Skywatcher refractor

[andyboy1970]

I have imaged with my ST80 here are two posts -

http://www.eastmidlandsstargazers.org.uk/topic/3621-m45-with-the-st80/

http://www.eastmidlandsstargazers.org.uk/topic/3617-the-great-orion-nebula-st80-style/

[dawson]

Very nice. Thanks.

I just read a bit about achromatics vs apochromatics. Not sure i understand the technical differences in their design, but i think i understand how their results differ.

Nice images.

James

[dawson]

Sorry, further daft and annoying question:

What is a doublet and a triplet? Can these be both achromatic and apochromatic? Are there any commercially available semi-apo's out there?

James

[catman161]

Hey James, this is a good read

http://starizona.com/acb/basics/equip_telescopes_refractors.aspx

[Guest Ely Ellis]

Hi James,

 

Well, I may be talking out my back end, but I think this is how it goes.

 

An Achromatic has two lens's and therefore is referred to as a doublet.

An Apochromatic has 3 lens's, the extra one for added correction and is referred to as a triplet.

You can buy filters for Acro's, which help to remove the fringing (Chromatic aberration), Baader do a filter which they call a 'Semi-Apo' which is supposed to be so good it makes an acro into something like an apo.

Not sure how well these filters work but there is quite a difference in price between acro's and apo's and I don't think the price of a filter could match the price of an apo, otherwise we wouldn't need apo's.

ED glass is something different and I think it is to help pass more light through the lens with less distortion.

 

I may be wrong in all that, I think PhilJay would be the best person to answer.

 

Cheers

 

Martin

Edited April 14, 2013 by Ely Ellis

[Brantuk]

Doublets have two lenses to focus two colours at the same focal point. Triplets focus three colours. The lenses also do a degree of correction but the field still gets bent cos you can't do both colour and correction perfectly at the same time (due to the nature of glass). So sometimes you'll find field flateners and reducers are necessary additions

[dawson]

Thanks felix and martin. I'll have a read of that link tomorrow. Just been reading other stuff. It's all so convoluted.

Cheers and good night.

James

[dawson]

Thanks kim.

James

[Orion]

I think I read somewhere that there is no chromatic aberration in reflector telescopes because there is no glass where the light passes through (apart from some aberration coming from the eyepiece). In reflectors, the light just reflects from the aluminised surface, so all colours should focus at the same focal plane (is that correct?). If there was no eyepiece, the light would focus on the CCD. I hope I'm right on this.

[Brantuk]

Mirrors come with their own problems like coma, cone spread, and inward/outward focus shift (depending on camera). Also more sensitive to collimation and transportation. Parabolic ones deal with the light best.

[Guest Ely Ellis]

Something I wondered:

I know what a parabolic mirror is, how its made and what it does, but understand that at the very cheap end, there are reflectors that don't have parabolic mirrors, in which case, what do they use and how does it work?

 

Martin

[Brantuk]

Mostly conical mirrors from what I've seen - which I understand shifts the different colours to "close" but different focal points. Parabaloids bring everything together at the same spot (or near enough). But double check me cos I'm no expert - very much a lay understanding lol

[Guest Ely Ellis]

For some reason, I know it sounds silly, but I thought they may have half a glass ball in the bottom. Don't laugh!

It was just the only way I could think of as to how they could reflect light and focus it on a point (the secondary / EP).

I see on reflectors, the letter P which possibly stands for Parabolic, but the ones that done, never mention what they do have.

Ah well, it was just out of interest anyway.

 

Cheers Kim

 

Martin

[Brantuk]

Yup - "P" is for parabolic

[andyboy1970]

Spherical mirrors are widely used in cheap reflectors, to make a parabolic mirror the first stage (fairly quick) is to grind a spherical mirror then continue grinding to a parabola (long slow process).

So a spherical mirror takes about 1/4 the time a parabolic mirror takes to make.

Spherical mirrors work great a long focal lengths as there is little difference between the parabola and spheric shape above about F8.

[Brantuk]

Oh that's interesting thanks Andy - I didn't know that.

[andyboy1970]

Oh that's interesting thanks Andy - I didn't know that.

I was asking a Stelafane telescope maker a few years back and sure enough put the mirror size and F ratio into a wavefront calculator and f8 upwards Spherical and parabolic are almost identical.

[Brantuk]

Some of this stuff has very curious properties - I'll certainly not forget that one

[tuckstar]

http://whuyss.tripod.com/zeiss/presentation.html

Here's some numbers for you. Found this while researching my Zeiss. Hence its a Zeiss website. But gives the measurement differences between an apo and an achro. Hope the numbers make sense to someone. And yes the Zeiss is 0.95 strehl.;-)