Solar imaging - apo vs achro, question

[Ben Cartwright SASS]

I currently have a Quark Chromo and normally use my ST80 and my XLT AZ 102, both Achromatics for viewing and for imaging.  I also have a Sky Watcher ED100 APO F/9 and have used it both for visual and imaging.  I need to do testing on this, but it seemed to me that they were both very close as far a sharpness goes, but the seeing has been very poor around here.

 

  I have read several places that with solar it doesn't matter whether you shoot Achro or APO as it is a very narrow bandwidth that is passed through the filter.  Even though I have poor seeing here I would like to have the best shot at getting sharp captures.

What is the consensus on APO vs Achro for Solar?

[Alien 13]

With any narrowband imaging the gap between Achro and Apo gets smaller but I have a sneaking feeling that the better lens figuring and coatings on the more expensive glass does have an impact.

Alan

[spaceboy]

If I remember correctly it was said some time ago quarks are recommended / advise for use in APO refractors. There have been reviews to suggest this is indeed true and that it certainly improves performance for visual so I guess that also rolls over to imaging also.

[Ben the Ignorant]

When I look at the Sun through colored glass filters (not interf) my eyepieces suddenly become sharper across the whole field, the difference is most striking at the edge but the center benefits, too. I don't remember if the gain was greater in the 80mm apo relative to the 80mm achro because I didn't do a comparo, but single-color viewing, even with the very broad band of simple colored glass, sharpens the whole field a lot.

I hear everyone who does solar imaging in my area uses large achros.

[Peter Drew]

It's true that for solar Ha visual/imaging the inherent CA of the achromat is not a great disadvantage but as with most things the generally higher quality of figuring found in the more expensive APOS can be epected to provide the best results. Despite the very significant supression of solar brightness to make it safe, the solar image tends to still be a little on the bright side, coloured filters which produce further attenuation do seem to enhance the visible detail. This is most evident when observing in Ha and thin cloud passes in front of the Sun temporarily giving views similar to the epensive double stacking.  

[Davey-T]

The one thing I have found that improves Quark imaging, all other things being equal, is aperture the 100mm doublet  is nice, the 110 triplet is nicer and the 152 doublet is awesome.

Dave

Edited October 31, 2016 by Davey-T

[ronin]

As previously mentioned the narrow spectral band will mean there is little if anything between the two designs.

The apo/ed may be slightly better as even on the narrow spectrum there will be a difference, and as the achro should have the Green at the low turning point then the Red will get spread over a greater depth then on an apo, an ED should be between the two. On an apo the short bit of the red spectrum used should all just about be on a common focal plane and flat. So sharper. I expect that the image through an apo will literally snap into focus, and snap out just as fast.

[Merlin66]

The Chromatic Aberrations of the Achro is not a big deal - the narrow Ha bandwidth takes care of that.

What can be an issue is the Spherical Aberrations - SA

Some achros suffer from SA more than others......

 

[Stu]

4 minutes ago, Merlin66 said:

The Chromatic Aberrations of the Achro is not a big deal - the narrow Ha bandwidth takes care of that.

What can be an issue is the Spherical Aberrations - SA

Some achros suffer from SA more than others......

 

I tried a Quark in a Skywatcher 150mm f5, and the results were disappointing. I always put this down to SA, the image was just always soft.

The 152mm f5.9 scopes have significantly better figuring I believe, and the results as Dave says are spectacular.

So, I agree that the CA of an achro is not an issue, but the generally better figuring you get with an apo means that they are likely to give better results. The exceptions will be achros with good figuring such as the 102 f11s and the 152 f5.9s

[deejay]

My very expensive Lunt 60mm Solar scope has a single achro lens as standard at the objective end with no corrector lens, I assume its well figured spherically. The instructions say that there is little point in any more expense due to the small bandwidth and field of view.