The Baader Semi-APO Filter

[Merlin]

I've had this filter for several years and wondered what the blocking strength of the filter is. Does anyone have any info' in this respect?

I reckon that the amount of blockage might be 30 to 40 per cent, but this is only a guess. To call the filter "semi-apo" is a bit of a misnomer, as it's only a blocking filter, whereas an apochromat lets all of the light through by bringing all of it to a focus, not by blocking undesirable wavelengths.

There have been wildly exaggerated claims for the performance of the Baader filter, but a plus point is that it's colour neutral. The filter is worth having for the job it does.

[John]

There is a transmission chart for the Semi-Apo filter on this web page:

http://www.eyepiecesetc.com/Baader_Planetarium_Semi_Apo_filter_2_2458399_p/1501055.htm

[Sirius Starwatcher]

Thanks for posting the question Merlin. This filter arrived yesterday and I have not had the weather to try it out.

[ronin]

Seems a strange filter curve for a semi-apo filter.

The visible end bears a resemblance to the visible bit of the Baader Neodymium Moon and Skyglow filter.

The curve shown blocks 700-1100nm now that is IR and cannot be seen, transmitting 1100-1500 is even further into the IR.

An apodisation filter should be a type of ND filter where the density is a function of the distance from the centre of the filter.

Usually not linear but more like a sine curve, or other similar function.

They basically end up acting like a aperture mask.

Used to "play" about making them and finding out what the results were, yes it was trial and error and guesswork.

None really worked - didn't expect them to I suppose.

[cotterless45]

In practical terms , this filter certainly produces results. It certainly helps with planetary and double star observing,

Nick.

[Stu]

Seems like a Neodymium moon and skyglow but with a slightly harder cut off in the violet which makes sense. Not tried one but sounds like it could be good in an achro

[damnut]

@sirius, it will work well in the st120, though I tend not to bother with any filters. Am in llanfair so if it clears I'll drop you a pm. Have brought some big ep's.:-)

[badhex]

As Merlin says I've read wildly varying claims as to the performance of this filter, so interesting to hear owners reports of how good it actually is. I have wondered if it would be worth the price tag of the 2" version in an F5 petal achro - anyone got specific experience?

[badhex]

Whoops, I meant F5 Petzval achro

[SpaceWalker]

Post #5  by Roy McCoy in this thread on CN contains attachment that compares three filter's spectrograms including the Semi-Apo. Ths is a link to the Semi-Apo spectrogram on the Baader site. It actually doesn't cut  off 100% of the unfocused spectrum that's probably why the views have neutral tone.

As Merlin says I've read wildly varying claims as to the performance of this filter, so interesting to hear owners reports of how good it actually is. I have wondered if it would be worth the price tag of the 2" version in an F5 petal achro - anyone got specific experience?

You may want to read Dave Russell's reports, e.g here or if you're a member of Astromart, here. IIRC he used Vixen NeoAchro 140 for his experiments which should be Petzval achro.  I used what Dave suggested, e.g. #8 Light Yellow filter on my f/9 Chinese Meade frac with good results, but I didn't like intensive yellow cast , so I prefer the Semi-Apo.

[badhex]

Thanks Spacewalker