Advice on Baader Semi-Apo filters

[spaceboy]

Hi SGL members

Any thoughts on the above filter for use in cheap long focal length refractors?

At the cost of these filters new I'm struggling to commit to checkout not knowing a great deal about them so was hoping we have some members who have experience using them. I'm finding these don't come up S/H too often so is it others can't bring themselves to pay out for new either, they are so good people don't want to give them up or is it they are not worth the money and no-one buys them???

Another SGL member told me they picked one up cheap as chips in an auction and they didn't find it made any real improvement with there scope. For me there is two obvious concerns so far 1) If the filter was known to be good you wouldn't be able to pick one up cheap from a famous auction site given the cost of these things new. 2) Although in a short focal length refractor the member told me they didn't see any real benefits from using the filter.

As always any advice would be of much appreciation.

SPACEBOY

[jgs001]

I have the Semi APO, and compared it with the fringe killer and contrast booster. The Semi APO is not as good at removing the CA as the other two, the CB possibly being slightly better, but the colour cast I found very intrusive. I ended up using the Semi APO exclusively... There was some visible CA, but it was much better than without, and there was a far more natural colour to the view. I was using an 80mm f/5 (ST80 clone).

You can see what I mean at

http://stargazerslounge.com/equipment-reviews/67599-baader-ca-filter-comparison.html

[Moonshane]

wow. I think that would settle it for me in terms of the filters. I had a WO VR-1 which did work on my old 120mm f8.3 achro but again for me the CA was preferable to the yellow cast.

the semi apo filter seems better in this respect.

personally though, I think I'd sell the achro you have and buy an ED (eg Celestron) if that's the route you are considering. but as they say on big brother 'you decide'.

[jgs001]

Good point Shane... I no longer need to use the filter with the 80ED... I still have it, as my little ST80 makes a great portable scope for travelling and it's used then

[spaceboy]

What you are saying is right Shane but I have already gone over this ground in another thread and coming from only using large apertures to using a 4" (only size Apo I'm ever going to afford) I think would only lead to disappointment. My thinking behind filters is they can be used in more than one scope so allowing the possibility of being retained if the refractor was to ever go. The SA seemed the obvious choice as it is said to basically be a moon / sky glo filter combined with a fringe killer. And Sky glo's get good reviews in other scopes and I can't see the fringe part as being all that obtrusive as it's neutral.

[John]

A CA filter works (I think) by filtering out (blocking) some of the offending wavelengths of light, specifically those which the achromat is not able to bring to focus at the same point as other wavelengths, whereas an apochromat brings all (or at least substantially more) wavelengths to focus at the same point.

To me this suggests that Shane's preference for an ED100 over an filtered 120 F/8.3 achromat might make sense because the act of applying the CA filter reduces the light reaching the eye, with a consequential reduction in contrast and resolution, which undermines the benefits or the additional aperture. Maybe ?.

Sorry, just thinking aloud really

[spaceboy]

You say visible CA with the SA but on what Jgs? The moon is not so much an issue and TBH the planets aren't the worst but they are lacking the clarity I get from my reflectors because of it. I take it you haven't tried the 3 filters in reflectors??

SPACEBOY

[spaceboy]

To me this suggests that Shane's preference for an ED100 over an filtered 120 F/8.3 achromat might make sense because the act of applying the CA filter reduces the light reaching the eye, with a consequential reduction in contrast and resolution, which undermines the benefits or the additional aperture. Maybe ?.

Sorry, just thinking aloud really

This is where I think my problem is. I love the views the 120 gives on open clusters. My reflector just won't give wide views due to restrictions with Exit Pupil on ep's. But if I'm going to commit a nights observing to the 120 alone I was hoping to enjoy the views from Planets as well. Wanting the best of all worlds again

[jgs001]

You say visible CA with the SA but on what Jgs? The moon is not so much an issue and TBH the planets aren't the worst but they are lacking the clarity I get from my reflectors because of it. I take it you haven't tried the 3 filters in reflectors??

SPACEBOY

Primarily the moon for visual use, that was the biggest offender for CA, although the filter lived in the diagonal. Nope never tried them in reflectors... there's not a lot of point, as you shouldn't get CA in a reflector anyway, and until this year, have never had access to one. The Semi APO did a fairly good job with imaging with the ST80, although on the brightest stars there was still a blue halo. The problem with imaging with it, the blue was removed from the image... so reflection neb like M45 and the blue in M42 was lost. The biggest issue... was, as you say, the resolving ability, my 80ED is far better than the ST80... not sure if that is down to the CA or the qualities of the glass in use.

[spaceboy]

I think I have had a lucky escape by the sounds of things. I'm beginning think there is no cheap way around getting quality views from refractors. You either have to pay out copious amounts of money on ED glass or settle for expensive filters in your achromatic that don't really cut the mustard. Sad as I really do like using the refractor "Such is life!!"

Thanks everyone for the advice but I can't afford or want to give up aperture for a smaller ED and I'd rather spend my money on BGO's than waste it on filters that are going to turn my views yellow or have no real impact on CA.

SPACEBOY

[jgs001]

It's worth keeping an eye out on AB&S, I got both my Semi APO and Contrast Booster from there... much cheaper than new too.

[ollypenrice]

When the cheap SW achromats hit the market there was a flurry of filter making to cater for them. The Fringe Killer, Minus Violet and Semi Apo, maybe others. I say this because in a big sort out I have just dusted off a very decent 6 inch achro that rarely sees the night, these days.

Since apo and semi apos have become cheaper I guess the achros have gone out of fashion and I bet the filters are lying in the bottoms of EP boxes. Have you tried a wanted ad?

Olly

PS I have just seen what I wrote there - barely sees the night these days... Oh dear.

[spaceboy]

I have been searching for well over a month now. Fringe filters of varying types have come up but upon checking reviews most are said to only change the purple halo to a yellow halo. I will keep an eye out for a SA filter to come up but I doubt I will find one at the price I now feel they are worth. I think I will just look in to improving the moon contrast in my reflectors and give up on the refractor as a lost cause when viewing detail on distant planets. It's great on the moon and DSO's (not really effected by CA) so I don't plan on letting it go but I no longer think it is worth throwing a lot of money at.