6 replies to this topic

[Tony_Gondola]

I'm looking for DIY designs for reducers, flatteners, coma correctors and such using "off the shelf" lenses.

[Astrojensen]

A reducer is just a plain achromatic lens. An objective from an old binocular works fine. Flatteners and coma correctors are trickier, but I know some people have made them from off the shelf optics, mostly from Edmund Optics lenses.

 

 

Clear skies!

Thomas, Denmark

[Tony_Gondola]

I have played around a bit with a simple reducer like that and once you get the spacing right it works surprisingly well. I have an older 90mm f/10 doublet here that's not doing much. I've considered using a large achromat, something around 60mm or so, permanently installed up in the main tube to have a shorter FL that I don't have to mess with.

[luxo II]

Stellarvue have some spare 80mm doublets - multicoated and mounted, f/9 or f/6 - that would do nicely as reducers for DIY experiments.

 

Other candidates would be binocular objectives, or even finderscope objectives (these are typically f/3.5 to f/4.5 but quality may not be great)

 

The simplest flattener is a weak singlet lens, either plano-convex (refractors, SCT, MTC) or plano-concave (newtonians) close to the focal plane.

 

And if you want the maths and some actual specs, see https://www.telescop...eous_optics.htm

 

At 11.22 the cemented doublet as a reducer is discussed and analyzed.

Edited by luxo II, 15 March 2025 - 11:55 PM.

[brebisson]

Hello,

 

I don't "understand optics"... so I am lost here...

 

But I WANT to be able to make a (cheap) 1.25" coma corrector, out of of the shelf parts, for a couple of 114*450 scopes which have WAY too much comma!

 

Could anyone help me?

 

Thanks,

Cyrille

[luxo II]

Er …that would be wonderful, eh ?
Unfortunately that is not so easy.

 

Such a thing does exist https://www.teleskop...komakorr1-18160

 

However there is an issue which probably makes it impractical on anything faster than f/8. As with all coma correctors it must be a significant distance in front of the focal plane - 75mm in this case. Add to that the length of its barrel and you’ll realise there isn’t enough space without this sticking right into the light path inside the OTA.

 

So, for a total length between focal plane and the front of this around 130mm, with a clear aperture under 30mm, it’s going to vignette everything off axis which could be pointless. The scope may well be better without it.

Edited by luxo II, 16 March 2025 - 02:21 AM.

[brebisson]

Hello Luxo,

 

Ha, crap!!! Yes, on a 114*450 it would be worthless!

 

Thanks for the link neverthless!

 

Cyrille