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PostPosted: Tue Feb 07, 2012 10:30 pm 
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Barry, in both cases a quick calculation shows a real field of about 20 arcminutes for your scope.

At 250x with a 6mm Vixen with the 305mm scope mentioned above and 45 degree apparent field gives a true field of 11 arcminutes.
Do you think you would be able to observe planets properly with an 11 arc minute field ?

Or would wide field eyepieces be needed with this telescope undriven although it has a GoTo driven mode if needed.

Regards, David


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PostPosted: Wed Feb 08, 2012 5:11 pm 
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Location: Long Ditton, Surrey, UK
After coming across this Sky at Night Group Test (high power eyepieces), the Vixen 8mm LVW is beginning to look very tempting:

http://www.vixenoptics.com/acc/lvw_eyepieces.html

The link is with the 8mm eyepiece.

65 FOV, 20mm eyerelief, around £199

Umar


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PostPosted: Wed Feb 08, 2012 5:30 pm 
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I read the test and as Martin Mobberley did the test I fully accept his findings.

However, and I will probably upset a lot of Sky at Night magazine readers.
I get this magazine monthly with several others.

But in many of the tests the scores are all very similar and I don't agree with their findings.

For instance they just tested 4 image stabilised binoculars and chose the winner as the 15x50 Canon IS. They made the correct choice, but stated that the images are good up to 70% to the edge.
In my opinion this binocular has the best edge performance of any binocular I have ever tested. I would rate it as good to 90% or even 100% of field.
I have used the Canon IS for years. Maybe they had a faulty example.

And I see similar things with say tests of other binoculars.

Anyway, I got that off my chest, sorry.
What I am saying is don't believe tests unless you know who is doing the testing and also that they have not been given specially selected items by the importer/supplier.
This commonly happens with binocular tests.


The 8mm Vixen wide angle would be good except it will give you 190x not 250x.
This is fine.
It also means you would get 20 arcminute field not 11 arc minutes with the 250x 6mm 45 degree Vixen.

If you told me you would only use the 305mm Dobsonian undriven I would say the true field with the 6mm is rather small. But as you have a motor drive then I would say the 6mm Vixen is O.K.

If you buy the 8mm Vixen wide angle then what will you use for a higher magnification?

Regards, David


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PostPosted: Wed Feb 08, 2012 5:40 pm 
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You do have 190x about with the 25mm and 3x barlow, but the 8mm Vixen wide angle will give a much better view.

Maybe buy the 8mm Vixen. They may be on offer at Astrofest.
Maybe the cheaper Williams have a 6mm or you might save up for another eyepiece for later.

Not all your eyepieces have to be the best. Sometimes value for money is also O.K. especially if you have a good drive and can use the centre of the eyepiece field.

It is a pity that they don't make 1.5x barlows.
They make 1.5x photographic teleconverters which are very valuable with camera lenses. These are basically the same thing.

regards, David


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PostPosted: Wed Feb 08, 2012 5:51 pm 
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The Williams SPL ? 6mm 55 degree field eyepiece is £69, maybe cheaper at Astrofest.
Perhaps you can find reviews as to its general performance if you cannot go to a 6mm Vixen on top of the 8mm Wide angle Vixen eyepiece.

David


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PostPosted: Wed Feb 08, 2012 9:42 pm 
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Astro engineering sell a 1.5x and 2x three element so called Apo multicoated barlow system for visual and photographic use.
Theoretically this would be ideal with the Vixen 8mm eyepiece giving 190x on the 305mm Dobsonian GoTo.
Also 280x and 380x.
At less than £50 in addition to the 8mm Vixen wide angle eyepiece.

I have had some astro engineeering items and although generally good the quality control was not that great.

If you can find someones independent review of this system it may work with the Vixen 8mm 65 degree eyepiece.

I suppose they are altering working distances to give 1.5x and 2x

It would be better if Vixen offered a matched similar system.

But this could be worth considering if you don't want to spend a lot on 2 further premium or not so premium eyepieces.

Regards, David


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PostPosted: Thu Feb 09, 2012 1:42 pm 
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With a 305mm scope with a motor drive, I actually don't think one needs to buy expensive wide field eyepieces for observing the planets.
Even at fairly high power the planet only occupies a small part of the field of view.
So if you stick to near the centre of the field simple well made eyepieces are quite sufficient.
My 1970s 8mm Edmunds 3 element RKE regularly showed me stars 0.5 magnitude fainter than complex eyepieces. It was also excellent on planets. Although a 7mm Nagler was amazing, in the centre of the field the RKE was probably better.
The 8mm RKE has only simple single blue coatings.

Well made Japanese Kelners are also very good, but don't put the planet dead centre as Kelners give ghost images.
Japanese Orthos are also good.

I am pretty sure my 8mm RKE will be just as good as modern 7 element wonders on planets.

With an undriven scope one almost certainly does better with wide angle eyepieces for planetary work as the planet moves fast across the field.

Regards, David


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PostPosted: Thu Feb 09, 2012 4:44 pm 
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Location: Long Ditton, Surrey, UK
I will take that into account next time I come across a review.

Vixen manufacture two barlows - however, I read somewhere that barlows increase eye relief, but if used with eyepieces that already have long eye relief, vignetting occurs. Have you come across this?

Do barlows preserve the FOV?

So the 45* FOV vixen NLV would be the better option if I always used my scope driven, and I only observed planets (due to price)?

Are high magnifications suitable for any deep sky objects? If so, wouldn't the 8mm Vixen LVW 65* FOV be more versatile, with a 1.5x or 2x Barlow for the planets?

Regards, Umar


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PostPosted: Thu Feb 09, 2012 5:21 pm 
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Barlows don't normally increase eye relief.
They preserve the eye relief of a lower power eyepiece, whereas normally a higher power eyepiece has less eye relief.
In normal eyepieces eye relief is related to focal length of the eyepiece.
That is why long eye relief eyepieces have more elements and are specially computed to overcome the normal situation.

Vignetting could occur if the barlows are not properly designed.
But high quality ones will probably be O.K.
With planets you are using short focal lengths so i think this would not normally be a problem.

Again if not properly designed a barlow could cause vignetting i.e. reduce the field of view. But again if properly designed will not.

You are right a Vixen barlow is probably designed to suit Vixen eyepieces.
I don't know if they make a 1.5x.

Normally deep sky objects need low, very low or medium eyepieces.
But things like planetary nebula and maybe globular clusters respond to high magnification.


I would think that the 8mm Vixen wide angle would be good and IF the Astro Engineering 1.5x / 2x barlow works it could be good.

Also a 2 inch eyepiece with large field lens, i.e. front lens will give you the best low power wide field views but here your eye pupil size comes into play.

If you are in light pollution 4mm or 5mm exit pupils are probably your best for low powers.

a 4mm exit pupil on a 305mm needs a 75x eyepiece.
a 5mm exit pupil needs a 60x eyepiece.
1500mm divided by 75 equals 20mm eyepiece.
1500mm divided by 60 equals 25mm eyepiece. I think you have this but the eye relief may not be great.

If the scope is in dark skies.
6mm exit pupil equals 50x which needs a 30mm eyepiece. Here a 2 inch eyepiece wide field is better.
There are various 28mm, 30mm, 32mm eyepieces in 2 inch fitting.

Regards, David


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PostPosted: Thu Feb 09, 2012 5:26 pm 
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If your astigmatism is large and complex you MAY not like large exit pupil eyepieces so very low power 2 inch eyepieces may not be necessary.
Some of these 2 inch eyepieces are quite cheap. Revelation? Skywatcher? TAL ?

David


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PostPosted: Thu Feb 09, 2012 5:32 pm 
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Yes vignetting could occur with barlow and LER eyepieces because of their complex optical structure.
But if Vixen barlows are matched to the wide field eyepieces it won't.
A small amount of vignetting may be acceptable.

Yes a cheaper 45 degree eyepiece with a driven mount should considered because of cost.
If you could afford the 8mm Vixen wide angle, a 5mm Vixen wide angle and a 6mm 45 degree Vixen that would be nice.

But there are cheaper options discussed above.

Regards, David


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PostPosted: Thu Feb 09, 2012 6:20 pm 
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If I start with the NLV 6mm, (eventually acquiring LVW 8&5 mm) will the restricted FOV be too tight for all planetery nebulae and globular clusters?


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PostPosted: Thu Feb 09, 2012 6:34 pm 
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Most planetary nebulae are tiny so no problem.
Also globular clusters no problem although the biggest may stretch the 11 arc minute field.

Look up M13 and M22 these are probably the largest visible from the U.K.
I think M71 may be a hybrid I have forgotten.
I cannot remember the Helix nebula size.
But I don't think I ever found it.

Anyway you have at least a 25mm eyepiece and you could pick up a budget but good 15mm somewhere.

Other deep sky observers here know much more as I last observed these properly in the 1980s.

David


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PostPosted: Thu Feb 09, 2012 7:39 pm 
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M22 given as 32 arcminutes.
M13 23 arc minutes
Helix nebula too big
M27 8x5 arcminutes.

Although the two globulars seem to large for a 11 arcminute eyepiece in light pollution you will be lucky to see half the given diameter. M22 is also low in our sky.

So in fact all the above objects and most planetary nebula and globular clusters willl look nice in a 250x 11 arcminute eyepiece. But in many cases this magnification is too high for these objects.
But you also need an eyepiece of about 15mm giving 100x.
And one around 180x but you don't need premium expensive eyepieces.

Regards, David


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PostPosted: Thu Feb 09, 2012 10:09 pm 
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In summary,
With a 305mm scope in some light pollution one would aim to have the following eyepieces.

75x
100x
150x
180 to 200x
250x
320x
375 to 400x

As a complete set.

As a beginning.

75x or 60x
180 to 200x
250x

Although premium eyepieces are nice to use they are not essential.

This is all very flexible and depends what you enjoy observing, the observing conditions and how much one wishes to pay.

In a way if one wears glasses the choice is easier because you just look at eyepieces with 20mm eye relief or more.

David


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