# Shortening a newtonian, need help with the math.

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So, I'm preparing to shorten the tube of my Orion Starblast 4.5 OTA to use it with a DSLR.

The process of doing so is simple enough, but I don't know how much I should cut. For every millimeter I need to move the camera on the focuser, how much of the tube should I remove?

Thanks!

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Exactly the same amount.  Move the mirror up the tube (or shorten the tube) by 20mm and the focal plane moves "out" of the focuser by 20mm.

James

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Awesome! I didn't know if it'd be a 1:1 or 1:2 ratio.

Thanks!

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Additional question: does shortening the tube change the focal length and f#? I plan to cut about 30mm off this tube.

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In theory yes, but if you are moving the camera by the same amount then the resulting focal length will be the same.

But, I'll refer you to this similar thread, with regards to the comments on moving the 1° mirror a relatively large distance and the resulting requirement for a larger 2°!

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No.  With a newt the focal length and focal ratio (unless the tube has been stopped down to reduce the aperture for some reason) are entirely determined by the primary mirror.

James

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In theory yes, but if you are moving the camera by the same amount then the resulting focal length will be the same.

You can do what you like with the camera  The focal length is a property of the curvature of the primary mirror.  It will not change if you shorten the tube.

This is the entire point of moving the mirror up the tube.  If the camera can't move inwards enough to reach the focal plane, 450mm "in front" of the mirror, then shortening the tube by 30mm will move the focal plane 30mm "outwards" with respect to the focuser, hopefully putting it somewhere such that the camera can be placed to have the sensor coincide with the focal plane.

Where this doesn't hold is when you have two or more optical components in the optical path, such as in an SCT or Mak.  They are focused by moving the mirror and in fact the focal length changes at the same time (because the effective focal length is a function of the distance(s) between the optical components).

James

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shortening the tube can alter the illuminated field, and you may need a slightly larger secondary as far as i believe, though i am not an expert on this at all, someone with more knowledge may confirm or deny this.

Ray

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Very interesting. Thanks for the speedy and specific responses guys!

In theory yes, but if you are moving the camera by the same amount then the resulting focal length will be the same.

But, I'll refer you to this similar thread, with regards to the comments on moving the 1° mirror a relatively large distance and the resulting requirement for a larger 2°!

Yeah, I've come to accept that I'll lose some of the light. This scope was built with a larger secondary though, and at f4 I don't expect this to be a huge problem.

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personally I wouldn't actually shorten the tube, that could effect the resale value,   I need to move the primary in my  astromaster, and am looking at using longer mounting screws with spacers or washers and again for the collimation screws to move the mirror up the tube

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Got some assistance from my dad, since four hands to hold it steady are better than two... sawed 30mm off the end. Mission success! It is incredibly windy, clouds are rolling in, and the moon is drowning out the sky, but I am indeed able to focus on the moon now!

Even better, I can still see all of my primary mirror clips during collimation, so I don't think I'm losing much light, if any.

I'm excited for some better astronomy nights now!

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Very quick test shot: http://i.imgur.com/ksUfCvU.jpg

This was done without any tracking, the wind blowing, and the telescope hadn't fully cooled off yet.I wanted to see if I could go beyond focus in both directions and quickly test for clarity before I turned into a popsicle. Not too shabby!

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I'm pleased to hear that it has all worked

James

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