Which type of scope is best at resolving doubles?

[rusirius]

Having learned about doubles a few weeks ago we were eager to get out and observe some. At the time, Epsillon Lyrae seemed a good choice along with Albireo nearby. What puzzled me though was that although I could split the secondary doubles in Epsilon Lyrae easily in a 5in refractor, it was somewhat more difficult with the 8in SCT. The refractor gave a much clearer and distinct view whereas the SCT was fuzzy. I now have a 12in Dob and although it can split the doubles easily, the view still does not appear nearly as sharp as in the refractor.

Is a frac the scope of choice for doubles?

PS, I spotted this Double and Variable Stars forum on SGL a while back (2 months maybe) but when I later came back to post this question I couldn't find it - I was of course looking for it in the Observing zone. I found it by chance last night here in the Science & News zone which I don't look at very often and where I wouldn't have expected it to be, which is probably why it eluded me. Was it moved from the Observing zone to the Science & News zone? If not, then I'm curious as to why is it not in the Observing zone] as that would seem to be the obvious place?

[russ]

Myself and Rob once did a double star comparison using an ED80, Celestron C8 and Skymax 127. We came to the same conclusion. Desite it's huge aperture deficit the ED80 easily provided the better view of the double double and Albireo. Stars were pin point and colour contrast was amazing. The C8 was as you describe...fuzzy. The 127 was also very good, certainly better than the C8. Haven't really used the Newt for doubles.

An SCT would not be my weapon of choice for double stars. A good refractor or at least a Maksutov. I'm not a seasoned double star observer, so my opinion doesn't count for much.

Russ

[MartinB]

Lunator will no doubt be along shortly and give you the answer!

My experience is identical to yours. My ED120 will resolve doubles much better than my 12" dob. There are a number of factors involved beyond just aperture. Resolution is aperture dependent but also seeing dependent. If the seeing is mediocre the resolution benefits of aperture will be lost.

Contrast is important in being sure of a clean split and here refractors (especially apos) come into their own.

Optical quality and tight FWHM are obviously important consideration esp if the optics aren't diffraction limited or colour isn't well corrected. So collimation is critical.

Optical coatings and control of internal reflections will also be factors in controlling glare.

Long focal lengths seem to be favourite - less work for the EP so fewer optical distortions

In moderate seeing conditions I would guess a high quality refractor with a relatively high F ratio will deliver best results. In pristine seeing a larger aperture Mak will take some beating.

[Merlin66]

Unfortunately, after having built and used large reflectors ( up to 29") I've got to say nothing seems to beat a good collimated refractor for close double stars.

I had some great views with the 18" reflector but usually the seeing made things difficult.

The best views I've had in 40 years are with the TV 4" Genesis.. the images are tack sharp, clear and bright; you can power up the magnification ( I use an old original Ziess 6mm Monocentric for this type of stuff).

It's also good fun to set up a webcam/ DMK etc and image double stars. There's a couple of very good freeware software packages designed for analysing and measuring double star images; well worth a go......

[rusirius]

Well my puzzlement initially stemed from my assumption that the greater angular resolution of the 12in Dob ought to make splitting doubles easier. Of course in practice it turns out to be otherwise. The Dob does of course come into its own with deep sky and the detail seen in globular clusters is amazing. Nevertheless the frac (Skywatcher Evorstar 120 achro) does better at doubles so I wondered whether there was an optical problem with the Dob and SCT. But then surely not with both of them....

I guess I'm beggining to learn from experience that each type of scope performs differently with different observing tasks.

I did a bit of googling but did not come across software for analysing images of double stars and I don't have a decent webcam/ccd yet. I've experimented a little with my unmodded 350D but not on double stars yet but again I've found the frac seems to perform the best with imaging. I'm still having rather variable success with getting the focussing right even with a hartmann mask so I'm perhaps not ready for doubles yet...

[Brinders]

SCT are notorious for requiring good and frequent (re)collimation and being allowed plenty of cool down time to ambient temperature. I have the WO Megrez 72 APO and on occasion found I can split doubles better with that despite is modest aperture than my C9.25 SCT. Having said that I remember one clear night recently where I was able to split the double in Polaris with the SCT but not with the WO!

Brinders

[Merlin66]

The best double star software I've seen is Florent Losse's "REDUC V3.8.5"

check out //astrosurf.com/hfosaf

The program is mailware so drop Florent an email at florent_losse (AT)yahoo.com

[Doc]

My TAL 4" refractor gives me a better split on tight doubles then my 16" Dob.

I find even though the dob has alot more light gathering capability the seeing and collimation must be perfect to match a refractor.

[Andrew*]

The Carl Zeiss Telementor was/is well known to be a fantastic double splitter, and could do far more than it's 63mm aperture would suggest.

IMHO this fact is testament to the idea that refractors do the best job for their aperture on double stars, and often out-perform instruments many times their aperture.

Andrew

[russ]

Put this to the test last night. Megrez 72 with 3.2mm TMB = top notch double star observing. 12.5" Newt far less satisfying.

[rusirius]

Thanks for the info on REDUC. I've e-mailed Florent and am awaiting his response.

Thanks also to those who posted their own observations. It seems we all agree that the frac IS best on doubles. Its reassuring to know that my own experience seems to correspond to that of many others and that there is in fact nothing wrong with my reflector scopes.

[FLO]

PM Lunator as well, there's not much he doesn't know about splitting stars

[GazOC]

Just to confuse matters theres also an element of different types of scopes for different types of doubles.

[solo]

My 4" frac far outperforms my 8" newt on doubles.No contest!

John.

[lunator]

Sorry for the late response to this but I have been travelling. Unfortunatly not to any dark skies .

This thread pretty much sums up how people find double star observing. The preferred type of scope is a refractor. This is due to the fact that they are stable and do not lose collimation. Double star observing requires excellent collimation if you are to split doubles below 2" (arc seconds).

Just to be different I have done most of my double star observing with an F6 reflector and whilst this does require more work to keep properly collimated it could beat my refractor when set up properly.

Martin has covered some of the points already but I will start from the theorectical and move to the practical/empirical.

I'll start with the Airy Disk.

The stars are point sources of light but due to the way light is affected as we 'manage' it throught the scope we observe a disk which is a spot surrounded by concentic rings. George Airy found that in a clear optical design (refractor) the central dot contains 84% of the light, the 1st ring 7%, the second ring 3% and the rest of the light (6%) goes into all the other rings.

Any system that has a central obstruction will take some of the light out of the central spot and put it into the rings.

The next question is how big is this spot. The spot size is dependant on 2 factors. The first is the aperture of the scope and the second the wavelength of the light. The accepted principle is to use the wavelength of 550nm as this is where the human eye is most sensitive, but if you are looking at very red/orange stars this will lead to a slightly different spot size although it may only be noticable if you are trying to split very tight pairs.

The Raleigh Criterion

Based on the wave length of 550nm how close can stars be before they cannot be separated?

The Raleigh Criterion is calculated where the peak of the Airy spot of the one star is in the centre of the dark ring of the other star. This point has 73% of the intensity of the peak point. This is a large enough difference for the eye to observe. How far apart is this?

Using the formula Theta res = 1.22 lamda/D and replacing lamda with the 550nm figure we can get the resolution in radians.

Theta res = 1.22x0.000055/D

To convert to arc seconds you multiple by 206265.

Theta res = 1.22x0.000055x206265/D

Theta res = 13.8/D

So a 10cm scope will split stars down to 1.38" based on the Raleigh Criterion. This equates to the drop in intensity of 27% in between the 2 peaks.

The next step is the Dawes Limit. The Rev Dawes discovered that in good seeing - moderately favourable as he called it- you could see a separation between 2 yellow mag 6 stars of 4.56" based on a 1 inch aperture. This limit has been proven to work up to apertures up to 30cm and only applies to stars of even magnitudes and fairly bight. The Dawes limit does break down with faint pairs as the drop in intensity is only around 3% so in faint pairs this is almost undetectable.

What does this mean in observing terms. The Raleigh & Dawes limits are a combination of theoretical & empirical and 'in the field' the empirical becomes an even larger factor in the ability to resolve doubles.

The other factors are

1- Scope collimation

2- Seeing conditions

1- Scope collimation

Most refractors are well collimated and do not lose it easily. This means that the performance of the scope will only be limited by atmospheric conditions.

With SCT's the Primary mirror moves to achieve focus and optical aberations are present at all focus except where the principle focus of the primary matches the conjugate focus of the secondary.

With Newtonians the effect of coma is the main issue although spherical aberation and astigmatism can be issues. If your scope is collimated to the equivalent of the Raleigh Criterion you will suffer from a quarter wave coma and this will destroy any close double star view... and even if your reflector is collimated well at the start of the evening then a cooling mirror and system can lead to the collimation deteriorating during the session and so the views will not be as expected. Also the only way to get collimation spot on is a star test at high power. This will lead to fantastic views but also alot of frustration as you attempt to make the smallest possible changes.

2- Seeing Conditions

The seeing of the atmosphere is basically how 'steady' the air is. From my location in London the seeing is usually between 1-2". To see how this affect the observing I will list the data for my 3 scopes.

Using my ST120 the Dawes limit is 0.97" and the Raleigh criteria is 1.15".

The Newt has a Dawes limit of 0.58" and a RC of 0.69"

The OMC 250 is 0.46" and 0.55".

Basically on an average night for brighter doubles the ST120 will perform as well as the larger scope. This does not make the ST120 btter than the other it is just able to match them under the prevaling conditions.

Where the larger scopes come into their own is when the seeing is better than average and/or I am investigating doubles that are fainter than Magnitude 9.

When the seeing is excellent I have separated doubles down to 0.8" in the newt. I would not be able to acheive a split in the ST120 no matter how much I tried. The Newt& OMC can pick up tight doubles down to mag 11-12 and the ST120 would struggle to achieve this.

In summary a small well made refractor will work just as well as a large scope on the brighter 'show piece' doubles. The Larger scopes will out perform the smaller scope when conditions permit and collimation is at its best.

Cheers

Ian

[Doc]

Great explanation Ian.

Thankyou.

[lunator]

One point to add is that the best explaination for this including more formulas can be found in the Bob Arygle (ed.) book "Observing and Measuring Visual Double Stars".

Chapter 9 written by Chris Taylor is an excellent explanation of how Newtonians are affected by optical and mechanical errors and how they can affect the capabilities of the scope.

Cheers

Ian

[FLO]

Thank-you Ian

[rusirius]

Wow! What a post! Thank you for taking the time to compose it and explain matters. I found it fascinating and informative. You did loose me on one thing though:

With SCT's the Primary mirror moves to achieve focus and optical aberations are present at all focus except where the principle focus of the primary matches the conjugate focus of the secondary.

I looked up that term 'conjugate focus' but I could not find a satisfactory explanation. Do you know of a reference that can explain this please?

... and even if your reflector is collimated well at the start of the evening then a cooling mirror and system can lead to the collimation deteriorating during the session and so the views will not be as expected. Also the only way to get collimation spot on is a star test at high power. This will lead to fantastic views but also alot of frustration as you attempt to make the smallest possible changes.

Yes, I had noticed on my dob that collimation can drift during the observing session, especially when not enough time has been allowed for coolling, but even when the scope has had an hour to cool down I still find it sometimes needs checking after an hour or so of observing.

With regard to the bigger scope being better for splitting faint stars that was an interesting point. I'm surprised though that you can pick out doubles as faint as mag 11-12 in London!

Although I'm aware of the effect of the coma effect with Newtonians, I found this part interesting:

With Newtonians the effect of coma is the main issue although spherical aberation and astigmatism can be issues. If your scope is collimated to the equivalent of the Raleigh Criterion you will suffer from a quarter wave coma and this will destroy any close double star view... and even if your reflector is collimated well at the start of the evening then a cooling mirror and system can lead to the collimation deteriorating during the session and so the views will not be as expected. Also the only way to get collimation spot on is a star test at high power. This will lead to fantastic views but also alot of frustration as you attempt to make the smallest possible changes.

Are you saying that the scope needs to be collimated to better than the equivalent of the RC to be able to split doubles well but this this is quite tricky to achieve? I have to say that I have tried doing a star test on my Dob but I have inevitably ended up making things worse and ended up going back to the laser! It didn't seem to be a problem on the SCT, but its much trickier on the newt. I presume that the adjustments here are only to the primary?

[GazOC]

When a scope is designed there is an optimal spacing between the primary and the secondary where the designer has (hopefully) made the scope as aberation free as possible. Because SCTS and Maks (can) use moving primaries to focus this spacing is not maintained and aberations are unavoidable in this type of design at all other points of focus except the one, optimal point.

[darditti]

I agree with what Ian has said, and kind of disagree with the thrust of the earlier comments, which were over-praising refractors.

The question was "Which type of scope is best at resolving doubles?" to which the correct, literal answer is: "A big one!".

However, if the question is meant to be "Which type of scope is best at resolving doubles for its aperture?", then, surely, refractors will win - few people will try observing them with a 3 or 4 inch reflector. However, again, if the question is "Which type of scope is best at resolving doubles for the money you pay?" I am not sure refractors win out, certainly not at the larger end of the size bracket. My experience (and general wisdom) is that you will get roughly the same resolution from a good 130mm refractor as from a good 200mm reflector or SCT. The former will be a lot more expensive.

I have three telescopes mounted in parallel, on the same mount, in my observatory, a C-14, a C-5, and a 100ED. It is thus very easy for me to look through one then another at the same double star, and compare the view through the scopes under the same conditions. The resolution of the C-5 is slightly lower than that of the 100ED, as I would expect. But there is no way the 100 will compete with the C-14 on resolution, even on an average night.

However, double star views through the 100ED are very attractive, in a way more so than view through the much higher resolution SCT, because of the good contrast, and the clearly-delineated Airy disks, and this points to maybe why some people are over-praising small refractors in this thread. You can ask "What telescope is best at splitting doubles?" or you can ask "What telescope gives the most attractive views of easy doubles?". These have maybe different answers.

The thing I notice when directly comparing views through refractor and SCT is that the clear and sharp delineation of the Airy disks in the refractor, though, in fact, they are a symptom of the low resolution and light-grasp of this instrument, in a way, deceive the eye and brain into thinking it has superior resolution. You don't see all that fuzz and mess of bright light you get with the bigger instrument. So the view looks nicer, but any objective test will show it is a lower resolution view than the bigger telescope gives.

So I enjoy looking at doubles through the refractor, even though there would seem to be no point in using it for this purpose, as I have a much higher resolution telescope available. I think it is this effect that is influencing the way people answer the question here, when the truth is you will split a lot more doubles if you spend your money on a reflector or SCT than if you spend it on a refractor.

David