CA, Detail and Contrast

[Regulus]

Questions
1. Is detail lost in a scope that suffers from Chromatic Aberration?
2. Does it effect Contrast while viewing or photographing the Moon or Jupiter for example?
And,
3. Does using a Correction Filter restore the Detail/Contrast, or just block the Red/Blue ends of the spectrum?

Trevor

[casstony]

A slow f/11 4" achromat will give quite nice lunar and planetary views if the lens is well figured. The united optics scopes are rebranded by all and sundry and have a reputation for good quality.
For imaging an ED scope would be more suitable.

[ericwbenson]

Hi Trevor,

CA means not all wavelength focus at the same plane. Defocusing a telescope decreases contrast and detail by spreading out the psf (point source become blobs), and so does CA in that you can never focus perfectly. By how much is what you need to know to make an objective call. So yes for 1)

2) yes when viewing and possibly not if imaging because you could possibly refocus between filters using a mono camera and filter wheel. But yes it would affect DSLR and one shot color. Also I don't think many people refocus between colors on Jupiter (or even use FWs), just not enough time.

3) The correction filters that block the extreme ends of the visual spectrum do increase contrast/detail by blocking the light that doesn't focus properly at a cost of less overall light transmission. But not a magic bullet though since scopes with CA are likely to have other optical defects, IMHO.

Best,
EB

[brian nordstrom]

:question: Great question's Trevor and as a primarily visual observer my self and having owned a very nice NG 127mm f7.5 ( 950mm fl ) triplet and now a very , very good Istar 127mm f8 doublet achromat ( 1000mm fl ) and I would say the difference between the 2 was suggestive , eg. different people have varying tolerances for CA in particular .
Both have awesome SA correction and I have had people comment on the lack of CA seen in the Istar , and even a friend in Darwin who owns a TV NP127 asked if my Achro was a triplet the first time he seen Venus thru mine , high praise .

On contrast the tube design , blackening , baffles and their placement predict better contrast in a refractor than the lense its-self ( given that's it a well made and designed one first ) .

I have used most CA reduction filters in the past and find that visually they work very well but like CA its self I don't like the yellow/green tints they impair , to me an ugly un-natural hue on the moon and planets , I prefer the slight CA and find after a while I don't even notice it .

I have a 'Chromocorr'd 150mm f8 Saxon refractor in NZ that my nephew is using and the correction for CA and more importantly SA is marketly in another league , as good as a good ED 150mm doublet , but at a cost of time and money ( if you can find one ) ..

Back to personal preference on CA alone , I find my Istar is as good at showing any details on the moon , especially on Jupiter as good as the NG triplet was , yes the triplet is basically CA free but details seen on the giant planet are basically identical at 300x or there about's ( 3mm TV radian ) in both and I did have the chance to test these 2 x 127mm refractors side by side a few times before selling the triplet .

If CA bothers you grab an APO ( at high $ 's ) , or newtonion , or SCT ;)/ if a bit does not any of the 100-150mm f8-f12 refractors available today are awesome value for money at prices un-heard of only 10 years ago , we are so lucky these days .:thumbsup:.

Brian.

[Regulus]

Thx folks that's helpful.

Brian these 'Chromocorr' are made for a specific scope brand and particular sizes, aren't they? Like Synta style 100mm and 150mm.
Do you find it gives ED performance in a fast Achro and, Is there any light loss using one?
I know they can cost in excess of $500, but are they worth the outlay?

Trev

[Kunama]

Hi Trevor, James Ling on CN has been working with Valery (who made the Chromacorrs) getting one set up for is huge Istar refractor and it makes for very good reading. From my reading, it improves things on axis. My searching for one came up empty when I had the Istar 150mm F10. I found that a Baader SemiApo filter did improve things quite a bit but not anywhere near true apo performance.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5921753/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

[Regulus]

Thx Matt, that was an interesting read.
Trev

[brian nordstrom]

:) Hi Trev , I found zero loss of light when fitted to my Saxon 150mm f8 but that scope is still in NZ , I had my 150mm f5 up in Darwin so sadly could not try it in that scope .
But yes they work very well , just very fiddly to get the spacing between chromocor and focal plane exactly right it has to be 161mm's from memory and I destroyed many cheep filters ( removed the glass) trying again and again and again ,,,,, over weeks to get it just right but when it was on ,, man you knew as the views are excellent now .
Brian.

[MattT]

f12 is where achro's start for me Trevor. Very little in the way of CA etc in my ATM Istar 6" f12 achro. A Fringe Killer cleans the view up if needed, not that it really does. The view of Jupiter, which I regard as a good test of an achro, is very much like my 100ED f9 scope, just as white but brighter with a tiny violet fringe.
As Brian says well placed baffles make a difference, as does a long (3x the objective size) dew shield. All adds up to a jet black interior.
If I ever want to change it to an f15 it's buy a f12 R35 lens from Istar and replace the f12 achro lens…easy.

Matt

[AG Hybrid]

I find that the Baader Semi-Apo filter cleans up CA on my 102mm F6.5 Achro quite nicely. Id say ~80% reduction on Jupiter. It does add a gentle yellow tinge though. But that's alright with me. I find without it Jupiter is almost non-observable in my opinion. But, with the filter, the CA is mostly removed I can I see subtle details in the colorbands I couldn't before. It really is a difference maker.

With the filter CA is on, objects like m45 is from almost, to approaching, practically invisible(Does that make sense? That sentence makes sense in my mind). The filter is attached to the diagonal all the time.

Mind you to be balanced. I have not tried it at a dark site yet where star brightness is turn up to 11.

[Regulus]

I laughed. It did make sense once I rearranged a few word :-)

I am looking at a 100 f11 and wondering if I will be overly disappointed with CA on bright planetary, and Lunar viewing.
I understand the fringe-killer filter is quite good and was wondering how much they effect contrast (eg Jupiter cloud bands and Saturn's rings).

I am surprised to read that a large f6.5 achro isn't badly plagued by CA and can only think the lenses must be very well made and coated.

The 150 f5 that Brian had was a demon for CA, extreme on the Moon and Jupiter/Venus etc.
If, as I have seen on some 100MM achros, the fringe is minimal then I think I will buy it since the price is good. It's just to bad that I cannot try before I buy, but that's a common problem everywhere - and worse in Tasmania.

It would still work photographically using RGB filters and Ha OIII

Thx everyone for you comments

Trevor

[MattT]

Fringe Killer v Semi -apo.
Had both. The semi-apo gives a more natural coloured image but dims quite noticeably.
The FK gives a slight yellow tinge and is a brighter image than the semi-apo.
I kept the FK.
Matt

[FlashDrive]

That's interesting...I have a 2" Baader Fringe Killer which I use ' sometimes ' on my Vixen NA140 f/5.7....( mostly on a very bright Moon )....there is a ' wafer thin ' green/yellow ring of colour surrounding the Moon.....BUT ...you have to deliberately look for it.....it is so insignificant that it is no distraction at all.
For the views this Scope gives, I am not at all ' picky ' about an ever so slight bit of CA.

Anyhow.....I have a Semi-Apo Filter heading to me in the Post....I shall make a comparison and see how it goes.....these are a good filter and have a necessary part to play in our hobby.

Col.....:D

[AG Hybrid]

I have both actually. I prefer the view with the Semi-Apo. The fringe killer's yellow tint is too strong for the and moon and planets imo.
If I want to a brighter image to see deep I use the 12".

[brian nordstrom]

;) Yes Trev the 150mm f5 had CA , but to me I found its sharp views of Jupiter at about 90x made up for it , very sharp otics that one .

I had a feeling you were looking at that f11 , 4 incher . Nice scope .
I beileve it would give very good luna planetary views with minimal CA , especially if you choose your eyepieces carefully and use a prisim (Tak or Zeiss ) diagonal , Orthos will show CA quite bad ( but super sharp ! ) , Plossls will be better,
but in my opinion TelVue Radians are the best high power eyepieces made for when you want the best natural colour free views thru an achromat .

Some say they impart a yellowish ting on bright objects when using a reflector , but that's a good thing , again my opinion , I love mine in my 127mm Istar , very clean , natural views .

Good luck Trev on what you decide .

Brian.

[Regulus]

It's been interesting reading these posts, and I thank you all for sharing.
Esp. interesting they way different people seem to see a bit differently and opt for one or other of the anti-CA filters, so it appears that a personal test is required to see which better suits me. Since I am one of those who see green in the Great Orion Neb it will be interesting finding out how me eyes respond to the filters.

Brian I have been considering the Radians as perhaps the best 'budget' quality ep so I take your advice as reinforcement of my own decision.
I have been considering that UO 100/1100 refractor as perhaps an excellent mid sized achro that would make reasonably easy viewing over a shorter scope on an eq mount - at least I can sit down more, rather than bend over most of the time :-). And it seems to be a reasonably well made scope/fittings, tho I am not certain the diagonal comes up to the quality of a TV or Zeiss (nothing much does except another TV or Zeiss). So that will be an additional cost - but what's new there?

I reckon I will be on the market in a few weeks time for a FK or Semi-Apo if the CA is too annoying.

Thx again for you input guys.
Trev