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Extra focal length


dawson
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1. In an SCT, the image is upside down. At what point does that happen in the path of the light travelling through the corrector plate, primary, secondary, down the narrow tube and onto the sensor of the camera?

2. Does adding say a 10cm (glass-free) extension tube to the back of the SCT, simply add 10cm onto the focal length of the set up (assuming you don't take 10cm off by moving the primary mirror 10cm nearer the corrector plate to achieve focus), or does it add proportionally greater focal length than its actual length?

Thanks

Jd

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If memory serves me correctly, I think you may have asked something similar in the past James, may I suggest a search on the forum re the replies, they were pretty comprehensive and Petersul answered with the technicalities.


 


Or draw it out on a piece of paper to work it out for yourself, that's what I do when Im trying to figure out stuff in optics.


Just remember an SCT mirror moves to achieve focus, therefore if you stick an extension tube on the back you will have to move the mirror forwards to achieve focus at the new point. This does not necessarily mean the focal length has changed ( I say necessarily because there are other factors which will affect the fl as answered in the previous post)


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Thank you both (and Kim); yes it is a similar question, and re-reading that thread (thanks for finding) has reminded me about that book which is still at home so I will have a look at the relevant pages over the weekend.


 


The context, is simply to work out how much extra optical length adding say the flip mirror makes to the scope and how much that alters the f/ ratio. If it is simply addative in terms of adding 100mm to the existing 2800mm, then it will have negligible impact upon the f/ ratio, but I was talking to someone the other week and they were suggesting it had a great effect than that, which made me think something else was happening.


 


I'll re-read the pages and feed back.



I do recall that the outcome last time was that the focal length of a Mak or SCT isn't fixed.


 


Thanks again.


 


James

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In simple terms and very genralised, it will not add anything to your focal length, the point at which your mirrors and lenses focus light is not going to alter by adding anything after that point, much the same way as me standing a metre away from the eyepiece is not going to turn my 1000mm F/L 200p into a f10 2000mm F/L scope.


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The SCT is different from a newtonian in that it has two curved mirrors, (primary and secondary). When focusing by moving the primary, (changing the separation of the primary and secondary), the focal length does actually change according to a formula and not directly proportional to the movement of the primary.


 


In a newtonian or refractor the focal plane and focal length/ratio remains fixed unless you use a barlow or reducer.


 


As James says, this should have negligible effect by only changing the focal ratio/length by just 5% or so.


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But by adding length to the back will have the effect of needing to move the mirrors closer, thus shortening the focal length, so I stand by what I said, adding a 50mm tube will not add to the focal length.


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Here's an interesting thread Mike...

http://stargazerslounge.com/topic/123243-sct-focussing-and-changing-focal-length-some-data/

 

The final post, #7, shows some measurements using a Meade 10" SCT F10 = 2540mm.

This shows the change in focal length when adding accessories and pushing the focal plane further out...

 

Scope focal length:
No diagonal 2460mm
1.25" diagonal 2730mm
2" diagonal 2840mm

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So that changes the f/ratio from native f/10 to:


 


No diagonal 2460mm f/10.3


1.25" diagonal 2730mm f/9.3


2" diagonal 2840mm f/8.9


 


So only by 1 in ratio terms at most.


 


That kind of answers my question. Thank you.


 


That book says:


 


"...The distance that the secondary is inside the prime focus, divided into the distance between the secondary and the actual focal point behind the primary, gives the amplification factor, typically between 3 and 6. The effective focal length of a Cassegrain telescope is the focal length of the primary multiplied by the amplification factor...."


 


Which also goes along with what the SGL guy is saying about a 0.001" shift in the primary mirror during long exposure astrophotography will result in a shift in the focus point/plane by 5x this distance, squared, which would be 0.025". Maybe another reason we don't see more people doing faint DSO imaging with large catadioptric scopes.


 


Thanks all for the discussion and links.


 


James

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"Maybe another reason we don't see more people doing faint DSO imaging with large catadioptric scopes"


 


I think it's more to do with the speed of the scope James. At f-10 it takes twice as long to gather dso subs than at f-5. It won't be anything to do with the notion of a .025" shift in focus point. :)


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Lol, no, not unless it gets permanently mounted! I'm no so interested in the DSO stuff, more the planetary and lunar at the moment. I just wanted clarification on the focal length issue.


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So that changes the f/ratio from native f/10 to:

 

No diagonal 2460mm f/10.3

1.25" diagonal 2730mm f/9.3

2" diagonal 2840mm f/8.9

 

James

I'd bin that calculator if I were you James :D :D

 

No diagonal 2460mm f/9.7

1.25" diagonal 2730mm f/10.7

2" diagonal 2840mm f/11.2

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Did i do the maths using the aperture of an 11" scope rather than a 10" one?! I was also doing maths thinking about the C11. Doh!

Jd

No, you actually must have done a correct intermediate calculation for the changed focal ratio on the 10".

 

Then in each case you took the number 10 and divided it by the correct F ratio you'd calculated and then used that result as the F ratio. :wacko:

 

So you just went an unnecessary step too far with the calculation. :)

 

Even though the numbers were wrong, your conclusion...

     So only by 1 in ratio terms at most.

 

     That kind of answers my question. Thank you.

was pretty much correct anyway :)

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Haha :) ok, i'm just logging onto flo's website....

I thought i'd divided the apparent focal lengths by the 10" aperture (254mm) but have no idea where i went wrong. I wasn't even drunk or under the influence of drugs (do multivitamins with iron count?)!

:)

Jd

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