Visual double stars: colour & aperture

[JeremyS]

We often discuss the merits of observing with a small telescope. And we sometimes discuss coloured doubles.

This comment in 1993 March S&T by James Mullaney is intriguing 

[lunator]

Hi Jeremy I'm not totally convinced by this view. 

From personal experience the most vivid  (& false) colours I have seen was in my ST120 but it was more to do with it being an F5 scope. I have seen some nice colours in the 70 & 80mm scopes I have but the best views have been in my 200mm newt with orthoscopic eyepieces. The colours are more realistic the view of Almach, Cor. Caroli or Albireo are stunning.

Cheers

Ian

[mikeDnight]

I've read the opinions of some who say colours are more vivid in larger apertures, but I'm sure there's  some variation in colour sensitivity involved between observers. Using a 4" I find the colours of some doubles quite vivid while others not so obvious. With the more subtle colours simply defocusing by a tiny amount can help differentiate the colour differences between the components. I'm not sure why good dark adaption seems to help in my case but it does?

 

 

Edited November 19, 2023 by mikeDnight

[tico]

Then, is more the Focal lengh, the aperture, the kind of Lens, ED, APO... In case of refractor? 

[Ags]

I do find that star colors show better in my 90mm refractor than my 150mm SCT, particularly reds, giving gorgeous renditions of carbon stars. But blues and yellows/creams are also good as for example Delta Cephei. The frac also shows better color on Jupiter, but probably for different reasons.

[Mr Spock]

I'm not sure what's at play here but I find my 120mm achro shows the colour of Izar better than any other scope I have. It's quite a capable double star scope, though of course the 4" apo has a much cleaner presentation. Perhaps the slightly defocused 'haze' of the achro brings out the colour somehow? 

Generally colour perception is related to brightness. If an object is too bright then it can wash out the colour. That's probably why brighter stars look better through smaller apertures. 

[Ags]

2 hours ago, Mr Spock said:

Perhaps the slightly defocused 'haze' of the achro brings out the colour somehow?

My theory exactly with my 90mm semi apo.

[tico]

Actually, I have read in different places that in the case of double stars the refracting telescope is superior to the rest of the designs..., I don't know if you have been able to compare different telescopes in this sense and what your impression has been...

[lunator]

Hi tico, refractors are excellent double star scopes. Their only real drawback is for tight doubles where you need resolution they tend to get e bit expensive.

If we take the Raleigh Criteria and use 5500nm as the wave length of the star then to achieve a clear. Split the equation equals 138/Aperture in mm. So a 120mm scope will cleanly split a 1.15" pair ( assuming roughly equal magnitudes 🙂)  Once you get up to 150mm scope or larger refractors get very heavy and expensive. My 200 F6 next will perform pretty well against a refractor and can split pairs down to 0.7" The conditions don't often allow this but when they co-operate it can be great fun to split them. 

A 120mm  refractor could get somewhere close to my newt at the Dawes or sparrow limit but the view show elongation of the pair rather than a clear split. 

If you want a refractor go for it as you won't be disappointed. If you want to chase very tight doubles you will have to consider a different design.

Cheers

Ian

[Ags]

On the other hand, there are so many doubles you can keep yourself busy for life without worrying about the sub-arcsecond splits. I find more of an issue is the limiting magnitude of the scope, particularly for those of us with Bortle 8/9 skies. For example, a relatively simply double like Polaris is difficult with my ZS66 at home, but not a problem from a dark site.

[Mr Spock]

27 minutes ago, Ags said:

I find more of an issue is the limiting magnitude of the scope, particularly for those of us with Bortle 8/9 skies.

With the charts I use for the 4" I don't mark any secondary below 10.0 as I just can't see them  There's still plenty to go at though.

[Mircea]

For about twenty years the 125mm F/ 6.9 reflector seen below was my main instrument. And double stars were and are my main interest.

The 125mm is a very good telescope, is able to split 52 Ori and is resolving STF 2 and STF 13. 

I tend to agree with what was stated by James Mullaney. The colors of double stars are more vivid in this reflector than in my 250mm Dobsonian.

Actually, Albireo and Almach have washed out colors in the bigger telescope. ( Which is not a telescope ''to be despised'' as Lockyer used to express himself about good telescopes. )

I use a funny method to see the colors of stars consisting in the use of a Huygens eyepiece of 16.7mm focus. Stars like Betelgeus, Pollux, Castor, Antares, Rigel, Procyon are providing quite a show !

Because of the strong spherical aberration induced by the eyepiece, the image of the star is expanding in a huge dandelion of light, radially furrowed by thin spikes of light , perpetually moving and pulsating due to turbulence.

 

There is color to be seen also at subarc doubles.

Like the orange hue of Bu 720 in Andromeda split last Summer with the 250mm Dobsonian.

 

Clear sky, Mircea

 

 

[RobertI]

Interesting article Jeremy. My experience has been that my 10x50s, 66mm APO and 100mm achro generate the most vivid star colours. In each case it’s the deep yellows, the oranges and reds that are most striking, but then I guess most colourful stars fall into this category.

I can understand why the 100mm achro shows vivid colours - the lens gives everything a slightly warmer tint which I think would exaggerate the oranges and reds. Not so sure why the 66mm should be good in this regard, perhaps we are getting into the low ‘illumination’ (brightness) explanation of the article, or perhaps there is another factor at play here, that I often use the scope for low power views which might tend to concentrate colours. And the 10x50s - well they’re achromatic, low power and small aperture, and perhaps two eyes makes a difference too!

It would be interesting to compare colours in my 102ED with and without binoviewers - I would expect the dimmer image and possible stereo vision of binoviewers to show more striking colours. 

Edited November 22, 2023 by RobertI

[Ags]

Not that I have a large dob, but my ZS66 can show nice colors in Albireo while the big dob will  'overexpose' them, but surely there are hundreds of faint doubles that are indetectable or colorless in my ZS66 but should show as moderately bright, richly colored doubles in the big dob?

[John]

Perhaps the larger airy disks delivered by smaller apertures help to see the colours more markedly ?

One trick I use at outreach events, for slightly older eyes, is to slightly defocus the star / stars being observed. This seems to make the colours stand out more. Maybe that is linked to the larger airy disk effect ?

Younger observers seem to see star colours more readily in general I've found.

[Paz]

5 minutes ago, John said:

Perhaps the larger airy disks delivered by smaller apertures help to see the colours more markedly ?

That's what I was thinking, smaller scopes give a dimmer image but the larger airy disc hits more receptors in the eye than a smaller airy disc so if there is enough brightness there then the larger airy disc enables more colour  to be seen?

[The60mmKid]

Yeah, it's a thing. https://www.cloudynights.com/topic/633339-put-your-stars-on-a-diet/

[The60mmKid]

A moment of revelation for me was the first time I observed with a binocular telescope. I could not believe how much more color I could see in stars vs. monoviewing.

[John]

1 hour ago, The60mmKid said:

A moment of revelation for me was the first time I observed with a binocular telescope. I could not believe how much more color I could see in stars vs. monoviewing.

Unfortunately I don't get on with binoviewers. I have tried a few of them over the years but somehow I can't take to using them 😒

I've no doubts about the benefits for those who do get on them them though 👍

Edited November 23, 2023 by John

[The60mmKid]

5 hours ago, John said:

Unfortunately I don't get on with binoviewers. I have tried a few of them over the years but somehow I can't take to using them 😒

I've no doubts about the benefits for those who do get on them them though 👍

Have you ever used a binocular telescope? It's a different beast. I have a friend who detests binoviewers but is head over heels for his Kowa Highlander. A binocular telescope can be a brilliant stargazing tool, including for doubles. Possibly worth trying.

[badhex]

11 hours ago, The60mmKid said:

Yeah, it's a thing. https://www.cloudynights.com/topic/633339-put-your-stars-on-a-diet/

I've not been pursuing double stars much of late but this thread, the CN post, plus the one it links to have renewed my interest, especially in digging out my SW 80ED. It had sort of always been my idea anyway to use it primarily as a double star scope since it's pretty compact but offers a bit more than the ZS73 and has a more suitable FR for doubles, but all this additional info has sealed it. 

Also, the book "In Starland with a three inch telescope" looks great. According to the linked thread, there are a few accuracy issues due to age etc but someone over on CN did a project updating the details and made a spreadsheet but the link to it is dead, so I'm trying to contact them for a copy. I'll share here if fruitful. 

Edited November 23, 2023 by badhex

[John]

3 hours ago, The60mmKid said:

Have you ever used a binocular telescope? It's a different beast. I have a friend who detests binoviewers but is head over heels for his Kowa Highlander. A binocular telescope can be a brilliant stargazing tool, including for doubles. Possibly worth trying.

I must try one someday !

I get on OK with binoculars so it might be what I'm looking for.

[badhex]

So the kind person on CN, PJ Anway, got back to me and sent me a PDF and Excel version of the updated star data; the original source is from the book "In Starland with a Three-Inch Telescope" from 1909, which they reviewed here. It's a nice review, and looks like a lovely book: https://www.cloudynights.com/articles/cat/user-reviews/books-software/observing-books/in-starland-with-a-3-telescope-r1874

The book itself is available on archive.org: http://www.archive.org/details/instarlandwithth00olcorich

With PJ's permission I've shared the data here as well for anyone who is interested. I appreciate it's a touch off topic being more about general double star observing with small scopes, rather than specifically about colour, but this thread sent me down the rabbit hole!

Starland.pdf Starland.xls

[Mircea]

badhex

I like double stars and everything about double stars: books, articles, magazines, journals, maps.

Thank you for the very interesting and  reviewed lists of double stars by Olcott. 

And my  thanks to PJ Anway for preparing them and for sharing them.

About twelve years ago I was using the book ''In Starland with a Three Inch Telescope'' as an observing guide. I used my 125mm reflector and my first 60x700mm refractor.

The only issue I had was not enough accuracy of the maps.

 

Please let me make just a comment about the list. Olcott star # 336 which is STF 795 or 52 Ori. The separation is not 1,1'' anymore.

The current separation is 0,97'' , going to decrease to 0,96'' next year and so on. Since year 2020 , 52 Ori is a subarc double.

Last night I struggled in vain to resolve it with my new 148mm F/5 Newtonian at 250x, the seeing was not good at all.

 

Related to the topic : StelleDoppie , a mirror of WDS, is stating 52 Ori consisting of A5V stars. Other A stars are Vega and Fomalhaut.

Well, maybe it is something wrong with me but always - this include the observation of last night - I saw 52 Ori as a deep yellow or light orange. Definitely not a sister of Vega.

How do you see this wonderful double star ? 

 

However, last night I was able to split Rigel / STF 668. The main star is of blueish white color , no doubt.

But what is the color of the companion ? How do you see it ?

Last night the seeing was bad, I was able just to confirm the existance of a companion, flooded by the intense, pulsating light of the primary star.

But in the past, in my 200mm Dobsonian, with a steady image, I saw the color of the companion as indigo.

Mircea

Edited November 23, 2023 by Mircea

[lunator]

On 22/11/2023 at 22:46, Ags said:

Not that I have a large dob, but my ZS66 can show nice colors in Albireo while the big dob will  'overexpose' them, but surely there are hundreds of faint doubles that are indetectable or colorless in my ZS66 but should show as moderately bright, richly colored doubles in the big dob?

Hi Agnes

In my 8" and 10" scopes colour is clearly visible down to mag 9.5 ang then the Purkinje effect becomes noticeable between Mag 10-11 where the fainter star in particular shows a blue tint. Below mag 11 I couldn't see any colour. These observations were predominantly from bortle 5-8 skies.

I have moved to a decent bortle 4 sky now so once everything is set up I might revisit some of these pairs to see if that makes a difference. I think it will but probably only 0.5-1 magnitudes.

cheers

Ian