17 replies to this topic

[TNAllen]

I recently joined your forum thinking I may assemble a Newtonian telescope. I have been looking through various books on telescope design and construction. One question I have regards supporting secondary mirrors. I have read about spiders, but wonder if secondary mirror supports are occasionally attached directly to focusing mechanisms rather than a spider. It occurs to me that alignment of the eyepieces with the secondary mirror could perhaps be done more accurately before inserting and connecting the assembly to the exterior of the telescope. This alternative would require a design sufficiently rigid to control vibration and maintain accurate alignment with the primary mirror and eyepiece, but this seems no more complex than maintaining alignment using a spider.

[izar187]

A sled focuser is perhaps one way this might be done.

But it is not very common, and there are usually good reasons why things are uncommon.

Here are some threads on sled focusers.

 

https://www.cloudyni...-focusers-dead/

 

https://www.cloudyni...d-focuser-help/

 

https://www.cloudyni...eed-help/#entry

 

https://www.instruct...er-on-EQ-mount/

 

https://www.cloudyni...s-more-popular/

 

And welcome to Cloudy Nights.

Though I have not built scopes, but just modified some I bought, lots of folks have.

be sure to check out the ATM, DIY forum.

 

https://www.cloudyni...-and-diy-forum/

Edited by izar187, 15 June 2025 - 12:08 PM.

[TNAllen]

Thank you, particularly for suggesting the ATM DIY Forum, I hadn't seen it, and it looks very interesting. I agree that common practices result for practical reasons. As I read about telescope design and construction, questions occur to me as a way perhaps of understanding the reasons for the choices people have made.

[Notdarkenough]

YouTube videos are often available discussing designs, flaws and Pros and Cons. I find that medium more conducive of detailed one-way information sharing.

[Jon Isaacs]

 

This alternative would require a design sufficiently rigid to control vibration and maintain accurate alignment with the primary mirror and eyepiece, but this seems no more complex than maintaining alignment using a spider.

 

A spider can be very rigid, the vanes are in tension like the spokes on a bicycle wheel.  The ease of adjustment of the secondary is also important, this is relatively straight forward with a spider design with a central hub.

 

A in single stalk spider are occasionally used in small Newtonians but a cantilever needs a lot more material. The other problem is the /height/room needed for the adjustment. 

 

Jon

[TNAllen]

Thank you for the responses. I have read through some, looked the photos, and now have a better understanding of the sled. It seems like a good way to manufacture a combined accurate and stable mounting for the eyepiece assembly and seconday mirror that could fascilitate aligning the two outside the telescope. If my thinking is correct, one could the adjust the primary mirror to complete alignment/collimation.

I am curious about the tolerances used in DIY manufacturing or fabricating of a telescope. I assume some are critical to producing good images, and some to easily focusing, and both are critical to producing a sufficiently stable image while locating objects in the telescope.

I would appreciate any information regarding tolerances. I have some background in machine shop practice, consequently, were I to fabricate a telescope, I would approach its design and fabrication from a machine shop perspective.

[apfever]

Thank you for the responses. I have read through some, looked the photos, and now have a better understanding of the sled. It seems like a good way to manufacture a combined accurate and stable mounting for the eyepiece assembly and seconday mirror that could fascilitate aligning the two outside the telescope. If my thinking is correct, one could the adjust the primary mirror to complete alignment/collimation.

 

Not so, and that is the main problem with your approach. Your focuser and secondary have to be aligned to the center of the primary. Aligning the primary does not do this.  You will have to shim your focuser/secondary assembly in order to get it correctly collimated to the center of the primary. The distance between your focuser and secondary should be carefully set to correctly center the secondary in the optical 'tube'.  Correctly centering the secondary in the tube is not critical but should be close. This would all be the case where your focuser/secondary assembly is considered rigid, without intention of additional adjustments after installation. 

Many single stalk systems that attach the secondary to the focuser still have a couple of adjustments for distance and rotation about the rod. 

[Dale Eason]

Tolerances depend on the parameters of the telescope like F#.  Faster scopes require tighter tolerances for good collimation.  Usually it is best to start with a scope that does not require tight tolerances.  Many people have no machining skills and none is needed to built the usual Dobsonian telescope.  A Dobsonian simplifies the mount of the scope.  Other types of mounts can require very tight tolerances.  The mount becomes the harder part to make.

 

Focusers can require tight tolerances but there are designs that most wood workers can achieve.

[TNAllen]

Thank you for your insightful responses. I understand what you have written, and it makes sense, particilarly for woodworkers, and no doubt produces the desired results.

However, say a person wanted to do make a single, combined assembly of the focuser and secondary mirror, and could measure the inside tube diameter and then produce the parts of the assembly, and mount it on the tube so that the secondary mirror center was aligned with the center of the primary mirror within a given practical tolerance. What would that tolerance need to be?

[TNAllen]

Let me add that I'm mot advocating a particular approach. I have no experience with building or using telescppes and recognize my limitations. By way of learning about building them, I am curious about the dimensions and tolerances required as, were I to decide to build a telescope, I'd need some sense of these.

[Jon Isaacs]

Thank you for your insightful responses. I understand what you have written, and it makes sense, particilarly for woodworkers, and no doubt produces the desired results.

However, say a person wanted to do make a single, combined assembly of the focuser and secondary mirror, and could measure the inside tube diameter and then produce the parts of the assembly, and mount it on the tube so that the secondary mirror center was aligned with the center of the primary mirror within a given practical tolerance. What would that tolerance need to be?

 

Let me add that I'm mot advocating a particular approach. I have no experience with building or using telescppes and recognize my limitations. By way of learning about building them, I am curious about the dimensions and tolerances required as, were I to decide to build a telescope, I'd need some sense of these.

 

The tolerances depend on the focal ratio, the aperture, whether or not you will be using a coma corrector.  Whatever you do, you will need to include adjustments for tilt and rotation of the secondary as well as positioning the secondary.  

 

The secondary normally is not quite centered in the tube.  

 

Have you ever looked through a Newtonian?  Collimated a Newtonian?  Focused a Newtonian?

 

There are good reason why integrated focusers and secondary mounts are extremely rare.. 

 

Jon

[TNAllen]

No, I have no experience with telescopes, however, as I explained, my interest is speculative. I am thinking of building a telescope, and therefore curious about the way one goes about building a good telescope, technique, dimensions, and consequently tolerances are important to me.

[izar187]

Here are some often recommended books on telescope making.

 

https://www.thriftbo...edition=3572577

 

https://archive.org/...00brow/mode/2up

 

For larger scopes.

 

https://www.amazon.c...s/dp/0943396557

[TNAllen]

Thank you for the recommendations. I just ordered the first, and recently received the last, which I'll look at more closely tomorrow.

[Oregon-raybender]

If you want to download free copies of the 

Sam Brown books (All About Telescopes)

 

Here is the total list I scanned, just chick on

the ones you want. 

 

Starry Nights 

 

https://archive.org/...out Telescopes/

[Dale Eason]

If you care to see how loose tolerances can be watch John Dobson build the telescope named after him.  It starts with him grinding a mirror but if you want to see the telescope being built go about half way in where he builds one.  His scopes revolutionized how telescopes are made and used and popularized bigger armature scopes because they really don't need much in special building skills.  

 

https://www.youtube....nz7JJlSZvw&t=4s

[TNAllen]

Thank you for both interesting suggestions. I am looking forward to looking onto these, and expect both will provide significant useful information. As you suggest, they will probably also provide answers to my questions regarding practical tolerances.

[Dale Eason]

The tolerances depend on the type of scope and more important the type of mount.  A Dobsonian used for visual work without goto or tracking can get by with 1/16 of inches or worse.  If you want go to  then a bit better.  If you want tracking then usually you are down to .001 inch in some of the mount components.  You can limit where the tolerance is needed like using an equatorial platform to give the Dob some tracking and even those don't require machinist tolerances.