DIY Refractor

[kbrown]

So I've decided to have a go at making a small refractor telescope mostly with what I already have plus some custom 3D printed and/or CNC milled bits and bobs. Peter from The Astronomy Centre was kind enough to donate the 72mm objective for this project which I'm ever so thankful for.

I've only got as far as printing a lens cap and digging around my surplus boxes to see if there's anything I could use. Here's what I've found so far:

The focuser is a bit big for this but it'll have to do for now. All I really need is a tube of the right length and I can start testing. What is the right length then? Well here's where I hoped you guys could help a little. Ideally I'd like to be able to do both visual observing and imaging.

I know I need to have enough back focus to be able to get the CCD sensor in the right spot with any optional stuff between the camera and the focuser. But where exactly is the sensor in focus?

I was looking at this picture and got a little confused: https://en.wikipedia.org/wiki/Refracting_telescope#/media/File:Kepschem.png

The eye piece seems to be behind the prime focal length (f1 + f2). Is this because of the design of the eye piece itself? Surely the CCD doesn't go there as there's no refracting lenses? But what is the correct position for it? Slightly in front of the prime focal length depending on the size of the sensor?

[Sunshine]

If only you could print up a set of triplet objectives! 

[Dragon_Astro]

Maybe you could ask Peter at the astronomy centre if he has an idea what the focal length for the lens might be? Guessing it's an achromatic objective he's given you, so at a guess it might be around 700mm-900mm?

you could do mock ups with cheap postal tubes, cutting them  to various lengths and test when things come into focus.

[johninderby]

The postal tube idea is a good cheap one to experiment with the tube length or you could go for two pieces of aluminium tube that telescope together.

[Cosmic Geoff]

20 minutes ago, johninderby said:

The postal tube idea is a good cheap one to experiment with the tube length or you could go for two pieces of aluminium tube that telescope together.

I was going to suggest a cardboard tube to start with. You can estimate the focal length of a lens by holding it up to a window so that it projects the scene outside.

[kbrown]

3 hours ago, Dragon_Astro said:

Maybe you could ask Peter at the astronomy centre if he has an idea what the focal length for the lens might be? Guessing it's an achromatic objective he's given you, so at a guess it might be around 700mm-900mm?

you could do mock ups with cheap postal tubes, cutting them  to various lengths and test when things come into focus.

He actually did measure it to be 420mm. I'm just trying to figure out where in that optical path the CCD will be in focus so I can make most of the focuser's travel to incorporate both visual observation and imaging. Is the CCD exactly at 420mm or is it closer to snugly fit within the light cone due to the size of the sensor? Hope that makes sense...

I'll do some mockups anyway :)

[Stub Mandrel]

If you fancy having a try at 3D printing a Crayford focuser, I've put my design on Thingiverse, with full build instructions.

https://www.thingiverse.com/thing:3051240

 

[kbrown]

1 hour ago, Stub Mandrel said:

If you fancy having a try at 3D printing a Crayford focuser, I've put my design on Thingiverse, with full build instructions.

https://www.thingiverse.com/thing:3051240

 

Saw this earlier in another thread. Looks really good. I'll probably save the time and use the one in the photo at least for now. I'm anxious to get this in a working order

[Stub Mandrel]

Makes sense to be honest, it looks decent quality.

[JamesF]

If it's a 420mm focal length, surely the camera sensor will have to be 420mm from the objective?  If you're thinking about a DSLR then the flange to sensor distance will probably be somewhere in the 45mm to 50mm range, so you're probably looking at a maximum of 370mm from the lens to the camera flange.  Might be sensible to allow another 30mm to 40mm for extra fittings and suchlike?

James

[kbrown]

2 hours ago, JamesF said:

If it's a 420mm focal length, surely the camera sensor will have to be 420mm from the objective?  If you're thinking about a DSLR then the flange to sensor distance will probably be somewhere in the 45mm to 50mm range, so you're probably looking at a maximum of 370mm from the lens to the camera flange.  Might be sensible to allow another 30mm to 40mm for extra fittings and suchlike?

James

I don't know what sort of brain fart I was having but having had a look at a few other schematics on how the light travels through the lens I now get it. Focal length is focal length. That's where the focal plane is where the light from infinity is focused on. Anyhoo... moving on

[JamesF]

Happens to us all :)

James

[Chris]

Yes it will be 420mm to the sensor then, but if you want it for visual too you'll need a bit of extra inward travel and even more off for the distance of the star diagonal. I can't remember how much a star diagonal adds, but it will be 420mm minus the distance of the star digaonal, minus a bit extra so you can travel through both sides of focus. 

Also if you go too short you can always use extension tubes, and if you go too long you can always cut more off the tube  

The suggestion of using cardboard tube to test is a good one.

[JamesF]

The ST80 has a focal length of 400mm, so modelling it on that and adding 20mm to the length might be a handy place to start.

James

[kbrown]

My thinking now is that since I (probably) can't print the tube in one piece anyway I'll split it into three sections that I can secure together easily. Front piece will be holding the objective, back piece will be holding the focuser. I'll design them so that I can temporarily attach them on a long dovetail to find the optimal final length. Then I can print the middle bit to the correct length.

This would then allow me to swap out the objective for a longer focal length if I so decide to do. I'd only need to extend the middle bit and/or the front bit if the new objective would not fit straight in.

[JamesF]

If you're splitting the tube into multiple pieces it might be worth thinking about an adjustable cell in case it is awkward to get the axes of the lens and focuser concentric.

James

[kbrown]

40 minutes ago, JamesF said:

If you're splitting the tube into multiple pieces it might be worth thinking about an adjustable cell in case it is awkward to get the axes of the lens and focuser concentric.

James

Should be fairly concentric if I do tapered couplings I'd think.

[Stub Mandrel]

My advice is flat shoulders for positive location, joined by tube in tube with a good overlap printed so internal and external nominal diameters are virtually identical, the printed texture will provide lots of grip.

[Gina]

I've used that principle too - for other things.

[kbrown]

Here's what I've done in FreeCAD so far:

[Stub Mandrel]

Excellent!

[kbrown]

First bit printed. My printer made a mess of the supports and I ended up doing a lot of post cleaning but the part itself is okay. 

I'm now thinking should I buy a star diagonal or make one myself? It's essentially just a mirror at an angle, right? I don't do a lot of visual observing so I guess it wouldn't have to be a top of the line thing. Maybe I could use a small secondary mirror from a newt... 

[Stub Mandrel]

Astromedia sell front silvered mirrors quite cheaply.

[Chris]

Looking excellent, well done so far!

[kbrown]

Long time no updates. This is because holidays got in the way ?

I managed to print the rear and front tubes so I could do a temporary assembly to find out the final length of the scope. I'm leaving enough back focus so I can stick my CCD and filter wheel to it. This won't quite give me enough outward travel on the focuser for visual use but I'm hoping the star diagonal I've ordered will be of enough extension. If not then I'll just print an extension tube. 

With this setup I was able to get my first images (of some lights on a crane a couple of miles away). Not sure if it was due to bad collimation or something else due to the flimsy temp setup but I was getting quite a bit of light scatter if that's the right term. Basically a glow around the bright lights. It could also be due to the objective not having any sort of AR coating a far as I can tell. I'm thinking of taking it to scientificmirrors.co.uk. They are able to do standard broadband coating which they claim to bring the reflection down to 2%.