How to calculate the focal length of a SCT or MAK

15 years 6 months ago #5405 by dave_lillis
Hi All,
This would be of great interest to all with a SCT or MAK.
Open this pdf file , dont let the maths daunt you, go straight to the last page. The graph shows how only a few inches in the position of the eyepiece can dramatically change the apparent focal length of the scope.!

For instance if I place my eyepiece 3 inches behind the backplate of the scope, my 12" F10 operates at F10.83 !!!

A F10 8 " SCT with the eyepiece the eyepiece 5.5 inches away from the back plate operates at F12.5.

www.brayebrookobservatory.org/BrayObsWeb...BOOKS/EFLMAKCASS.pdf

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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15 years 6 months ago #5406 by dave_lillis
Replied by dave_lillis on topic Cheap Ronchi tester
For those who are hypochondriacs when it comes to their scopes optics.
here is a a ronchi tester to test their scope with.

schmidling.netfirms.com/ez-testr.htm

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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15 years 6 months ago #5407 by michaeloconnell
Replied by michaeloconnell on topic Re: How to calculate the focal length of a SCT or MAK
Very Interesting stuff there Dave.
This would imply that when using a 2" diagonal on a SCT, you're also increasing the focal length considerably. I must give this a test.

As for the ronchi tester, I always thought they cost a couple of hundred euro. I'm surprised they are that cheap.
BTW, I hope you're not suggesting I'm a hypochondriac! :wink:
(I just ordered one!!)

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15 years 6 months ago #5408 by michaeloconnell
Replied by michaeloconnell on topic Re: How to calculate the focal length of a SCT or MAK
Right, I just had a read of it.
So, if I measure the distance from the backplate of the scope to the centre of the diagonal (allowing for the thickness of the mirror and supporting plate) and then up to the bottom element of the eyepiece, I should have the required distance.

For my 8" LX90 I tried it for two conditions:

1) With a 2" diagonal, a 1.25" adaptor, a Borg non-rotating helical focuser and a 1.25" eyepiece
In this case, the total back distance was 9". This implies that when using a 1.25" eyepiece or a webcam the scope is actually operating with a focal length of 110 inches. 110/8 = f/13.75 as opposed to the accepted value of f/10 which is really surprising actually!

2) With a 2" diagonal and a 17mm Nagler. One of the things about some 2" eyepieces is that they have several lenses. The total distance between the top and bottom lenses in these eyepieces can in some cases be around 2". I assumed that the scope's focal length ended at the bottom eyepiece element i.e the element closest to the scope. The back distance in this case was 4". This implies a focal length of 94". 94/8 = f/11.75

Very interesting stuff Dave. Thanks!

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15 years 6 months ago #5411 by dave_lillis
Michael,
It is very surprising, I thought that moving a few inches would only add a fraction to the focal length, but because we're moving the primary mirror WRT the secondary, the system acts like a variable barlow apparently.

To make things worse, the correcting effect of the corrector plate is tuned, i.e. you'll get spherical abberation if you go too far from the back plate of the scope, I've asked this on the mapug group and they say stay within a few inches and you'll be ok, nobody was able to precisely quantitate this.
Any optics professors out there able to figure this out ??

I was tinkering with the idea of getting the ronchi tester, it would be a very interesting test to see the pattern for a near and far position from the back plate. I had a feeling you'd go for it :wink:

Michael, next time you out, try those 2 setups and you should be able to see discrepencies in the size of the field of view other then those induced by the differently sized eyepieces..

Of course this all rides on the premise that the perfect F10 position is exactly at the back plate.
For all we know, they might have designed the scope that the F10 point is where the eyepiece is when using the supplied star diagonal ???
I currently cant find info on this...

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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15 years 6 months ago #5760 by dave_lillis
Well, I officially joined the ranks of the opto-chondriacs last night.
I took out the 4 inch last night with the ronchi tester and there was very mild overcorrection visible, phew...
I didnt think that it would show up an error as the scopes images are satisfactory, but then again i never push this scope to high magnifications.

I'll never buy a scope again without atleast attempting to try this out first!
This is a very useful tool.

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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15 years 6 months ago #5771 by ayiomamitis
Dave,

It is indeed scarry how a little change after the rear cell of the SCT can dramatically change the focal ratio of the system. This is something I first realized when doing work with a variable projection adapter where my camera at prime focus would have a moon that just fits perfectly or is way too big by just moving the tube of the variable projection adapter ever so slightly.

Anthony.

Anthony Ayiomamitis
Athens, Greece
www.perseus.gr

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15 years 6 months ago #5784 by dave_lillis
Anthony,
Next time I'm out and if I have the time, I'll try and figure out using skymap and a 26mm eyepiece where the F10 position is.

You can superimpose the field of view of an eyepiece in skymap, telling you what you can and cant fit into the field of view, so I'll try and match up the scopes image to that.

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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15 years 6 months ago #5974 by dave_lillis
After pouring the Mapug archives I found this.
www.mapug.com

"it was felt that the best primary to secondary spacing was achieved when the focal plane for an object at infinity was about 3 to 4 inches beyond the back of the telescope back plate.
"

"Even then there are compromises in the design since not all aberrations come to their lowest values for the same spacing. "
which is no surprise.

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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