Is an Apo overkill for splitting double stars?

[Neil English]

Hello friends,

Since my teenage years, I have maintained a passion for looking at double stars. After spending many happy years looking at my favourite doubles using 4" F/10 achromats, I took the plunge into the brave new world of colour pure observing. Now, it has been said that once you get a taste for the colour free, there’s no going back to achromats. But it wasn’t as easy as that for me. You see, after trying several Apo models (premium and budget) I found no advantage in regard to their ability to split even the toughest pairs for this aperture class. In fact, I have NEVER been able to split a pair in an Apo that I couldn't in my humble achromats. Disillusioned, I sold my F/9 Apos and invested in a high spec 4" F/15 Fraunhofer refractor and I must admit it has been a revelation. It has been the best double star instrument I have ever had the pleasure of looking through.

I, like many others here, are well aware of the awesome views Apos serve up on high contrast objects like planets and luna, but it intrigued me why my achromats, despite showing modest secondary spectrum, were in no way hindering my ability to split doubles. And that seriously puzzled me. One way to think about resolution in respect of double stars is the size of the Airy disk which scales directly with wavelength and inversely with aperture. So a 2” refractor will display Airy disks twice as large as a 4” scope of the same F ratio. Smaller Airy disks make tighter double stars easier to resolve and you always get that with larger aperture. That’s easy to confirm in the field.

But what about wavelength? This, to my knowledge, is a less vigorously investigated variable. The size of the Airy disk scales directly with the wavelength of light used. So bluish pairs (shorter peak wavelength) should be easier to resolve then reddish pairs (longer peak wavelength) and my experiences over the years seem to confirm this. What’s more, the high power views of the Airy disks of reddish pairs – have a look at 61 Cygni for an example – appear distinctly larger to my eyes at high powers than bluer pairs.

Then, it occurred to me that the imperfections of the achromat in respect to colour correction could actually be a decisive advantage over, dare I say it, the perfect colour correction of the Apo. To see what I mean, take a 4” F/10 achromat and charge it with a high magnification- say 200x – and look at the colour of moderately bright white stars (spectral class O, B &A). Invariably they take on a greenish-yellow cast. Thus, although it may not be entirely accurate to describe it as such, the idea that the achromat is acting as a kind of ‘green filter’ seems appropriate enough. Because my 4” F/10 achromat can’t focus the deep red inside the Airy disc, it is effectively cutting off an appreciable amount of the longer wavelengths that would otherwise increase the size of the Airy disk. In contrast, the near perfect achromatism exhibited by the Apo brings these longer visible wavelengths to a common focus within the Airy disk and thus should produce a slightly larger Airy disk than its achromatic counterpart. In practice, however, the difference will be small and probably not easily measurable.

There, I’ve said it. Now I need to go face the firing squad.

Neil English

[brianb]

There is no doubt whatsoever that the smeared light from imperfect colour correction ruins the chance of getting a decent split on close double stars. However not all achromats have noticeably imperfect colour correction - long focus achromats are unfashionable these days, but they do work very well (when properly mounted).

If you have a close double star to split where both components are tolerably bright, it might be worth trying a deep blue or violet filter e.g. Wratten #47 to shrink the diffraction pattern. This will be most effective in a scope with minimal or zero chromatic error. With a refractor, it might also be worth trying a central obstruction stop too - yes, you heard me right; a central obstruction diverts light from the disc to the rings (reducing contrast on extended objects i.e. planetary surfaces) but a 33% CO actually shrinks the central disc by ~ 10%.

[Ant]

I had an 80mm F11 achro a few years ago and the views that it gave showed that it could punch well above it weight.

But it was big - longer than my 8" newt - so I think that one of the biggest issues of long FL refractors is the length of the tube and the weight of it - meaning you have to have a large mount to errr mount them on.

Personally I hate lying on the floor to look through the EP. Thats one of the reasons I sold it.

Welcome to SGL BTW

Ant

[acey]

An apo is not inherently better than an achromat, it's just that the apo can have shorter focal length and give comparable results. People want shorter telescopes for imaging, wide-field gazing and convenience. Your experiences bear out the continuing usefulness of long-focus achromats for double stars.

[NickH]

I have to say the best views I have seen for double stars have been on the FL102 and an F11 A80 (latter being a £100 achromat s/h).. though for wide field, nothing so far has come close to the TV102/TMB's I have personally looked through/used, so I guess that supports the above!

[lunator]

Neil

To be honest I have found an 8" F6 Newt with orthoscopic eyepieces the best way to view doubles.

I do use an OMC250 as well but the vcolours are not as pure as the Newt/ortho combination.

The secondary spectrum may be a factor but generally the seeing conditions will out weigh this.

Cheers

Ian

[Steve]

My thinking too Ian. The 8" Newtonian will have greater resolution, zero chromatic aberration and significantly more light gathering ability.

[Macavity]

I have always assumed that "Sir Patrick's" historical (early 60's!) references to the MERITS of the 3-inch refractor were to a (fairly long focus) achromat. I think, even the humble ST102 (with a bit of extrapolation!) illustrates what an achromat MIGHT be capable of... So, for the moment, my ambitions to buy a 5" APO are shelved.

Re. double-splitting, I now rather regret parting with a onetime MAK90! I'm not even convinced that my MAK127 is THAT much better at this task. But I suspect recent observations with the latter were dominated by "cooling" and, above all, SEEING? I'd still like to TRY a MAK180, despite...

To perhaps echo brianb's comments, I suspect the most sigificant (astronomical?) investment I have made in recent months was to buy a Skywatcher PILLAR to go under my Giro III - And to build a (ongoing, secure!) outside telescope store. But I digress...

[MartinB]

It would be interesting to try using an apo to try to split tight doubles and compare filtered with unflitered. I think I'll give it a go with my ED120. Just wondering what filter would be best. I guess a simple blue or green although a cyan filter would be better.

[Macavity]

Just wondering what filter would be best.

One can (might) speculate? Perhaps, if there is sufficient colour contrast, one of these (Baader etc.) "contrast" type filters? The merit of these seems that they pass RG( and cut intermediate colours. Perhaps their general "minus-violet" property would limit the brightness of a typical blue-white primary star. But, who knows...

[Neil English]

Hello folks,

Thanks for all your posts; I guess what I'm getting at is that an Apo can't outperform a well made achromat on doubles. It pays to remember that virtually all the grreat tally of double stars discovered over the last few centuries were done so with old, high quality glass. The great 36" Lick refractor could and continues to resolve doubles separated by 0.1" despite the fact that it swims in a morass of secondary spectrum.

If you want the ultimate then it's always best to make the focal length as long as possible. When the F ratio is very high (F/15) you get an entirely new affect kicking in; STEADIER views caused by the scope's DEPTH of FOCUS. The huge depth of focus on long focus achromats (and Maksutov's too) allows the observer to better accomodate the unrelenting movements of the earth's turbulent atmosphere. When you make a scope too short, as is the trend today, you lose this effect but VERY FEW people have even considered it since we live in a generation that has not been informed by looking at the sky through long focal length scopes. If you want to hear the fully fleshed out arguments on "beating the seeing" you can read the threads that have been started on Cloudy Nights; just google depth of focus, atmopsheric defocus and long focal length and you'll get there. They are actually on going threads. I am currently in talks with optical theorists to provide a fully worked theory on this new phenomenon.

Of course reflectors give great views too and an 8" F/6 reflector is a fantastic all round scope. I have merely concentrated on refractors in this thread as I believe too many people snubb the remarkable properties of achromats without really appreciating their many virtues.

My up-and-coming book will give Apos and achromats of all genres a fair hearing; so you'll get it all in there.

Kind regards,

Neil.

[MartinB]

Hello folks,

Thanks for your posts. Firstly Brian B, have I offended you in another life? I don't recognise the smearing out of colour you recognise. Maybe you need to have your achromats recollimated? If you'd like to put a 30% obstruction on your refractor objective, then that's your look out. Personally I prefer unobstructed views.

Your reference to undermounting? Eh? I take it you are referring to my 4" F/15 achromat? With all due respect to your comments, what right have you to tell me the kind of mount to use?

Crikey Neil, I guess you two have some previous. I didn't pick up anything to get irritated about in Brian's post. There are some awful achromat's out there which I doubt are great double splitters but others of high quality and with a long focal length you are looking at great performance. It will be a long scope with long levers and will need to be well mounted. Seems reasonable, what's with the attitude?

A very interesting thread and some very useful thoughts and information. It is nice to know from the outset whether there is any personal commercial interest when these posts are made. That certainly wouldn't invalidate the information given in any way but does help members evaluate comments. Not to do so could be seen as discourteous.

[Neil English]

Hi Martin,

Brian's comments on my mounting options, I thought, were a bit condescending. I would invite Brian to come try it out before pronouncing judgement.

In regard to commercial interests. I never mentioned any brand have I??

Concerning themes: ever read the "Emperor's New Clothes"?

Kind regards,

Neil.

[acey]

I too am surprised by the response to what seemed a perfectly reasonable comment.

"Commercial interest" could be taken to refer to the forthcoming book.

[brianb]

Firstly Brian B, have I offended you in another life? I don't recognise the smearing out of colour you recognise. Maybe you need to have your achromats recollimated? If you'd like to put a 30% obstruction on your refractor objective, then that's your look out. Personally I prefer unobstructed views.

Your reference to undermounting? Eh? I take it you are referring to my 4" F/15 achromat?

Steady on!

1. I was not referring to ANY specific instrument. Colour smearing is common with short focus achromats sold at bargain level. I would not expect to notice it when using a 4" f/15 visually ... though I would expect to have to refocus when imaging with a parfocal colour seperation filter set with such an instrument (or a short ED achromat), whereas I'd expect to get away without refocusing when using a triplet achromat.

2. I was not criticising your mounting, of which I have no knowledge. I have, however, seen several attempts to mount very good vintage long focus achromats on modern mounts which have been thoroughly unsatisfactory because of the large moment of inertia of the long heavy tube.

In regard to commercial interests. I never mentioned any brand have I??

Neither did I. Why do you assume that I was criticising your production model?

I apologise unreservedly for any offence taken ... but if you go back and read my post carefully, you will see that there is no need to take offence in the first place.

[NickH]

Brian is a good guy Neil... I doubt any offence was intended

[Mark at Beaufort]

Very interesting thread Neil. I also love refractors and have a 4" f6.9 APO which I greatly enjoy viewing doubles. In addition I have the Meade 6" AR6 f/8 achro which sits on an alt/az mount with a pillar extension.

It would be interesting to compare my 4" with a f/12 or f/15 frac at some time. I was only aware of 2 new f/15 fracs available for general purchase a Skylight about £1500 or an Antares about £1000.

Was I correct that Nick H was going to use the Skylight at SGL5? If so, would this be a good time for a number of experience observers to give an opinion on the APO and f/15 Achro debate?

Mark

[Neil English]

Hi Brian,

OK, no problem. I had inferred (perhaps erroneously) you were referring to my decision to mount my instrument on a Gibraltar mount, which is in the public domain. You also referred to a "production model" which I never actually stipulated.

My apologies here. Looks like we have a passionate bunch of people out there.

Of course you can get pretty grotty achromats. My point is that there are fairly inexpensive achromats out there that do a perfectly good job at splitting doubles. In this arena of observational astronomy there is little to be gained by buying an Apo.

For the record, I am a big fan of both genres and I own a lovely little 3" F/6.3 doublet Apo which goes everywhere with me.

Kind regards,

Neil.

[NickH]

I hope to have it at SGL5 yes...

not sure about the mount option

[Mark at Beaufort]

Nick you are welcome to try it on my Skywatcher Alt/Az mount with the pillar extension. I use the same mount on your old Meade AR6. As you know I only live 8 minutes from Lucksall so I can easily bring this and an EQ mount over.

Mark

[NickH]

Excellent. I should have either a CG5 or an EQ6 with me for my kit (not sure which yet), but any mount donations for this beauty gratefully received.

The idea is to let as many people try it as possible!

[ollypenrice]

Brilliant thread. I too think the long focus achromat needs a comeback. Nothing to do with achro versus apo, but rather as a way of prioritizing the spend. You spend on an apo to get a fast focal ratio without chromaic abberation. But you could spend the same sum on a really well made very slow achromat which would do some jobs better.

There was a post about a big new achro on sale recently, a strange black device. Anyone remember?

Glad the spat is over! I could see nothing to worry about in Brian's post either.

Olly

[brianb]

There was a post about a big new achro on sale recently, a strange black device. Anyone remember?

This one? ... Zerochromat Home Page

Devil's advocate here. What about a folded long focus achromat - with a couple of flats you could have an unobstructed f/20 10cm/4" scope in a tube 1m long with the same viewing arrangement as a Newtonian i.e. eyepiece at the top of the tube, needing no diagonal and allowing a short tripod to be used (good for stability). Not classical, but maybe more practical.

[barkis]

There was a post about a big new achro on sale recently, a strange black device. Anyone remember?

This one? ... http://http://www.zerochromat.com/

Devil's advocate here. What about a folded long focus achromat - with a couple of flats you could have an unobstructed f/20 10cm/4" scope in a tube 1m long with the same viewing arrangement as a Newtonian i.e. eyepiece at the top of the tube, needing no diagonal and allowing a short tripod to be used (good for stability). Not classical, but maybe more practical.

I have just sold a pair of octagonal flat mirrors to a guy who is making a folded refractor. Beautiful they were too. Sold them for peanuts I did.

Ron.

[acey]

There was a Sky and Telescope article by Ernie Pfannenschmidt about folded refractors (March 2001). Certainly a nice idea, with the potential for very high perfomance. Don't think the article is online but there's a photo of it here:

http://victoria.rasc.ca/events/2003/mars/ernie05.jpg

I have a 100mm Yukon spotting scope that is a folded refractor - the optics are unexceptional and I only use it for low-power deep-sky, but it makes for a highly portable instrument.

Yukon 100X 6-100 x 100 Spotting Scope