The difference between an Achromatic and an Apochromatic

[Darth Takahashi]

Whilst I'm current working overseas and have no opportunity to do any imaging at the moment I thought I'd organize my photos. I came across some older photos made with a Vixen 120N neo-achromatic telescope. This telescope is an achromatic type with a built in field flattener which is perfect for imaging, or so I thought. Unfortunately, it didn't live up to my expectations.

You see a lot of discussion regarding the two different type of refractors, however, they say a picture is worth 1000 words so lets give it ago!

Vixen 120 NEO-Achromtic Telescope

(click to enlarge)

TAK TOA130F Apochromatic Telescope

(click to enlarge)

Big difference! Yes the exposures are not the same and there is a tracking problem with the first photo. However, this still nicely demonstrates the difference between the two types. You don't need to buy a TAK to see the difference, any well correct doublet from Vixen, Borg, WO & others will do.

Yes Vixen do make good telescopes its just that I personal wouldn't list this version amongst them!

Chromatic aberration, the purple hazes that appears around the stars is unavoidable went imaging. The build up of the signal also means an equal build up of purple haze. Normally when discussing chromatic aberration people talk about viewing Venus or the limb of the moon because its only detectable by eye on the brighter objects.

So the moral here is if you intent to take this hobby further and into imaging you might as well save a little more money for Apochromatic telescope. I move from this telescope to a WO Zenithstar 110 and was very satisfied. Its was only some years later when I started working in Japan that I had the opportunity to purchase my dream scope, the Takahashi TOA130F.

I hope this helps which the most critical decision that you will make in this Hobby.

Clear Skies, Neil.

[John]

Thanks for that report Neil. I've read somewhere else (Cloudynights I expect) that the Vixen 120 neo-achromat was not that well colour corrected. There is a marked difference in your photos - I wonder if it would be as marked visually ?.

I have a Vixen ED102SS which is pretty well corrected - it's a plain F/6.5 ED doublet but Vixen have done well to control the colour as well at that focal length I feel.

John

[Whippy]

Thanks for that Neil but John does have a good point. I recently sold a TAL 100R (4" f10 achromat) and saw very little false colour (if any) whilst observing but I didn't even bother to try and image with because it'd show all the colour that your Vixen shot shows. The Skywatcher/Celestron/Orion ED80/100/120 scopes show that you can have a capable scope for imaging and observing for relatively little outlay but if all you're interested in is visual and you have a tight budget, perhaps an achromat may well serve your needs?

Tony..

[GazOC]

I think the Vixen neos were achromatic Petzvals, they should be slightly better colour corrected than a doublet of the same aperture/ focal ratio but nowhere near an apo or ED scope. They should also show a flatter field than and apo or achro doublet.

[Darth Takahashi]

I believe that the main point we agree on is that yes an Achromatic telescope can work well visual so long as the focal ratio is not faster than approx F9/10. Anything shorter than this needs a much higher degree of colour correction. Again, personally I believe it necessary to use ED glass for shorter focal ratio's such as F8/7 or lower etc...

The Petzval design helps to flatten the field but it does nothing to improve the colour correction and at F6.7 these telescopes are just too fast for their own good.

That's a real pity since these telescopes are obviously aimed at imaging. Perhaps a minus violet filter would have helped, in the end I went down another route.

Neil.

[GazOC]

The focal ratio needed to control CA also depends on the aperture. An old rule of thumb for achros used to be a focal ratio of 3 times the aperture in inches was acceptable and 5 times the aperture in inches was pretty much colour free.

[canuck]

Hmmm...I think this test is a bit hard on the achromat.

As has been pointed out, the colour issue will always be present with an achromat (unless it's a reaaallly long one), so it's not unexpected to see it on this shot. Comparing different shots with differing exposure times and a tracking issue is an uneven comparison.

What would be very interesting, is to see pictures taken one after the other, using the same mount & camera, compensate exposure times (or numbers) so the differing focal length of each scope has enough time to burn in similar detail.

A challenging test to do, and who has this kind of time? :scratch:

Anyway, I still wouldn't recommend an achromat if imaging was a possibility, but they do have their place. I love the star images through quality, long focal length achromats.

[Tiny]

Nice article in this month's (feb) Astronomy Now on this and the various glass types used.

[MartinB]

I wouldn't bother with an achromat if using a colour CCD/DSLR however, they can be ok with monochips. That Vixen has quality optics apart from the CA. So you could mono image using a green filter, or you could do RGB adjusting the focus for each filter. A clear luminence filter wouldn't be good. It would certainly be a great scope for narrow band imaging

[Ben Ritchie]

It would certainly be a great scope for narrow band imaging

I don't know the answer to this for sure, but i've always wondered about this statement - it seems to me that spherochromatism is going to be an issue, especially for H-alpha and SII which are quite far to the red while an achromat designer is probably going to optimise for the green.

So for narrowband with an achromat you're obviously not going to get longitudinal color (because it's narrowband!) but for H-alpha it still seems to me that you'll potentially suffer from spherical aberration? i.e. your image could still be quite soft in H-alpha, even though there's no false colour.

But I don't know for sure, and i'd be interested if anybody does.

[MartinB]

Streuth mate, got me there :shock: You been reading a fancy optics book Ben?

[GazOC]

If the scope is well designed I don't think SA nessarcerily] need be a concern if you focus to the wavelength you are imaging at.

[Ben Ritchie]

Streuth mate, got me there :shock: You been reading a fancy optics book Ben?

Too much time in University ... problem is I always fell asleep in Optics

[Brass n glass]

I once had a very, very old Dollond four-inch f15 achromatic that showed far less false colour than the four-inch f10 Vixen (not neo) that I bought in 1997. Venus in the Vixen was like a bright Christmas tree while with the old brass banger it was a consistent rather dull light grey without the Vixen's flashing reds and blues - Hugh.

[Darth Takahashi]

That's right, there are always going to be trade off made to achieve faster optics. F15 is fine for visual use on bright objects but its of limited use when imaging. Coming back to the point regarding focusing for each colour RGB or narrow band, I think you could only do that automatically. Its difficult enough to achieve the correct focus for one image so doing it 3 three times or more would be problematic at best?

Here is another image taken with the Vixen Neo N120.

(click to enlarge)

Neil.

[Whippy]

Coming back to the point regarding focusing for each colour RGB or narrow band, I think you could only do that automatically. Its difficult enough to achieve the correct focus for one image so doing it 3 three times or more would be problematic at best?

Erm, I would imagine anyone with as mono (including me) has to re-focus when they're doing RGB even if their filters are parfocal just to be on the safe side. I know I do with both my refractors (one triplet apochromat and one ED doublet) and I don't find it a problem at all, it takes a couple of minutes per filter maximum.

Tony..

[GazOC]

Out of curiosity Tony, how far are the 3 focus points from each other on the ED doublet? Are they evenly spaced?

[Whippy]

There's hardly anything in it Gaz. I think the Blue and the Green are virtually in the same place whilst the Red is a little off but it's hard to tell when you're moving the focuser in and out whislt looking at the monitor at the same time. I'm 90% certain you wouldn't notice any colour visually with an object in focus unless you really looked for it.

Tony..

[GazOC]

Cheers Tony, blue and green being close and red a little further away ties in with what most people seem to be saying about the ED120.

[DaveGibbons]

"The difference between an Achromatic and an Apochromatic".

Hundreds of £/$, visually not as much difference as you might think .

cheers

Dave

[Andrew*]

"The difference between an Achromatic and an Apochromatic".

Hundreds of £/$

I agree Dave,

take Skywatcher's for example:

120mm f/8.3 Evostar - £189

120ED PRO f/7.5 - £949

I know there is a little difference in f/ratio and the ED comes with an extra bell or whistle, but still the price difference is more than 4x! And that increases by 4x again when you get into the serious apos.

Andrew

[canuck]

"The difference between an Achromatic and an Apochromatic".

Hundreds of £/$, visually not as much difference as you might think .

8)

[Darth Takahashi]

To be fair I think there is a little more to it than these simple price comparison would lead you to believe. Such as the quality of the glass and the figuring / polishing of the lenses etc... However that said there is currently too much of a premium loaded on the top end APO's.

It would be nice to see a few more images uploaded from the group demonstrating the qualities of the different refractors available today. Perhaps we could all agree on a particular target and exposure time etc... I know it quite subjective since the camera plays an important part, however there must be enough of us using a DSLR of one kind of another?

What do you think?

Neil.