if necessary).  Focus on a object with and without the digaonal/spacers an see what the difference is in the amount of focuser and spacers and this is the diagonal light path lenght.

 

It is really much easier just to estimate it though. In most 2" diagonals, the distance from the front of the mirror box to the mirror center and to the top of the mirror box is typically about  about 75mm.  Measure the distance from the top of the mirror box to the top of the eyepiece holder and add it to 75mm. This is approximate, but there really is no reason to be that exact here.  Most 2" diagonals have a light path of about 100mm, but some are longer. 

 

How to accurately measure the inside and outside of focus distance.

 

First you need something of the correct thickness or some way to measure the correct thickness (about 3.1mm in this case).  This is not exact, but it is close enough and if you err, go a bit bigger).

 

Here are the steps:

 

  1. Focus the scope on a bright star as carefully as possible
  2. Slip the eyepiece out of the eyepiece holder by the space you need (again, in this case, about 3.1mm or a bit more)
  3. Make your observation.  Try to see how wide the shadow is with respect to the entire pattern
  4. Without moving the eyepiece in the holder, refocus the scope for best focus
  5. After you have used the focuser to get in perfect focus, loosen the eyepiece and slide it all the way in..  This automatically puts you at the exact same distance inside as you were at outside.  It is the most exact way to do the comparison
  6. Make your observation and note if the secondary shadow is the same size in proportion to the shadow, or if it is larger or smaller, and if larger or smaller, try to estimate the ratio of the difference
  7. Repeat as necessary until you fell you have a good feel for the measurement

Some notes.  The 3.1mm figure is not the primary goal here.  If you can't find something exactly 3.1mm thick, get as close as you can (a bit wider is a bit better).  

The real goal again is not to have the exact distance required for 10 waves of defocus, but rather to have the exact same amount of defocus inside and outside.  It can be 3.15mm or 3.2mm (or whatever) so it is not the exact distance that is critical but that whatever distance is used, it the exact same distance is used on both sides.  The eyepiece method I use is (I think) the most precise way to make sure that the view is done at exactly the same distance on the caustic cone before and after the focal plane. 


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#15 Dave Bush

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Posted 22 June 2019 - 05:15 PM

Thanks Edggie.  That method is pretty straightforward.  

 

So with regards to my situation, since the back focus when using the reducer is 105mm per Celestron, measured from the back of the reducer (also per Celestron) and if most diagonals are 100mm then I’ll be barely inside that back focus.  I’m attaching the diagonal directly to the reducer having first removed the nosepiece.   

 

I’ll measure the diagonal when I get it though just to be sure.  It’s a Baader BBHS by the way. 



#16 Eddgie

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Posted 22 June 2019 - 05:16 PM

This is about what 10 waves of defocus should look like at f/7 with a 33% obstruction. Note that the left set of images shows a shadow that is about 1/3rd (33% obstruction) of the width of the width of the whole pattern.

 

On the right, you can see that the shadow appears to be slightly larger as a percentage of the pattern than on the bottom.  This is what 1/4th wave of SA will look like (as modeled in Aberrator 3.0, an easy to use free download on the www.)

 

Now to be fair, the image will not look this crisp and clean in the eyepiece, and the edges of the shadow may appear indistinct, but the point is that if they look pretty even in and out, then the SA is pretty good.  If they appear much worse than shown here in terms of size difference in and out, this indicates that there is probably more meaningful SA present.  In that case, you would want to pull off the corrector for planetary observing, or try a Barlow to see if the SA is reduced.  A Barlow will have the effect of making you move the mirrors further to reach focus. 

 

SA test reduced.jpg


Edited by Eddgie, 22 June 2019 - 05:19 PM.


#17 Dave Bush

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Posted 22 June 2019 - 09:21 PM

Thank you for the examples.  That will help.  



#18 MikeMiller

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Posted 24 June 2019 - 10:56 AM

I've been using the reducer visually and it works perfectly.  I prefer 100º eyepieces and with the 21mm Ethos being the lowest made, I needed the reducer to get wide enough low power views.

 

As for the locking ring, get this one and save some $$.  I did...

 

https://agenaastro.c...-ring-l-02.html

Based on your advice, I gave it a try this weekend. The Baader 2" Clicklock diagonal has female SCT threads and will thread directly on to the .7x reducer without the lock ring. It can't rotate, but that isn't a problem on an alt-az mount. It also gives it enough clearance to avoid bumping against the mirror lock knobs or focuser.

 

It worked great, and gave extra wideness on the big WO XWA 20mm 100º and ES 30mm 82º. Helpful when exploring things like double cluster, wild duck cluster,  Big nebulae like M8 and M17 were very nice.

 

Sounds like this will be an extra tool in my eyepiece box going forward. Thanks!


Edited by MikeMiller, 24 June 2019 - 10:58 AM.


#19 MikeMiller

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Posted 24 June 2019 - 11:01 AM

I just realized that some people will need the lock ring even with alt-az. It just so happens that the orientation of my scope on my mount with my diagonal and the .7x happens to result in the eyepiece pointing straight up. My scope has tube-rings that don't rotate, so it was probably complete blind luck that it worked so well. :)



#20 Dave Bush

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Posted 24 June 2019 - 11:12 AM

I just realized that some people will need the lock ring even with alt-az. It just so happens that the orientation of my scope on my mount with my diagonal and the .7x happens to result in the eyepiece pointing straight up. My scope has tube-rings that don't rotate, so it was probably complete blind luck that it worked so well. smile.gif

Luck indeed.  I'm ordered the lock ring (the cheaper one) just in case.  But it sound like I might get lucky as well.  We'll see.

 

Glad to hear visually all looks great. 


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#21 bobito

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Posted 26 June 2019 - 10:43 PM

...

To your question though, I would only modify your statement to say that if the size of the secondary shadow is the same size in proportion when compared at the exact same distance inside and outside of focus (about 3.1mm at f/7) then this would suggest that the SA is very close to perfect.

  ...

If I may indulge your knowledge on this.  What is the formula to come up with 3.1mm?

 

I have a 12" f/10 with an aftermarket R&P focuser and filter drawer so I'm pushing it a bit. I would like to try your test in and out of focus so am wondering what is the optimal distance in my configuration?



#22 Eddgie

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Posted 27 June 2019 - 06:58 AM

The formula is in Suiter's  book on star testing, which I have sold.  I never used the formula though because Suiter posted a table.  The formula is based on wavelengths of light for a given focal ratio (aperture independent) and the table provides a bunch of focal ratios.

 

The only measurement I remembered from the table was for f/10 since I have tested SCTs so much and for f/10, 10 waves of defocus is a bit over 4.4mm, so I just multiplied 4.4mm by .7x, came up with 3.08 and rounded out to 3.1mm. 

 

Again, one does not have to be exact with the distance (though it is desirable to do so).  What is more important is to be exact that whatever distance is used is identical on the inside and outside of focus.   


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#23 Eddgie

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Posted 27 June 2019 - 07:15 AM

Another way to do it is to use the freeware program Aberrator 3.0.  This is an excellent program for modeling optics and it is the program I used to generate the above images.

 

The program has a setting for defocus and I have done a screen capture on it.  If one starts the program and puts in the number of waves of defocus, it will calculate the distance.   I have done that here for f/7.

 

I think there is a bug in the program.  It only seems to work properly for the first input.   If you put in 10 and then change it to 7 waves, it will report a bogus number.  If you close and restart, and 7 waves in, it will give the correct number.

 

Anyway, this is the program I use for modeling, SA and Astigmatism.   It will also provide an MTF plot of the instrument!  That is the best part in my opinion.  Knowing there the obstruction and quality is one thing, but knowing how it affects the image is the real goal of doing any of this, yes?

 

 defocus.jpg

 

Once again, the defocus is independent of aperture.  Only focal ratio is needed for defocus.  All f/10 instruments will be 4.4mm at 10 waves (again it is just a fraction over 4.4, but 4.4 is close enough).


Edited by Eddgie, 27 June 2019 - 07:17 AM.

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#24 bobito

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Posted 27 June 2019 - 12:10 PM

Thank you, Eddgie, much appreciated!




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