Baader Semi Apo v Neodymium

[russ.will]

Bit of a niche question this one, but here goes!

I had been pondering the Neodymium filter to give me better views low down to the south east, where Cambridge lies some 7 or so miles distant. The sky above is pretty damn good from my garden, but time constraints tend to mean I can't really enjoy some subjects until later in their season when they shift from south east, through south to west at humane hours of the night. It appears that the Neodymium is probably the best in this class without spending ridiculous (to me) money.

Whilst viewing the moon last night and enjoying some remarkably crisp views between the clouds, that it would be nice to nip the remaining minimal CA in the bud, that a longish frac gives. However, I noted through the night that either I'm not sensitive to violet (weird?) or that the AR127L has minimal violet fringing, but slightly more pronounced green.

So my thoughts were two fold. The Semi Apo/Fringe Killer filters seem to all major on reducing violet, so based on user experience, would that be money wasted. Second, I noted that the Semi Apo has a "Neodymium Substrate", so do you in effect get a bit of LPR thrown in for free with this filter anyway, as opposed to the full monty Neo which may be a little dark for a 5" frac?

Thanks in advance,

Russell

[TwoPi]

Hi Russell,

I've not used any of the filters you've mentioned but it looks from the transmission curve here that there is some light pollution rejection in the Semi APO.

That said, if you take a look at a typical high pressure sodium lamp transmission curve here it looks like the main peak is around 570nm (5700Å on the graph) which looks right on the border of what's blocked... I'm guessing they've designed it with that in mind and so it does block this wavelength but wouldn't like to say for sure.

Hope this helps (if only a bit!).

[rwilkey]

Hi Russ, the semi-apo filter is better than the fringe killer as this gives a yellow hue, where as the semi-apo does not.  However, I have a great respect for the Baader Neodymium filter as it is a very good all-rounder, and is also good at dimmimg the Moon, but does not deal with CA as far as I know (as I have only used it on an ED doublet which does not suffer from CA).  On the aperture of your scope, yes, the Neodymium Filter does darken the sky quite a bit and will not be too dark or narrow a transmission for a 5" frac, I use it on my 4".  I think you will find it very useful for light-polluted skies.

[russ.will]

Thanks chaps. I downloaded the pictures from the front of the filter boxes and compiled into one image. According to the transmission graphs and the description on the boxes, The Semi-Apo combines the Fringe Killer and Moon & Skyglow in one filter.

Unless I'm reading this wrong, the Fringe Killer only seems to affect violet, which I'm not really troubled by, but not green which I can see. The Semi Apo seems to cut green, but not quite as much as the Moon & Skyglow and also transmits more light in the light blue region which should make it a touch brighter.

I must say, that I do want to over emphasize the scale of CA I'm seeing. It's miniscule and irrelevant on anything that isn't the moon so far. But if I can nibble at it and squash the skyglow down a bit too, all in one filter I can leave on the diagonal, it seems the Semi-Apo does this with the least light loss?

Russell

[SkyGibbon]

My Es-127, which is a rebranded Bresser I am pretty sure, does pretty well with CA. I also see the green around the moon (basically on the green inside of it, violet on the outside) depending on where I set my eye. It doesn't bother me very much and I have to go looking for it to see it pretty much. The semi-apo is a mainstay in my diagonal as I think it does some LP reduction also. I have also heard you can stack a #8 light yellow with a #82a Light blue and you have a cheap mans semi-apo. The light blue gives the moon a bit more contrast around craters too. I didn't like the fringe killer much, except for it does ok with Jupiter if you don't mind the tint.

[Stargazer_00]

I find the es eyepieces themselves introduce a little yellow on bright object edges so that might be acting to counter some violet fringe if you're using them.

[russ.will]

My Es-127, which is a rebranded Bresser I am pretty sure, does pretty well with CA. I also see the green around the moon (basically on the green inside of it, violet on the outside) depending on where I set my eye. It doesn't bother me very much and I have to go looking for it to see it pretty much. The semi-apo is a mainstay in my diagonal as I think it does some LP reduction also. I have also heard you can stack a #8 light yellow with a #82a Light blue and you have a cheap mans semi-apo. The light blue gives the moon a bit more contrast around craters too. I didn't like the fringe killer much, except for it does ok with Jupiter if you don't mind the tint.

To a large degree, you are confirming my thoughts.

I have the Bresser AR-127L , which at F9.4 is a fair bit longer than the AR-127 Bresser/ES F6.5 scope you have. Like you, I find eye positioning cancels the issue to a large degree, except that I don't find violet to be an issue at all. That is possibly a factor of the longer focal ratio.

The fact that, through personal experience, you are using the Semi-Apo with good results in the same aperture and getting some LP reduction into the bargain, is valuable first hand comment.

I think I will take a punt on the Semi Apo in 2".

Russell

[russ.will]

I find the es eyepieces themselves introduce a little yellow on bright object edges so that might be acting to counter some violet fringe if you're using them.

When I had the BGOs, I found much the same, but the colour shift was really minimal in the ES82s. My interest in filters was generated whilst using the Maxvision 16 & 24mm (with ES FE) which I find to be touch more neutral than the 82s.

Russell

[Stargazer_00]

Best of luck with it. Please write one of your reviews, it'll be helpful for others

[Donkeiller]

Bit of a niche question this one, but here goes!

I had been pondering the Neodymium filter to give me better views low down to the south east, where Cambridge lies some 7 or so miles distant. The sky above is pretty damn good from my garden, but time constraints tend to mean I can't really enjoy some subjects until later in their season when they shift from south east, through south to west at humane hours of the night. It appears that the Neodymium is probably the best in this class without spending ridiculous (to me) money.

Whilst viewing the moon last night and enjoying some remarkably crisp views between the clouds, that it would be nice to nip the remaining minimal CA in the bud, that a longish frac gives. However, I noted through the night that either I'm not sensitive to violet (weird?) or that the AR127L has minimal violet fringing, but slightly more pronounced green.

So my thoughts were two fold. The Semi Apo/Fringe Killer filters seem to all major on reducing violet, so based on user experience, would that be money wasted. Second, I noted that the Semi Apo has a "Neodymium Substrate", so do you in effect get a bit of LPR thrown in for free with this filter anyway, as opposed to the full monty Neo which may be a little dark for a 5" frac?

Thanks in advance,

Russell

Lucky you! I live in the centre of Cambridge. The L.P. is Hellish.

It actually got worse since they changed the large sodium light at the bottom of my garden for a new "high efficiency" white metal halide lamp (http://en.wikipedia.org/wiki/File:Metal_Halide_Lamp_Spectrum.jpg)

I used to use my my Semi-Apo filter  (which is near useless as a minus-violet filter, by the way) to fillter out the sodium emission lines.

[ronin]

Oddly this appeared yesterday on CN:

CN-CAF-Review

Short descriptions and i suppose just one persons opinion, but may be useful.

[russ.will]

Good spot. It's all grist to the mill.

And yet another chap with one of the short tube Bresser/ES Achros. He seems concerned with violet only, which is odd considering where he says he lives.

Russell