What Magnification?

[gareththegeek]

We got first light on our new C100 EDR on Saturday and had a great session and that has lead to some questions over magnification.

We were using the 20mm Celestron Plossl that came with the scope, an 8mm TeleVue Plossl and a very poor quality 2x Barlow.

When reading advice such as 'Eyepieces The Very You Need' and various posts I have got the impression that in the UK seeing restricts magnification to 150-250x and that 2/3*Aperture in mm is the maximum usable magnification.

On Saturday night we were able to magnify Saturn, The Moon and Stars to 225x without any major loss of image quality even when using a low quality Barlow. When viewing M81 though we could not go above 112.5x.

So is the 2/3 rule mainly applicable to DSOs and in terms of buying eye pieces in future what would be a good maximum for viewing planets with our 100mm aperture ED refractor?

Sorry if this question has been asked to death but I have tried reading other posts and ended up a little confused

[Edit]I meant 2*Aperture*2/3 not 2/3*Aperture - oops[/Edit]

[jgs001]

The theoretical usable max is 2x aperture in mm... so for your scope it's 200x. Generally... it's considered to be a bit less than that in real terms as that's for ideal conditions. However, I've done much the same as you and pushed harder than your supposed to be able to (240x on an 80mm scope), and not noticed much of a problem... Whether that's just luck or the eye making up a bit I really don't know.

I'm not sure on M81 though...

[GazOC]

With my ED100 I found I could use a 5mm eyepiece for x180 on the planets most nights, a 4mm for x225 some nights and a 3.5mm for x257 very rarely.

[John]

While you can use powers over 200x with a good 100mm scope (which the C100ED is) you will not actually be revealing any more detail by doing so. On good nights I get nice sharp views of Saturn at 343x with my 6" refractor but I find backing off the power to say 240x leads to clearer views of subtle contrast features, faint moons etc.

[pvaz]

On DSOs specifically many times you need to reduce the magnification or else you'll make it dimmer.

On planets you can go to the max mag, or even slightly above it, although you won't see more detail. However sometimes a bigger image helps notice details better. As in all things it's a bit trial and error. It's hard to say what kind of views you'll enjoy better. I personally prefer to stay at low mag for DSOs but when it comes to planet I like to use as much mag as I can.

[gareththegeek]

Thanks for the replies!

So would you say as a rule of thumb (although lower mag may often be better) that maximum usable magnification would be 100-130x on DSOs and up to 250x on planets? This would obviously be on a good night...

How about the moon? Can you achieve higher magnification since it is so bright?

[NGC 1502]

Hi gareth, I find that with my scopes that the max mag varies according to how steady the atmosphere is, and what I am looking at. For me, Saturn seems to take more mag than Jupiter which fuzzies out at a lower power. Double stars, a favourite of mine, often take a surprisingly high power. Globular clusters look better at higher power but galaxies do not. Hope this helps, Ed.

[pvaz]

Well the best thing is to have 2, 3 carefully chosen EPs and a quality barlow.

That gives you 4/6 options and you can then change accordingly to the situation.

If I had your scope I would be looking for the following: Good 25 or 26mm EP for DSOs. Also perfect with a good barlow for the smaller DSOs.

I would prefer a 10mm instead of your 8, but has you have it already it will provide a good mag for planets on most nights and a bit over the top mag when used with the barlow on excelent nights.

In alternative to the barlow, if you share the line of thinking that says "the less glass the better", you could get a 25 or 26mm, a 13mm and a 5mm for the very good nights. The moon also handles excessive mag ok. Still I usually prefer to stay within my scope's limits. Most of my dso observing is done at 38x and 75x.

Don't expect good views of DSOs with too much mag. Only ones that handle it (up to about 120x) are globular clusters and some planetary nebula, such as M57, but those are the exception. The brightest area of M42 is also nice at high mag.

[gareththegeek]

Thanks for the very well considered reply it is very helpful.

I was thinking of replacing the Barlow next because I think a good Barlow is good value since it doubles all your other pieces - don't know whether to get the TeleVue or the Celestron Ultima...

I guess I will have a better idea after a few more sessions with the lenses I have already. I agree about the 8mm, I stupidly bought it thinking I would buy an Explorer 200P then bought a C100 instead nm it is a beautiful eye piece.

We're mainly interested in viewing planets and the moon at the moment. I was thinking of getting a large aperture reflector further down the line if we get more interested in DSOs - and it can share the EQ mount.

[GazOC]

The Orion Shorty Plus is another very good barlow (same model as the Celestron Ultima) that can be had at a reasonable price. Make sure you get the "plus" model though.

[pvaz]

A good barlow is definitly a good purchase. Specially if you pair it with a couple of good EPs as it doubles their use and allow you to have quality views while keeping the cost down.

I just want to give you a heads up about a potential problem I had: When you use a barlow with an EP that haves a long eye relief, usually the wide field ones, the barlow haves the effect of increasing the eye relief and some times that forces you to keep your eye in a very specific position or you'll experience blackouts.

I have this problem when I use my Nagler 31T5 with any of the 2 barlows, although the bigbarlow 2x makes it worst then the Antares 1.6x (btw in regards to quality they are both excellent). To solve this I ordered the TV pan interface for the big barlow, which is another lens that corrects the light cone to remove this extended eye relief. Basically it transforms the bigbarlow into a powermate. I'm still waiting for it so I can't comment on how it works. All my other EPs don't have this problem and can be used comfortably with any of the barlows.

In resume, if your widefield EP haves a long eye relief, you should consider the tv powermate or the meade teleextender to avoid blackouts. You could also test that EP with the barlow you already have to see if it will be a problem or not, before making a decision.

[gareththegeek]

So the Orion Shorty Plus Barlow is pretty much identical to the Celestron Ultima it looks like..

Interesting you mention the blackouts, I have noticed with the 20mm Plossl that came with the scope that if you don't put your head in the right place it blacks out and I was finding this annoying. This is even without the Barlow. People said the 8mm lens would have tight eye relief but I find it to be exactly the right eye relief for where I want to put my eye. The 20mm I seem to keep getting too close.

So the Barlow will make the eye relief longer? I have never looked at the powermates/teleextenders, I don't know what they are.. so the pan interface shortens eye relief?

I wasn't planning on using wide field of view lenses since they are so pricey compared to plossls. I could get two or three plossls for the same price as a wide angle lens.

Thanks for the help!

[pvaz]

So the Barlow will make the eye relief longer? I have never looked at the powermates/teleextenders, I don't know what they are.. so the pan interface shortens eye relief?

The pan interface is a televue product that only works with their "big barlow".

A powermate is a design Al Nagler invented, basically it's a barlow that haves another lens on top and changes the angle of the light path back to the original. This guarantees it always gives the same magnification, the eye relief stays the same and the EP focuses on the same place.

Meade have a cheaper copy of this design, called Tele extender.

If you have trouble with blackouts, then barlowing the higher FL eyepieces may not be a good idea. Maybe getting a set of 4 nice plossls will be better then a barlow. The teleextender and powermate are quite pricy and you can get a couple of good 2nd hand TV plossls for the price.

[gareththegeek]

I'm not sure if the 'black outs' are a problem, might just be something we can get used to and we will probably replace the 20mm lens at some point in the future.

Weather is looking promising for toninght so I'll have another chance to play about with the current lenses - fingers crossed.

[pvaz]

Definitely, it's all a personal matter and you need to see if you adapt to it or not. Try out your 20mm with the barlow and see if the extended ER is a problem or not.

On my Nagler it's quite uncomfortable. The original eye relief is 19mm so I have to keep my eye centered at a 38mm (almost a full 4cm!) distance when I barlow it, that puts my eye well outside the rubber guards. If I move the eye just a bit off center, or closer, I experience blackouts and it's very annoying.

You need to test 1st and make a decision later. Still if your goal is to build a decent plossl set, then the barlow won't allow you much of a saving as a good barlow cost about the same as a 2nd hand quality plossl. The real savings from a barlow come when you either have 2/3 very expensive EPs which you want to reuse or when you want a lot of options (say 6 to 10 mags) where a barlow will allow to cut the amount of EPs in half.

Let's say you buy a nice 25mm plossl for about 55-60£ (usual price for a 2nd hand TV plossl). A celestron ultima will cost another 50£ or so, 2nd hand. so that's 105-110£ for the following options 25mm, 12.5mm, 8mm and 4mm with the risk of excessive ER on the 12.5mm option. And the 4mm will be pushing your odds a bit, so in reality you get 3 adequate mags for your scope.

If on the other hand you forget the barlow and get 3 2nd hand plossl, a 25mm, a 15mm and a 5mm (this one could be a nice, but cheaper, ortho) then you have a similar solution without blackout problems for about 150-180£.

Anyway you should experiment a lot more with your EPs before deciding on new gear. I'm just pointing out some options according to my personal taste/experience, you may well go on a different direction.

[Moonshane]

if you are going to buy a new decent quality barlow like the one you mention then I personally wouldn't. You can get a 1.25" 2.5x powermate for maybe £100 used. I have used a few barlows (although admittedly not the standard of the three mentioned) and the powermate is just incredible in my view - like it's not even there. you get the reduced light due to the smaller exit pupil as if you have bought a shorter FL eyepiece but that's all - no degradation - if you insert a 25mm eyepiece, it's as though you have inserted a 10mm eyepiece but with the same eye relief as with the 25mm.

on magnification, I think the only real answer is ti use as much as you can get away with, or need, to see the object you are observing clearly. my eyepieces are summarised as follows for my two scopes - the second magnifications are with the 2.5x powermate and the effective focal length of the eyepiece with the powermate in brackets:

f8.3 refractor FL 1000mm

14mm Radian 71x 179x (5.6mm)

18mm Radian 56x 139x (7.2mm)

24mm Panoptic 42x 104x (9.6mm)

35mm Panoptic 29x

f5.3 dobsonian FL 1600mm

14mm Radian 114x 286x (5.6mm)

18mm Radian 89x 222x (7.2mm)

24mm Panoptic 67x 167x (9.6mm)

35mm Panoptic 46x

Although I often get good enough seeing to use the Powermate plus 18mm Radian in the dob to give 222x, it's rare that things are stable enough to use the 14mm at 286x apart from the moon.

As I see it there's a 'gap' in my magnifications between 139x-179x on the frac 222x-286x on the dob. Therefore I am likely to by a used (all my lenses are used) 16mm T5 Nagler next which will give me 156x on the frac and 250x on the dob.

I could probably get away without this and that will be what will bring me to a decision over the next few months.

I'd definitely recommend buying fewer good quality eyepieces as you'll keep them for longer and it's therefore cheaper in the long run

[gareththegeek]

I don't think the black outs are so much of a problem. Had a quick go on the scope last night and I think it was just that my head was at the wrong angle when viewing near the zenith (because I was half lying down on the floor!).

The Powermate sounds cool but if you haven't tried a good quality Barlow I am not sure you can compare How am I to decide whether I want 2 or 2.5x

It's my Birthday in a months time so I'm going to have to put some serious thinking into this Hopefully we can get some more scope time in before then!

Paulo: What are the colour doubles from your signature? Is there a list somewhere? We're planning on making a start on the Lunar 100 as the moon waxes, its nice to have to do lists!

[pvaz]

I compiled this file excel with all the lists I use:

http://stargazerslounge.com/attachments/primers-tutorials/35962d1272729597-dso-checklist-messier-herschel-400-caldwell-colored-doubles-observerschecklist.zip

Each list is on a page and when you fill in the date observed column it updates the counters on the 1st page.

Try using the filters, specially the one on the constellation column to plan your sessions. With that list and a good star atlas you're on your way.

[gareththegeek]

Amazing, thanks!

That's some nice work

[pvaz]

BTW, if you stay up past midnight check Albireo, which is a very nice double in Cygnus with an orange star and it's smaller blue companion. It's also easy to split past 50x or so.

[gareththegeek]

Yes, I have heard people mention that that's a good one. I have been waiting for it to slide overhead more since my view to the North is restricted.

Plenty of other stuff on this list to be getting on with though eh?

[Moonshane]

The Powermate sounds cool but if you haven't tried a good quality Barlow I am not sure you can compare How am I to decide whether I want 2 or 2.5x

It's my Birthday in a months time so I'm going to have to put some serious thinking into this Hopefully we can get some more scope time in before then!

fair point! but if you get a chance try one out. if you are near Manchester, you could always bring your scope and eyepieces here (or just the latter) and try the powermate versus a 2x barlow I have (Cape Instruments which is supposedly as good as the TV ones). I'll never part with my powermate.

the 1.25" fitting comes in 2.5x and 5x and the 2" fitting (with a 1.25" adapter) comes in 2x and 4x. the 1.25" is a lot lighter as you'd expect.

take your own advice too and take your time before buying again. this will 1) allow you make the right decision for you and 2) save up more dosh (I don't think equipment lust ever disappears from astronomy!).

good luck mate.

[gareththegeek]

Thanks for the offer but Manchester is a bit far to come from Wiltshire

With my current lenses a 2.5x would give me magnifications of 45x 112.5x 112.5x (again) and 281x doh! Really wish I'd waited before getting the TV lens but nm. If I had waited then I would have missed out on a brilliant night on Saturday as I'd only have had 45x

[Moonshane]

Thanks for the offer but Manchester is a bit far to come from Wiltshire

With my current lenses a 2.5x would give me magnifications of 45x 112.5x 112.5x (again) and 281x doh! Really wish I'd waited before getting the TV lens but nm. If I had waited then I would have missed out on a brilliant night on Saturday as I'd only have had 45x

ha ha I agree! never regret anything mate - just learn it, remember it, use it and move on