Filters to help achromats split doubles

[John]

I came across this interesting piece comparing the effects of a MV type filter with a #58 Wratten filter when used with an achromat refractor to split double stars:

http://www.cityastronomy.com/58-filter.htm

Has anyone tried similar experiments with filters on doubles ?

[DirkSteele]

That is a really interesting piece of work.  Wish I had an Achro to try it with as the writer did.  Be interesting to see if there is meaningful difference for Apos as well, given that some Apos are more Apo than others. 

[Peter Drew]

I've never tried filters other than variable density polarising filters, these reduce the glare to the point where just the intense "core" of the star image shows and stops it from flooding the separation gap.  

[Stu]

I always assumed something like a Continuum filter which gives a relatively narrowband pass on green would be good for doubles in an a group, and possibly even to reduce glare in an apo.

[cotterless45]

Very interesting article . I use a Baader semi apo filter, as well as controlling ca, it cuts down glare and helps particularly with greatly contrasting close diffraction rings. In my opinion , it's boosted the performance on binaries (and planets).

Theres a decent comparison of Baader fringe , semi apo and contrast booster on the Baader - planetarium uk website (PDF). 

Nick.

[vlaiv]

I've used Baader Continuum filter on F/5 achro for moon observation and results are superb. It minimizes effects of seeing induced blurring and sharpens optics to great level.

I guess that most blurring due to seeing happens because different wavelengths refract differently. By using narrowish band filter most of the light bends equally so instead of blur we get shimmer effect. That helps a lot for rendering of small details (they are there just bouncing around and instead of being smeared with lower contrast, contrast remains the same).

I guess that there is a balance to be struck for using narrow band filter for double star work. We want filter that passing just right wavelengths to:

1. minimize airy disk (this means prefer blue part of the spectrum vs red part)

2. has the least impact of seeing (opposite from above, reds are the least affected by seeing)

3. falls into region where eye is the most sensitive (green)

4. is narrow enough to suppress seeing blur

5. is wide enough to allow enough light to come thru and let us actually see the stars (4nm band pass will have effect as attenuating star almost 2 mags)

6. falls into range where optics is well corrected.

Huh, easy?

I guess Continuum filter does fare well with above list being centered around 530-540nm and having band width of roughly 8-10nm (FWHM width).

 

[Fozzie]

That is an interesting read.. I recall reading something similar, albeit in a imaging context, in a book I picked up recently.. I'll have to dig out the actual wording a logic but essentially in "shoot the moon" by Nicolas Dupont-Bloch there is a chapter on Imaging with an achromatic refractor. (all to it's self, so it doesn't contaminate the APO sections with their less bendy light paths!) 

With in it the sequences of processing is to extract the green layer, as theoretically I guess, the objective coatings and focus point of the achro lens is sharper in this narrower wavelength, and the out focussed red/blue spectrum will actually cause a degradation of the image through the achro lens.  So in essence using only the green channel should yield a better crisper image.. Now I did try imaging with a solar continuum filter after I read this, but the data overwhelmed the PC and I never got any final results.  I usually convert to monochrome in pipp to reduce any residual chromatic noise anyway..

I know one of the "sales points" of the achro I have is that the multi coatings are to the benefit of the green channel most visible to the human eye, henceforth a clearer image..

It would be interesting to see a comparison of a image run with a written or solar continuum in place and one with out on lunar to see if there is any impact, albeit greatly enhanced than you would see visually.. well maybe interesting..!

Did I ramble on aimlessly then... Not even sure if this is relevant to TBH..

Ta

Fozzie

[chiltonstar]

I've used a green filter with my 102mm f13 achromat to reduce the violet twinkles with Sirius A and B, and tried others including a sodium D-line filter. The jury is out whether there was any real benefit..... An ND filter certainly works in the sense that it lowers the mag you can split a double with, for the reasons above, although throwing light away never seems a good idea.....

A blue filter will of course (theoretically) improve the separation of doubles with any scope because the diameter of the Airy disk is wavelength-dependant, and decreases with shorter (blue) wavelengths.

Chris

[Paz]

This is an interesting idea.

To date the only filter I use for doubles with the st120 is a semi apo. I will give my solar continuum and polarising filters a go the next time I get a chance and see what happens.