Random thoughts that I felt like sharing ...

[vlaiv]

 Here are a few interesting ideas from past couple of days:

- Making eyepiece from old camera lens with aid of 3d printer. Here is needed info on eyepiece design:

https://www.skyatnightmagazine.com/advice/make-a-50mm-eyepiece/

I reckon that 1.25" nose piece - to - lens filter thread adapter can be easily 3d printed. So can eye guard that will go instead of back cover.

- Making DIY telescope requires optical parts. I've stumbled upon interesting source of small diameter achromat lens (up to say 4"-5") that are more than affordable:

https://www.aliexpress.com/item/4000843113053.html?spm=a2g0o.store_pc_groupList.8148356.29.13645edeQEzrMI&pdp_npi=2%40dis!EUR!€ 36%2C98!€ 33%2C28!€ 33%2C28!!!!%400b0a182b16611130503026215ecb8f!12000016656249049!sh

For example - you can choose 80mm F600 lens (same as in this telescope: https://www.teleskop-express.de/shop/product_info.php/info/p7935_TS-Optics-80-600mm-Refractor-Teleskope---optical-tube-with-rings.html) at very reasonable price and free shipping (at least to my destination - much better than those crazy "deals" where you have to pay shipping 2 or 3 times more than the item itself). You get to choose if you want green or blue coating as well 

- 3d printed focuser. Well, there are plenty of examples floating around, but I've noticed that most use regular bearings that are a bit chunky in that role.

Searching around for good bearings for that role came up with this:

Look up FF 2010 or FF 3020 (difference is size - former are suitable for smaller 1.25" focusers, while later are more suited for 2" with longer draw tube).

 

 

 

[DaveL59]

That's very interesting, plenty older M42 lenses about that can be had for peanuts, tho some may be best avoided, the Meyer-Optik Domiplan 50mm f2.8 for example. Its a dinky little lens but this one you need to centre the rear element cell using 3 set screws to get best image, tho they are often very cheap, possibly why. Also not great in infinity focus as the one I have just doesn't quite get there. Many of these older M42's may be manual/pre-set aperture which makes them even simpler to use, no aperture pin to poke you in the eye 😉 

I'd considered Ali for objectives when I was looking for a replacement for the vintage 3-inch scope but they didn't have anything in the 76/1000mm range, certainly a good low-cost source if they perform well optically.

Edited August 21, 2022 by DaveL59

[kbrown]

Just out of curiosity I tried my cheap Canon 50mm f/1.8 as an eyepiece. I think I managed to attach it to the 2" focuser directly without adapters. It worked great but hasn't had much use since I don't really do visual observing.

[vlaiv]

3 minutes ago, kbrown said:

Just out of curiosity I tried my cheap Canon 50mm f/1.8 as an eyepiece. I think I managed to attach it to the 2" focuser directly without adapters. It worked great but hasn't had much use since I don't really do visual observing.

Which way around did you try it?

Article says that front of the lens should go inside focuser and that back of the lens should be eyepiece side, but I wondered what would happen if we reversed lens.

[markse68]

16 hours ago, vlaiv said:

- 3d printed focuser. Well, there are plenty of examples floating around, but I've noticed that most use regular bearings that are a bit chunky in that role.

Searching around for good bearings for that role came up with this:

Look up FF 2010 or FF 3020 (difference is size - former are suitable for smaller 1.25" focusers, while later are more suited for 2" with longer draw tube).

 

Hi Vlaiv, how would you keep a loose bearing like that in place? It would have to roll between 2 surfaces and would creep. I think you would need something like this? https://uk.rs-online.com/web/p/linear-slides/0749301?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Bearings_%26_Seals_Whoop-_-Linear+Slides_Whoop-_-749301&matchtype=&aud-827186183726:pla-297200285488&cq_src=google_ads&cq_cmp=12814832524&cq_term=&cq_plac=&cq_net=g&cq_plt=gp&gclid=Cj0KCQjw0oyYBhDGARIsAMZEuMsJi-ZKYkvamLmxeH5wGLFTvPyEPTYPyhL-mHI0b2LS8l2b8r6aDBEaAnjAEALw_wcB&gclsrc=aw.ds

Mark

[DaveL59]

16 minutes ago, vlaiv said:

Which way around did you try it?

Article says that front of the lens should go inside focuser and that back of the lens should be eyepiece side, but I wondered what would happen if we reversed lens.

Would be a bit like looking thru binos in reverse, the image would be tiny if perfectly formed. At least looking thru from the back/camera/film end you'd get a lot of eye relief, given the lens to film plane is around 43mm generally.

Actually I'll correct that having pulled my 50mm F1.7 out to check. Looking in from the camera end you get a larger image. In reverse is about 1:1 or whatever macro-scaling the specific lens is capable of. In fact photographers do mount lenses in reverse for macro work which I'd forgotten for a moment when I first posted 🙂 

Edited August 22, 2022 by DaveL59

[vlaiv]

1 hour ago, DaveL59 said:

Would be a bit like looking thru binos in reverse, the image would be tiny if perfectly formed. At least looking thru from the back/camera/film end you'd get a lot of eye relief, given the lens to film plane is around 43mm generally.

Actually I'll correct that having pulled my 50mm F1.7 out to check. Looking in from the camera end you get a larger image. In reverse is about 1:1 or whatever macro-scaling the specific lens is capable of. In fact photographers do mount lenses in reverse for macro work which I'd forgotten for a moment when I first posted 🙂 

I think that lens is lens - it has same focal length from front and from rear so it can't really provide you with different magnification depending on which side you choose to look at.

Only difference is in correction - sharpness.

Lens is usually best corrected for some distance - telescope optics is corrected for infinity, but most lenses are not, but are rather corrected for some sensible distance like - 5 meters or so.

Macro lens are corrected for closer distance.

If you think about it - eyepiece takes diverging light rays from very close focal plane and turn those into collimated beam - like this:

Telescope focal plane to the left and exit pupil / our eye on the right.

Camera lens works "in opposite" direction:

here light goes from right to left - object at the distance to the right (at infinity - we get collimated rays) and then it converges in camera body towards the sensor (on the left).

If you look at rays in both images - it stands to reason that we need to replace sensor with focal plane of telescope and object at infinity (or far away) - with our eye.

We should be looking at the front of camera lens and turn back side (which sits in camera) towards the scope.

In linked article - it is other way around, and that is why I asked if there is difference (maybe a bit of sharpness due to spherical correction or something).

[vlaiv]

1 hour ago, markse68 said:

Hi Vlaiv, how would you keep a loose bearing like that in place? It would have to roll between 2 surfaces and would creep. I think you would need something like this? https://uk.rs-online.com/web/p/linear-slides/0749301?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Bearings_%26_Seals_Whoop-_-Linear+Slides_Whoop-_-749301&matchtype=&aud-827186183726:pla-297200285488&cq_src=google_ads&cq_cmp=12814832524&cq_term=&cq_plac=&cq_net=g&cq_plt=gp&gclid=Cj0KCQjw0oyYBhDGARIsAMZEuMsJi-ZKYkvamLmxeH5wGLFTvPyEPTYPyhL-mHI0b2LS8l2b8r6aDBEaAnjAEALw_wcB&gclsrc=aw.ds

Mark

Good point. I was thinking of making a slot in 3d printed part for that plastic housing to sit in. Rollers won't make contact on the other side - but you are probably right, these are made to slide rather than to sit in single position.

Maybe groove could be made for them to "float" back and forth as draw tube moves. They would only move half of draw tube travel. Will need to think about it.

They are less than euro each, so I'll get few of them just for fun and to try out things.

[kbrown]

2 hours ago, vlaiv said:

Which way around did you try it?

Article says that front of the lens should go inside focuser and that back of the lens should be eyepiece side, but I wondered what would happen if we reversed lens.

It was a long time ago but I'm pretty sure it was the front of the lens to the focuser. I didn't even think it could work the other way as well...

[Alien 13]

There are a few eyepieces out there that disguise there previous life as CCTV camera lenses by the fitting of a sleeve, some are zooms too. The lens is always mounted with objective end towards the focuser.

Alan 

Edited August 24, 2022 by Alien 13