# Thread: Secondary distance from primary

1. ## Secondary distance from primary

Hello,

Can someone calculate the theoretical distance between my primary and secondary mirror? Specifications are below.

153mm f/8 primary mirror
31mm secondary mirror
190mm inner, 197mm outer tube
81mm focuser minimum rack height

Where am I trying to aim the focus? Where my focuser is at minimum, medium or maximum height? Am I right if the secondary mirror should be placed (1200-190/2-81)= 1024mm from the primary?

2. ## Re: Secondary distance from primary

If eyepieces were standardized so that all had their focal plane in the same spot relative to the barrel (i.e. if all were parfocal), then it would be possible to give an exact answer. But there is no standardization. If you already have a selection of eyepieces, you could determine where their focal planes were, and then arrange the position of the secondary to put the focal plane at that spot when the focuser is at mid-range.

Your calculation will put the focal plane at the shoulder of the eyepiece when the focuser is fully retracted. I would be inclined to add (subtract, actually, the way you have the calculation set up) half the focuser's range so that the focal point is at the shoulder of the eyepiece at mid-range. But, like I said, it is very eyepiece-dependent.

3. ## The Following User Says Thank You to KathyNS For This Useful Post:

FelixG (12-19-2014)

4. ## Re: Secondary distance from primary

I can't help with that without doing some serious math but Vic Menard's Catseye collimation website has a Newtonian design tool that may help: NEWTONIAN TELESCOPE DESIGN PLANNER (I.E. version) . See if that can help provide an easy solution.

5. ## The Following User Says Thank You to CamelHat For This Useful Post:

FelixG (12-19-2014)

6. ## Re: Secondary distance from primary

Thank you! I do have my eyepieces already. So placing my secondary mirror 984mm from the primary gives me a couple centimeters of room to find the focal plane of the eyepiece, correct?

7. ## Re: Secondary distance from primary

I'll check that out! Thank you.

8. ## Re: Secondary distance from primary

Originally Posted by FelixG
Hello,

Can someone calculate the theoretical distance between my primary and secondary mirror? Specifications are below.

153mm f/8 primary mirror
31mm secondary mirror
190mm inner, 197mm outer tube
81mm focuser minimum rack height

Where am I trying to aim the focus? Where my focuser is at minimum, medium or maximum height? Am I right if the secondary mirror should be placed (1200-190/2-81)= 1024mm from the primary?
I have always found the following website useful for these matters:

https://stellafane.org/tm/newt-web/newt-web.html

You ask about "where to aim the focus?" Well, if I understand the question as just the obvious question it is meant to be, you're asking "where to aim the focuser?" Aim it at the secondary mirror.

As you look thru the focuser, you should see the secondary mirror centered in the circle defined by the end of the draw tube.

Clears,
Joe

9. ## Re: Secondary distance from primary

I agree, the question wasn't very clear..

I meant to ask where to aim the focus of the primary mirror. If I'm not mistaken, the focal distance 1200mm, determines where the focusing point will be situated. Now, the question is.. where do I want the focusing point? In the middle of my focusers racking height?

10. ## Re: Secondary distance from primary

Originally Posted by FelixG
I agree, the question wasn't very clear..

I meant to ask where to aim the focus of the primary mirror. If I'm not mistaken, the focal distance 1200mm, determines where the focusing point will be situated. Now, the question is.. where do I want the focusing point? In the middle of my focusers racking height?
As Keith BC already said: "arrange the position of the secondary to put the focal plane at that spot when the focuser is at mid-range." The term focusing point is usually understood as the focal plane of the primary mirror -- the plane where the cone of light from the primary comes to its sharpest focus. So, in other words, the answer to your question is (more or less) YES -- "In the middle of my focuser's racking height".

There is a problem though.

I have a 13mm TV Nagler eyepiece. It's my most important eyepiece. I had to measure carefully when making my latest dob because the 13mm Nagler I have requires much more length between EP and secondary than my other EPs. So the focal plane of the primary for my 12.5" dob does NOT fall in the middle of my focuser (racked in). I had to make adjustments for my Nagler.

As Keith pointed out, everything depends on which EPs you are planning to use.

Clears,
Joe
Last edited by BABOafrica; 12-20-2014 at 03:38 PM. Reason: spelling

11. ## The Following User Says Thank You to BABOafrica For This Useful Post:

FelixG (12-20-2014)

12. ## Re: Secondary distance from primary

Both the Stellafane and Menard's tool present pictures resulting in a non-centered secondary because of the asymmetry caused by the distance of the reflection of the primary in the secondary when viewed from the focuser. So their results will be off by a few mm because non-offset secondaries will always result in an angle between the optical and mechanical axis when collimated properly. So if you have a non-offset secondary, add a few mm to the optical path.

13. ## Re: Secondary distance from primary

Thank you! I really appreciate any help I can get.

I'll be mounting my equipment soon. I believe the easiest way to do so is to start with mounting the focuser and the secondary at theoretical positions? Then use a friend who looks through the eyepiece as I'm moving the primary back and fourth to determine where it's to be mounted.

I suspect my questions are pretty common. Is there a large post somewhere that I've missed, where I can find help?

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