which refractor filter

[bomberbaz]

I am pondering buying a short FL (F5 probably) refractor and am aware that this would probably benefit from a filter to help with CA isssues on brighter objects. I have not the cash to shell out on a more expensive triplet so a filter would be the cheapest option. But which one, please help.

There is the Neodymium, Semi Apo and fringe killer as well as the televue. Which does anybody use and why.

http://www.rothervalleyoptics.co.uk/baader-neodymium-ir-cut-filters.html

http://www.rothervalleyoptics.co.uk/baader-semi-apo-filters.html

http://www.firstlightoptics.com/achromat-semi-apo-filters/baader-fringe-killer-filter.html

http://www.televue.com/engine/TV3b_page.asp?id=56&Tab=_back#.VYz-d_lViko

They are all roughly the same price except the televues so open to suggestions. 

TIA

Steve

[ronin]

Not sure it is really worth bothering, they all have to alter the colour balance or if a proper apodisation they reduce the amount of light passing through.

Try just using the scope first and seeing if it really causes you a problem. Just thinking the CA may be no worse then having the filter in line.

[Moonshane]

Having just spent £200 on a used 80mm triplet I'd be surprised if you'd beat it's performance with e.g. am 80mm ST and a filter albeit at a small saving. Having done both, personally I'd honestly save a bit more/wait a bit longer and buy a small ED/APO. This will provide  sharper views at higher power for sure.

[Moonshane]

http://www.astrobuysell.com/uk/propview.php?view=99646

[SpaceWalker]

How much it'll benefit from minus-violet filter you can find from this table.

According to Dave Russell the best one is the Baader 495 yellow longpass, followed by #8 light yellow and others, check out post#4 by Dave in this thread on CN.

The better solution seems is the Aries Chromacor but it's ~$1K, so a good used triplet or ED doublet should  be cheaper solution.  As I understand, you are safe if the aperture in inches multiplied by 3 is equal or exceeds the scope f-ratio. If it's below than the more is the difference the less CA can be corrected by the filter.

[damnut]

Hi Bomberbaz,

I agree with ronin, there was no way I was paying almost the same as I paid for the ST120 on a filter to correct it!! However I was given a baader semi apo as payment in kind. I don't use it, it colours and blocks light. The only issues are with bright objects, planets & the moon. The widefield it gives otherwise at decent aperture outweighs the disadvantages.

If you want to borrow the filter drop me a pm.

[Chris]

http://www.astrobuysell.com/uk/propview.php?view=99646

I know this guy and he's a pleasure to deal with, also he looks after his kit. I doubt you would find a better optics on a refractor for that money. 

Well, apart from Shanes 200 pound triplet, what a deal that is!

[John]

I tried a couple of MV type filters on my Meade 6" F/8 but the tint that they added to the view was, to me, not as easy to bear as the CA. The fringe of violet around brighter objects was reduced by around 50% by such filters so they do something if you don't mind the side effects.

The Chromacors are designed for F/8 - F/10 achromats so I don't think would benefit the type of scope you are looking for, even if you could get hold of one. They need to be matched to the spherical correction of the objective too which is quite a ticklish business.

In the end I sold my Chromacors and got an ED120. Much simpler and a very satisying scope and one where the native optics put the vast majority of the light where it ought to be

[Pig]

Steve,

I have only tried one fringe killer on my old TV Pronto ( the 3rd link in your list ) and it turned everything into a Greenish colour !!!!! It killed everything and not just the fringe. More of an indiscriminate killer in my experience

[JED-E3]

If you're thinking of buying a 120 or 150mm f5 scope you'd probably find an 80mm aperture mask would clean up the views of bright objects as much as a filter, and you can make one for next to nothing.

James.

[rwilkey]

Hi Steve, not seen you around for awhile, I trust you are well?

The Baader Semi-APO filter is the only one specifically designed to combat CA, but you will never eliminate it completely.  Many people don't mind CA, you may find you can live with it, after all, it's only on bright objects isn't it?

Good luck with your new scope, however, I would not buy an f/5 refractor unless it was for AP.