5 replies to this topic

[eyeoftexas]

People use filters like the Baader Semi-APO or Fringe Killer with fast achromats to minimize CA.  Are such filters compatible with a NVD mounted on the achromat?  I notice that the Semi-APO has an IR cut to it.  This seems like it would defeat the use of a 685nm pass filter, etc.

[sixela]

It minimises CA to get the best possible image centred on green. That's of little comfort if you're wanting to look in H-alpha or near-IR.

You'd need an achromat centred on red, or for H-alpha with spherical aberration nulled on red. You can actually do that on some doublet by adjusting the spacing between the lenses -- people do that for QuarkStar solar H-alpha setups.

But H-alpha is usually done at fairly low power, and then the spherical aberration on red is usually not that bothersome, even if a scope with less spherochromaticism in red is still sharper (but at quite a price).

[The Ardent]

For testing IR filters (block, pass) with a refractor use R Cygni to determine the best filtering. In the winter use V1376 Orionis.

Examine focus and appearance vs other field stars.

People use filters like the Baader Semi-APO or Fringe Killer with fast achromats to minimize CA. Are such filters compatible with a NVD mounted on the achromat? I notice that the Semi-APO has an IR cut to it. This seems like it would defeat the use of a 685nm pass filter, etc.

[bobhen]

I use a 102mm fast F5 (also reduced to F3.5) achromatic doublet. I only use a 685 Pass and a  6nm Ha filter with that scope. Nothing else. I use the scope for very low power, very wide filed NV observing. CA at those magnifications is just not an issue. At around 15x and 25x most all aberrations including CA are just not apparent visually.

 

I also have a Tak TSA 120 and a Tak FC-100DF that I use with NV. But I still use the 102mm F5 because of its very wide field, low power capability. The views are great.

 

Bob

[eyeoftexas]

Thanks everyone for your guidance. My plan is definitely wide field, low power (scope due to arrive tomorrow).  I got it mainly for NV use so I’ll forego the achromat filters for now.

[GOLGO13]

Fast achromats work quite well. I use a 150mm F5. Very nice with the 67mm setup.