High Power EP for ED80

[Special K]

I need to bridge a big gap in the collection to achieve somewhere around 180x with the ED80. I have a decent Barlow so could get a 6 or 7mm EP and have use of the EP with and without the 2x Barlow [7mm = 85x and 171x]. I quite like the idea of a TV Nagler 7mm but wondering if the Barlow will slightly compromise performance? I have hardly used the Barlow since moving from the 130P. Perhaps there is a preference thing here: it's easier to swap EPs without a Barlow involved too.

I wouldn't want to go less than a 3mm I don't think. That's 200x in my scope.

[michael.h.f.wilkinson]

A Delos 3.5mm should fit the bill (or 3.5mm SLV, if you want to save some money)

[Charic]

If power calculations are similar to Reflectors, doubling the aperture gives a desired160x under perfect conditions, whereas your scope is capable of 80x. Pushing a little further is ok, but there soon comes a limit? A 7.5 mm EP will achieve 80x A 3mm will give you 200x , maybe a little too much?

[Piero]

Refractors are generally less affected than reflectors by bad seeing. If the optics are good enough you can push them beyond the *threshold* of 50x per inch aperture. I have pushed mine (a 60mm) at 144x and even at 206x and still got some nice views for the Moon and Jupiter. Of course there were limitations in terms of resolution due to the aperture but it was feasable. Those magnifications correspond to exit pupils of 0.4mm and and 0.3mm. I achieved these with Vixen SLV 5mm and Nagler 3.5mm T6 in combination with a Bresser SA 2x (a tele-extender). Those magnifications are also useful for splitting doubles particularly when these have different brightness or colour. If so, the eye can still work out that there are two objects and go beyond the telescope resolution limit. 

The Nagler 7mm T6 is a lovely eyepiece, very powerful but still in a compact design.

If you decide to go through the barlow route, just be careful that the barlow barrel enters in the diagonal holder properly, otherwise you might have do some inward refocusing. Not a big problem for most refractors, but just something to be aware of.

Piero

[YKSE]

According to BillP's book, Naglers barlow very well, no vignetting or such as some other eyepieces may suffer, there should be no optical compromise if the barlow is a good one(that's a hard definition question in ifself  depending who you ask ).

As high mag is more seeing dependant, a zoom (Nagler 3-6 .e.g.) can be an good alternative. My approach is barlowing a zoom though.

[Piero]

As high mag is more seeing dependant, a zoom (Nagler 3-6 .e.g.) can be an good alternative. My approach is barlowing a zoom though.

Slightly off-topic, what is the maximum useful ep focal length you used with this approach? 3mm, 2.5mm, 2mm, 1.5mm?

Thanks, Piero

[YKSE]

I've not used 80ED in high power though, mostly with C8 and 120ED for splitting tight-doubles, in C8 t up to 370x-380x, 120ED to around 200x.

Planetary work are usually done with bonoviewer in about 140x to 300x in C8.

[michael.h.f.wilkinson]

Refractors are generally less affected than reflectors by bad seeing. If the optics are good enough you can push them beyond the *threshold* of 50x per inch aperture. I have pushed mine (a 60mm) at 144x and even at 206x and still got some nice views for the Moon and Jupiter. Of course there were limitations in terms of resolution due to the aperture but it was feasable. Those magnifications correspond to exit pupils of 0.4mm and and 0.3mm. I achieved these with Vixen SLV 5mm and Nagler 3.5mm T6 in combination with a Bresser SA 2x (a tele-extender). Those magnifications are also useful for splitting doubles particularly when these have different brightness or colour. If so, the eye can still work out that there are two objects and go beyond the telescope resolution limit. 

The Nagler 7mm T6 is a lovely eyepiece, very powerful but still in a compact design.

If you decide to go through the barlow route, just be careful that the barlow barrel enters in the diagonal holder properly, otherwise you might have do some inward refocusing. Not a big problem for most refractors, but just something to be aware of.

Piero

There is no reason seeing affects a reflector differently from a refractor. Scopes with no (or a very small) central obstruction (given equal control of aberrations) have a more contrasty image which can be pushed a bit further, but the seeing does not enter into it. Much more important is the observers visual acuity. If you have a high visual acuity, you tend to tolerate higher magnifications less well, because of the blurring you can see much earlier than people with low visual acuity.

[Piero]

There is no reason seeing affects a reflector differently from a refractor. Scopes with no (or a very small) central obstruction (given equal control of aberrations) have a more contrasty image which can be pushed a bit further, but the seeing does not enter into it. Much more important is the observers visual acuity. If you have a high visual acuity, you tend to tolerate higher magnifications less well, because of the blurring you can see much earlier than people with low visual acuity.

I thought that because of the smaller aperture, a refractor looks through less turbulent atmosphere than a telescope with larger aperture (e.g. a reflector), and therefore is less affected by bad seeing.

Also, the exit pupil at magnifications between 100x-200x in a refractor is smaller than the exit pupil at the same magnifications for a larger reflector. This means that the observer would see less glare on these planetary objects. In an average seeing, I would think that to our eye the view is therefore less disturbed on a refractor than a larger reflector. 

The same happens when a green filter is applied when watching the sun. Same seeing, but the reduction of light makes the image stabler in my opinion and to my eye.

I am not saying that there is a physical reason where a bad seeing affects less a refractor. I am just saying that I believe our eyes see better planetary targets with a refractor when the seeing is average. Possibly I have bad explained. 

Why would this be wrong in your opinion? 

Regarding what you said about the lack of (or very small) central obstruction and visual acuity, I agree with you.

Another problem related to a person's eye which can affect the observation of planets, Moon or the Sun at high magnifications is floaters. 

[ronin]

Kevin, being in St Albans I suppose you are not a member of Hertford Astro are you?

Just wondering if Alan from Skies the Limit would bring along a 3.2mm BST for you to try if you were.

Or do you know anyone with one ?

[Moonshane]

If you can take short eye relief and use a 50 degree field the the nagler 6-3mm zoom is my high power choice in my 80mm f6 apo.

[Special K]

Kevin, being in St Albans I suppose you are not a member of Hertford Astro are you?

Just wondering if Alan from Skies the Limit would bring along a 3.2mm BST for you to try if you were.

Or do you know anyone with one ?

That's a good idea, thanks. I've been along to Herts Astro once or twice and have met Alan as well. I got my other BSTs from him!

The zoom sounds interesting and I'll put this down on the list too. Never used one but it looks good.

[Special K]

Another question to help me consider: I was looking at double stars last night and wondering what max separations I can get at 171x. Being realistic I didn't think my scope could give a resolution on doubles below 1".

I've been thinking about the most pleasing views I've had of Jupiter and really of a mind to get a 7mm. The 12mm gives a lovely crisp image of the planet even on a below average night, but it stays in briefly as I want a bigger disk. The next jump up for me currently is 120x which can look slushy in poorer conditions. The 7mm at 85x is directly between those mags, and on good nights I could Barlow this to 171x and also use for splitting tight doubles.

Piero, I was toying with the Barlow last night and looked really nice with the 17mm Hyperion. At 71x it gave a crisp view and began to split the ε Lyra doubles. The Barlow works with all my EPs so don't expect a problem fitting it with a Nag 7mm unless the barrel of it is longer than 30mm and creates a focus issue.