The need for APO

[butlermike]

Hi all,

Being fairly new to all this imaging (and astro generally) I was wonder if an apo chromatic lens is needed for good images of DSO's? I'm more interested in those than planets or lunar imaging but wondered if the CA meant that you would get fringing on stars in images?

considering you can get 120mm vs 80mm for approximately the same price is the CA going to effect DSO's?

thanks

mike

[steppenwolf]

CA does indeed mean that you will get colour fringing on bright stars so an APO is required for the best quality deep sky images. However, you don't have to spend a fortune - some fantastic colour aberration-free images can be taken with doublet lenses that use ED glass and an 80mm refractor is a great point to start from. An 80mm ED doublet will take MUCH better quality images than a similarly priced 120mm achromat.

CA will spoil your images so do all you can to avoid it if you want to capture deep sky objects.

[butlermike]

Thanks for the reply and info.

i thought that might be the case, I take it you don't get noticible CA with reflectors?

does anyone have links to images that might compare APO scopes to achromatic scopes, just out of interest.

[steppenwolf]

I take it you don't get noticible CA with reflectors?

You don't get any CA with a reflector - CA is caused by diffraction and a reflector works by, well, reflection.

[StarSteve]

<snip>

An 80mm ED doublet will take MUCH better quality images than a similarly priced 120mm achromat.

Hm, similarly priced? Which 80 ED APO would that be?

A SW 120mm F5 achro is 259 Euro.

The cheapest SW 80mm APO I can find is 417 Euro, hardly similarly priced.

In fact I haven't seen any 80mm APO at around what the 120mm achro costs.

I'm not arguing that a 80 APO wouldn't be better for imaging than a 120mm achro (I know very little about AP so I will take your word for that), but I don't think it's correct to say they aren't any more expensive.

Steve

[StarSteve]

Actually reading your post more closely, I think I misread you (wish I could edit ).

You were probably saying that a 120mm for the same money as a 80mm APO cost would still not produce images as good. Not that there are 80mm APO for what the cheapest 120mm achros cost.

My bad, forget what I wrote

Steve

[butlermike]

Thanks for advice. Just saw from your sig who you are Steve (steppenwolf Steve that is), I'm enjoying your book!

[laser_jock99]

You don't get any CA with a reflector - CA is caused by diffraction and a reflector works by, well, reflection.

Unless of course you have some kind of lens in the system (e.g. an eyepiece, coma corrector, corrector plate or balow lens) which if badly designed could introduce some CA. But yes, in principle mirror based telescopes are apochromatic by nature.