What is optimum magnification for doubles with small scope?

[MattJenko]

I'll bring over my Xmas Vixen SLV 6mm that I don't know about when the festivities have calmed down.

[RobertI]

I find for planets x1 per mm gives the right balance between brightness, contrast and resolution.

For close doubles you can go to x1.7 - x2 per mm.

Thanks Michael, that suggests a 6mm for planets and a 3.5 mm for doubles, which is in line with some other advice I have been given, so seems like the way forward.

I'll bring over my Xmas Vixen SLV 6mm that I don't know about when the festivities have calmed down.

Thanks Matt, that would be really great, would give me a good idea of what to expect with a 6mm is like through the scope.

[John]

I'll bring over my Xmas Vixen SLV 6mm that I don't know about when the festivities have calmed down.

The 6mm SLV is a superb eyepiece. One of the very best 6mm's I've ever used

[RobertI]

The 6mm SLV is a superb eyepiece. One of the very best 6mm's I've ever used

That's good to know John, Matt will be please to hear that too!

[MattJenko]

The 6mm SLV is a superb eyepiece. One of the very best 6mm's I've ever used

That's good to know John, Matt will be please to hear that too!

High praise indeed. Not helping my impatience though

[Charic]

Robertl......Depends on what you read, what advice you accept, and what feels right during the session. 

As first referenced by ronin, and similar to other remarks, my opinions are similar and my suggestions are based on what's written on the telescope specification plate.

Using this information gives me the magnification of the telescope and the eyepiece focal length required for that magnification?

On my telescope, D-200  (the Aperture ) gives  me  an idea of the magnification.

F-1200 ( the Focal length ) divided by D-200 gives me my focal ratio of f/6......this gives me the eyepiece focal length to acquire that magnification.

Using this information whether fact, gospel, myth, rule-of-thumb, wiki, seems to work well for me and does fall into many suggestions as to what to buy, and what is 'optimum'.

Using  a 6mm eyepiece will give me 200x with  a 1mm exit pupil? and again, depending on what advice you follow, a 2mm exit pupil is 'optimum' for Planetary observations, so now I would have to use the 12mm eyepiece at 100x giving me the 2mm exit [ F-1200/12=100x D/x= 2mm ] but for splitting those doubles, your saying you need higher magnification, which in-turn, reduces resolution, but any one feature, can and  will affect another?

I have also read ( for my telescope)  that not only is a 2mm an optimal exit pupil size for Planetary work, its also the 'Sweet-spot' for Skyliner telescope.

With this in mind, I now have  200x magnification, my HIGH 6mm EP, my MEDIUM 12mm for the Sweet-Spot, and by multiplying the f/ratio by 7 ( 7= generally regarded in calculations as the widest pupil dilation ) giving 6x7=42 for my LOW EP.

Now I don't have a 7mm entrance pupil any more, so I chose a 32mm as my LOW eyepiece, giving me a comfortable +5mm exit pupil. Not only that, at such low powers on a reflector, the secondary mirror becomes an issue? A refractor allows you to go much lower.

Using this idea, Folk can select their 'minimum' three eyepieces based on the technical specifications of their telescope. You can quite easily exceed these limits to some extent. From my signature you will see that I have a 5mm giving 240x and 0.8mm exit pupil and I'm even considering a smaller focal length of 3mm giving 400x and 0.5mm exit pupil ( Again, depending on what you believe and read and/or have experienced,  with a 0.5mm exit, this is regarded as the minimum exit pupil size one should use. I will use the 3mm eyepiece, as I do with ALL my oculars. I find its just better to use any EP that frames my subject, giving the best view at the time). I don't record my observations at present.

Back to your question, what is the 'optimum magnification'....  If this information  works, and you work by or around the telescopes specification, you will, at some stage, find your own OPTIMAL solution, which suits you best. Even my 'optimal' will probably be different to yours,  if we had the same scope, as our eye sight could be different.

Your f/6 with 10mm EP is giving about 1.6mm exit pupil @ 43x. With a  6mm EP you'll get 72x and 1mm exit pupil, which is a 'guide' maximum of your telescope. Its a slightly higer magnification, but as you increase the mag the image will darken. A 3mm EP would max your magnification ( must have the best seeing conditions available ) giving 144x and 0.5mm exit pupil, and a further darkening of the image ( its just physics ).

Barlowing an EP is also an option to get higher magnification,  retaining the original eye relief of the EP.

As you already mention......" what actually works in the real World"?  sometimes things do, but not always, but if you follow the guide lines,  you won't go far wrong.

lastly, as is often mentioned, having the most 'expensive', which can equate to having the 'best' ( this is down to personal choice ) and even down to using the 'correct' eyepiece design,  can better afford you to push these limits, but no matter what the costs, and the choice of optics from Objective/Mirror to the eyeglass of your eyepiece,  there is, and always will be,  a limit, that we reach far to soon! As Olly points out its a balancing act, you have to balance all the information, find the solution and enjoy the results.

I see your after the 5mm BST. I like that EP. I'm also considering the 3.2mm ED Starguider to complete my 'set'.  I have a 6mm WO SPL, to match my focal ratio, as BST did not have a 6mm,  and from what I have seen from this EP, I do fancy  getting the 3mm WO SPL, but on On price and spec, I will probably get the 3.2mm? Most folk would say that it will hardly get used or I need those 'Special seeing conditions'.....I don't need any of that just for the Moon.

[RobertI]

Thanks for the detailed thoughts Charic. Your point about eyesight also being a factor was a good one.

For information I had a chance to briefly test the 5mm BST (x86) and the 4.5mm TMB (x96) recently using the Megrez 72mm. Whilst not intended to be an eyepiece review it may be of interest.

Firstly Jupiter. The image in the TMB was obviously larger than the BST but was dimmer and more orange. Floaters were apparent when using both eyepieces and of the two the BST with the wider exit pupil was more comfortable to use. I couldn't see any more detail in the TMB than the BST but to be honest i was slightly disappointed by the level of detail using both eyepieces. Seeing was ok and during moments of clarity there were hints of detail on the edges of the two main equatorial belts and one or two lighter belts visible but nothing more. I discovered afterwards that the great red spot was in the middle of the disc but it did not jump out at me and i didn't see it at the time. I think perhaps i am expecting too much of 72mm (or perhaps my eyes are shot). However i believe both eyepieces gave more potential for detail than my Baader Hyperion 10mm (x43) purely due to increased object size. Perhaps for a more scientific test i should have used some colour filters.

Of the two eyepieces for planetary i would undoubtedly choose the BST; the slightly larger exit pupil made viewing more comfortable and the TMB had an annoying 'ghost' image which kept flitting across the FOV as you tried to centre your eye in the FOV - generally not a pleasant experience.

I then tested on three doubles: Rigel (mag 0.3 & 6.8 sep 9.3"), Castor (1.6 & 3.0, 5.1") and Polaris (2.0 & 9.1, 18.2"). Both eyepieces managed to split all three doubles but i felt that the BST was marginally better than the TMB at picking up Rigel and Polaris's fainter companions. Both eyepieces were a significant improvement on the 10mm for doubles as would be expected.

These are obviously not very detailed or scientific tests but i think, regarding my original point about magnifications in my Megrez 72, i can conclude that 5mm (x86) is comfortable for me to use in this scope and is definitely worth having for observing doubles and possibly planetary (more testing required). This equates to 30x per inch of aperture. 4.5mm is starting to get uncomfortable for me due to the narrower exit pupil.

I have still to try Matt's 6mm Vixen SVL and hope to do so soon. It will be interesting to see how 6mm premium eyepiece compares to a 5mm less-premium eyepiece.

Rob

[Charic]

Robertl........thanks for that. I do like my BST Starguiders. I also tried a 6mm TMB Planetary II, but after using the 60° afov afforded by  the BSTs the TMB felt too tight at 58°? I know its only 2° difference, but the TMB just did not feel right, ( like looking through a tube! ) and knowing what I know now, I don't really need or require 60° afov to view Jupiter.

The best image I ever had was on Jupiter, the conditions were perfect, and I was at about 360x using a  1.5x Barlow + 5mm BST. I`ll never forget that image, but I'm still waiting for Jupiter to be in the same part of the sky, same conditions,  to fully test and appreciate my 6mm WO SPL  or my next dedicated Planetary EP?

Most of today, I've been studying as to whether to acquire  a new 3mm EP, the WO SPL or the BST 3.2mm or just some  11mm+ Tele Vue Plossl EPs (  I've tried the 8mm - too tight for me with the 6mm eye relief )  also  an Edmund RKE  or an Orthoscopic in say about 12mm. The 12mm hits the sweet spot, and can be Barlowed to 6mm. I just want to try these EPs to see if there is any improvement over what I have already. There's no getting away from the Tele Vue quality, but for me its the eye-relief and comfort that I look into,(excuse the pun ) these tests are solely for what I might call my dedicated planetary EP. I won't know what their like until I try/test them. I did have a loan of the 11mm TV, but foolishly now ( I could have kept them longer ) I returned them too soon, but were looking and learning all the time, and constantly making new decisions. The only thing I bought for my system this Christmas was a drum throne to sit on, whilst observing, and one addition to the Philips series of astronomy books.

[John]

I have a feeling you will find that the 11mm Tele Vue plossl has eye relief and a field of view that still seems restrictive compared to the BST's. Plossls and orthoscopics do have limited fields of view and eye relief, the later of which is around 80% of their focal length, as a characteristic of the optical design. Al Nagler was not able to change that when he did his modest re-working of the plossl design. If you can borrow one again by all means try it though.

The Tele Vue Radian eyepieces would probably suit your preferences better but they can only be bought used now having been recently superceded by the Delos range.

[Charic]

Cheers John. My Starguiders are fairly flatfield across their field, but not fully. One or possibly two EPs that are truly flatfield will be sufficient for my Planetary needs. So far my choice of EPs have good eyerelief, widefield, and most of all good value. I don't think I need such a wide view just for Planetary. I still have'nt actually had a chance to fully experience my WO SPL and may not need to invest in anything else, but I'm inquisitive to see what else there is, and to compare with. I have looked at the Delos range.

[John]

I've heard nothing but good things about the WO SPL eyepieces - I'd like to try some myself one day.

[Pig]

I do not tend to go any higher power than 6mm when splitting doubles, that's about 83 x magnification in my 80mm. I too find the Vixen SLV's perform superbly. I have just purchased a Radian 6mm and hopefully I will get the chance to try it on a few doubles tonight  

[GavStar]

Shaun,

What doubles do you think you will try tonight? I'm keen to target some different ones?

Thanks

Gavin

[Pig]

Gavin,

I will try the following jut to see the difference between the SLV & Radian, all fairly straightforward with some colour differences involved

Polaris

Mizar

Almaak

Albireo

I will also try them both inline with the 1.4 x Barlow (the lens from the Tal) on various areas of the lunar termination

[GavStar]

Thanks Shaun, I will have a look

[Pig]

Your most welcome.

Its gone and gotten cotton picking cloudy here   Huge halo around the Moon too, maybe it will clear as the night goes on

[Stu]

Same here Shaun [emoji20], was looking nice earlier

[Stu]

Shaun,

What doubles do you think you will try tonight? I'm keen to target some different ones?

Thanks

Gavin

Gavin, if you haven't tried it, Sigma Orionis is a lovely quadruple star, the fourth being very faint. The trapezium in M42 is lovely obviously. Rigel is a nice Double with a faint secondary.

Also Beta Mon is a lovely triple, well worth a look.

[cotterless45]

Yo ! There's plenty suggestions hidden further down in the double star section, hurrah !

Recently got given a 6mm Circle T, nearly fell off my perch when I saw how sharp it is. Even kept my goggles on and got about two inches away,

Nick.

[jetstream]

Gavin, if you haven't tried it, Sigma Orionis is a lovely quadruple star, the fourth being very faint. The trapezium in M42 is lovely obviously. Rigel is a nice Double with a faint secondary.

Also Beta Mon is a lovely triple, well worth a look.

Great suggestion Stu, I saw Sigma Ori last night and it is a nice and kind of unusual system, one that will be visited often when sky cruising- 3 stars in a tight row and a pair- I'll have to try Beta Mon too.