Observing tight double stars with colour filters

[Mark at Beaufort]

I have been reading a number of articles regarding the best scopes for viewing tight double stars. There is an opinion that an Achromat refractor with a f number greater than f10 or f11 is the ideal scope. In addition to this I have read about the benefit of using different colour filters.

A quote from 'Washed out Astronomy'  states that colour filters can also be used with double stars to help bring out fainter secondaries if the two stars have significantly different spectral types (colours). In such cases, a filter that matches the color of the secondary may dim the primary and make the secondary easier to see. It doesn't always work, but when it does the effect can be quite spectacular. For example, try a blue filter on Izar (ε Bootis).

There is also the suggestion that a Wratten Red 25 helps to view the 'Pup' with Sirius. Other suggestions are a ND96 (13%) or a Wratten 58.

I enjoy splitting doubles and use either my 4" Astro Tech f6.95 APO or the 180mm Mak/Cass the latter being the better scope for viewing doubles.

I am wondering whether anyone has experience using filters to split tight doubles and what is your opinion on the various ones.

[John]

Interesting topic Mark

I have occasionally tried filters to split tight doubles but I can't say I the results have, thus far anyway, been that encouraging. I did try a red 25 on Sirius to try and split it with my ED120 but it did not work for me. I figured that, while the A star was "tighter" the dimming effect of the filter put the B star below the level needed to see it with the 120mm aperture. I've also tried some odd stuff such as an O-III or even an H-Beta but again I didn't seem to get much from the experiment.

I can see the logic that the right filtering could help either by removing some of the glare from a bright and close primary or by enhancing the contrast between a pair. I've just not found a formula that works as yet.

I have a "whole sky 100% cloud filter" in place here at the moment so I can't experiment tonight !

[ronin]

It would make a degree of sense for the fainter secondary if it were a sufficently different spectrum. So maybe work on Sirius and the pup but not on Capella.

How well it operates in practise I would not like to guess. Too many time I see that the assumption is that a filter will just remove the inconvenient bit but on say Sirius and the pup it reduces light from both.

I guess you would need a fair selection of narrowish bandpass filters as well  not at the imaging level of narrow but narrower then coloured glass. Also you either have to go by trial and error as in swapping filter round or have a good reference to the colour temp of each star in a binary pair and so preselect the best filter.

[neural]

Really interesting idea. I only have two filters, neither of which are really suitable (Nd moon+skyglow and UHC), but I'd be very interested to hear others' experiences. 

Looking at the WDS it doesn't seem very good on spectral types, but a quick search suggests:

STF 644 1.7" 6.96,6.78 B2,K3

S 802 3.9" 7.22,7.15 A2,K0

42 Cet 1.6" 6.45,6.99 A2,G4

STF 652 1.7" 6.26,7.44 A,G2

44 Boo 1.2" 5.20,6.10 F7,K4

HJ 4757 0.8" 4.94,5.73 B5,F8 (southerners only!)

I'm sure there are others but I'll have to find a suitable database...

J.

[chiltonstar]

Traditionally, a red filter helps reduce the blueish scatter around a bright primary such as Sirius making B more apparent; works for me with my 5" Mak, but not my ED80 - presumably because the B component is made too faint to see (as above).

A blue filter works with a tight pair of similar brightness stars by reducing the size of the Airy disk and therefore improving resolution. I can certainly see this with doubles say 1 arcsec or less apart, but the seeing needs to be very good!

Chris

[neural]

Traditionally, a red filter helps reduce the blueish scatter around a bright primary such as Sirius making B more apparent; works for me with my 5" Mak, but not my ED80 - presumably because the B component is made too faint to see (as above).

A blue filter works with a tight pair of similar brightness stars by reducing the size of the Airy disk and therefore improving resolution. I can certainly see this with doubles say 1 arcsec or less apart, but the seeing needs to be very good!

Chris

That makes sense, even if there isn't a difference in colour. Last night I looked at eta Ori with my ED100 and saw the central Airy disks overlapping -- I wonder if I tried with my 'moon & skyglow' filter if the discs would be smaller? I'll give it a go!