Why the purple halos?

[KISA]

Hi,

I've only just started playing with my modded 350D on my AOE 80ED and had a go at the Eta Carinae last night. The picture I've attached is the unprocessed raw which shows the noticeable purple halos.

The 350D has been modded by simply removing the IR filter, so I'm using a Hutech IDAS LPS filter as its replacement. (Hutech has informed me the filter completely blocks NIR up to 1000nm and will work as a UV/IR blocking filter for the camera.)

My AOE 80ED is a ED Doublet Fully Multi-Coated Refractor. (http://www.aoe.com.au/ed80.html)

So my question is; why do I have the purple halos? Apparently they are caused by Achromatic Refractors, excessive IR or I think poor focus. I have an ED/APO refractor and my IR is filtered so I’m a bit lost. I did spend a lot of time trying to the focus it correct, but that’s not to say I succeeded.

Any suggestions would be much appreciated, I’m hoping it’s my inexperience and not my scope :shrug:


Thanks

Markus

[Geoff45]

I expect it's chromatic aberration. Try using a minus violet filter (http://www.nexstarsite.com/Reviews/BaaderFringeKiller.htm)
Geoff

[[1ponders]]

Yep, definitely CA. :(

[KISA]

Thanks for that Geoff and Paul. The spec’s for my scope say the ED glass doublet optics should virtually eliminate chromatic aberration. Is the purple halo I’m getting considered to be a virtually eliminated chromatic aberration or has my scope been falsely advertised? Or (and still hoping) is it possibly something else:confuse2:.

Thanks,

Markus

[spearo]

While i agree a UV filter will indeed resolve a problem of residual CA associated with the refractor,
i get a sense that the color balance is a bit off in the raw.
Typically, one would go into PS select a few points including one on a star one KNOWS is supposed to be white and another point into an area that should be black
Then adjusting levels for each RGB ensure the white point hits 255 on each (careful dont go past it the number wont go higher than 255 but the impact of stretching the point will continue)

I think you might find the effect less pronounced after doing this
hope this helps

frank

[[1ponders]]

It may not be the scope. Do you have a focal reducer or anything else in the train that may cause the problems?

[KISA]

Thanks Frank, I'll give PS a go and see how it comes out.

Can I get anyones opinion regarding the statement made my AOE about their/my 80ED:-

"With superior air-spaced Extra-low Dispersion (ED) glass doublet optics that virtually eliminates chromatic aberration to deliver true color and edge-to-edge sharp images."

Is it true based on the CA I'm getting? If it is not performing anywhere near the statement (and it is the scope), I'll seriously consider returning it and getting a Saxon ED80, which is cheaper and from the quality pictures Mike Salway produces shows next to no CA (If you read this Mike please let me know if that's true or you've just processed it out).

Thanks,

Markus


Sorry Paul, missed your reply, there is nothing else in the train; it's just the scope, LPS filter and modded 350D

[[1ponders]]

I hope it's not the scope and it's something else, but if that's virtually eliminated chromatic aberration then I'll stick to my Orion ED80. :(

[AlexN]

I would definitely give it a go without the LPS filter before jumping to conclusions.. I've seen similar halo's come out of APO's due to reflective filters..

Give it a go without the filter, see if the problem persists, if it does, then I would definitely be finding out what can be done with regards to returning it... as that is a pretty serious case of CA...

[KISA]

Here is an image I took previously without an LPS filter.

It looks about the same to me, so I guess it the scope :(.

[Kal]

An ED doublet will reduce chromatic abberation very well for visual, but for digital photography the camera has extra sensitivity at wavelengths the eye can't see. Since they don't implicitly state FPL-53 (not that the glass type alone makes a refractor, but it does play an important part) I would assume it is based on an FPL-51 design. To remove CA an FPL-53 doublet, or an FPL-51 (or better) triplet will be better suited.

[KISA]

Thanks for the info Andrew, I'll try and find out the specifics for the scope from either AOE of York Optical (they sell the same scope) and see where I stand and what my options are.

Cheers,

Markus

[allan gould]

Markus
Unfortunately as a beginner you have believed the advertising hype about ED glass refractors. When they call it an apo and "virtually eliminates" colour, beware. An apo should not show any colour visually and only an extremely small amount (if any) on a long exposure photo which would include bright stars.
My latest purchase was made under the condition if it did not come up to my definition of an Apo and the claims of the vendor - it would go straight back. Fortunately I bought from a vendor whose opinion I implicitely trusted and I was delighted with my scope. It was all that was claimed and more.
To be honest from your photo it would appear that your scope has the amount of colour that I would expect to see in a very well corrected achromatic doublet.
But thats my opinion and may not be worth beans.

[KISA]

Thanks Allan.

Very true, the issues with being new can certainly be frustrating and painful :(.

I suppose what makes me particularly disappointed with this scope is that AOE also sell the Saxon 80ED, which is $200 less, so I would have expected the AOE 80ED to perform at least as good as the Saxon. But from what I've seen of Mikes pics (he uses a Saxon), there is no comparison, the Saxon is far better. (As well as the Orion 80ED, from Paul's experience).

So based on the claim from their website and an email I have from one of their staff saying the AOE 80ED is based on the Williams Optics platform and I wouldn't be disappointed, I think I'll try for the return option and get a Saxon. Hopefully AOE will be understanding and not make this difficult.

Thanks to everyone for your help and information, it's much appreciated.

Cheers,

Markus

CS (with hopefully no purple)

[RB]

Marcus, it looks like CA but the problem may well be your focus after-all.
How do you focus the 350D?

I know you mentioned that you did focus properly but with modded DSLRs it is absolutely critical to get focus spot on or else you run the risk of halos.
Perhaps you can give it another go using a focus mask and see if it happens again.

[dugnsuz]

My first thoughts too Andrew, but Markus's shots look very sharp with focus dead on IMO. I think as Kal said, we're seeing the differences in FPL51 and 53 type glasses when compared to the ED80. I know that the AOE scope doesn't use FPL53 glass as I enquired a while back when they first appeared.
Those ED80's are an absolute bargain for us budget conscious imagers!!
I still look at some of those early shots I took with mine and think "why did I sell it...D'Oh!!":doh:
Sorry Markus, not really what you want to hear, but try and exchange it - I've always found the AOE lot very fair to deal with.
Doug:thumbsup:

[gts055]

Fpl 51 or Fpl 53 means nothing. Its the mating elements used in the design that give maximum correction to the parameters sought. The only advantage of using FPL53 over FPL51 is that a shorter f ratio may be achieved in using FPL53 glass. The same color correction can be achieved using FPL51 at an albeit longer focal length. Mark

[dugnsuz]

Not too sure how it can mean nothing and then you go on to state exactly why FPL53 glass is preferable in the short tube designs discussed!
Isn't this the point - Markus has a scope which uses glass suited to a longer FL design and is suffering CA as a result and FPL53 type glass might perhaps be better?

Doug

[Kal]

In a hypothetical debate I would agree with you, but in this case we know exactly the parameters of the scope in question. I'm sure everyone has seen what is capable from an F7.5 FPL-53 doublet like the orion ED80, and it is very plausible that the CA is present here because you have a slightly faster focal ratio of F7 combined with a glass that has a significantly reduced abbe number, FPL-51.

The pictures posted above reminded me alot of pictures I have seen taken with the WO ZenithStar 80 II f/6.8 ED Doublet, which also uses FPL-51.

[gregbradley]

There are 2 possible causes;

1. your scope is not APO and no doublet is by the way.
2. your lens is out of collimation even though it is a refractor.

Doublets fall into semi-APO category which is not to say they can't be good. But that much violet fringing is unusual. There are plenty of ED80 images that are quite sharp.

The Tak FS series is one of the higher quality doublets made and they give blue halos on bright stars.

What it means is green is in focus and blue and violet are way out of focus. APO means all 3 are in focus within a tight tolerance. Usually this means a triplet lens with the middle element made of exotic glass (like FPL53 which is expensive and are hard to make, hence the high cost).

Overzealous marketing from telescope sellers have hijacked the definition of APO and degraded it to a lesser meaning. The original standard is quite high and unfortunately only a very few scopes would qualify and even more unfortunately they are very expensive. Tak TOA, Astrophysics, TEC, some Stellarvue, TMB/APM triplets, some William Optics (most are doublets) are the bulk of them.

Having one element of FPL51 or 53 is only meaningful if the mating elements glass is also known. Better results in doublets come from a higher quality mating element. Roland Christen says its the mating elements that determine colour aberration.

The other possibility is your lens is out of collimation. If the 2 elements
are not aligned properly then they will also cause that effect.

Try a free download of CCD inspector and see what it says about your collimation after it analyses that image.

Either way your scope is not performing well enough and you can reasonably expect better performance from a doublet. You should consider sending it back. If enough people did that it would give the false marketers a black eye.

Greg.

[gts055]

My apology :) It just gets me when I read about glass types being quoted and then its implied that FPL53 or similar is the only way to get near color free images. Glass quality, mating elements, design objectives (cost) all come into play. Slinks away :) Mark

[dugnsuz]

Understood:thumbsup:

[KISA]

Hi Andrew,

I did the focusing initially using FWHM and Half Flux Diameter, and then did the long and tedious photo by photo inspections for every 1/8 turn of the fine focuser knob. I went backwards and forwards over the in focus area well over a dozen times and took over 180 photos, very painful. But couldn't find a point that that produced pin point stars without a halo.

I'll also give the fee version of CCD inspector a go and see what it determines (thanks for that Greg).

I did contact AOE and full credit to them that they are more than happy to replace my current scope or to exchange it for a Saxon ED80. They were also surprised CA had occurred with that particular scope.

Markus

[Chippy]

Yeah I have also found AOE very good to deal with. Sounds like you should just swap it for the saxon. Maybe spend the $200 on upgrading to a nicer focuser or something else you want.