eyepiece or barlow

[Kevdan]

Hi All,

I have been getting somw very good but very small images of sturn and the orion nebula and was wonderinghow to go about getting bigger images, at the moment the smallest eyepiece i got is a 9mm plossl and with a 2x barlow on my 150 x 750 scope therefore my magnification is around 160x.

Is it better for me to get near the 300x max mag for my scope with a 3mm eyepice alone or by getting a 4mm or 5mm eyepiece and use the 2x barlow which in effect takes it over the max mag. Also if I use a eyepice of around 5mm with a barlow will objects race across the field of view faster than a wide FOV 3mm. Only reason for asking is that me not having a RA motor keeping track of stuff can be awkward.

Thanks,

Kev.

[John]

180x - 220x will be the most you can practically use with your scope under the typical UK seeing conditions. You won't find 300x any use I'm afraid - I rarely go over 250x with my 10" scope.

The Orion Nebula is a large object though - it should pretty much completely fill the field of view at around 100x.

[Kevdan]

The Orion Nebula is a large object though - it should pretty much completely fill the field of view at around 100x.

I can see it and the 4 stars at the centre are probably about .25mm-.5mm apart does that sound right ? why is the mag so low I was under the impression that you go for around 50x per inch of primary mirror?

Kev

[John]

You need low power (30x or so) to see the whole of the Orion Nebula - the 4 stars (the Trapezium) are just in a small, central, part of it. The nebula spreads out for a long way beyond those, rather like a fan, to my eyes.

The 50x per inch is a theoretical figure only - in practice the most you can use is limited by atmospheric conditions - hence my suggestion of a lower practical maximum.

[JamesM]

Kev you might well ask the question if you can only normally see around a maximum of x250 what's the point of a big scope. The answer is that a larger aperture can 'resolve' more detail on a given object which doesn't always require the maximum magnification.

James

[brantuk]

The limit is set by the atmospheric conditions and in the UK is around 200x to 250x as mentioned above. Looking through a high magnification eye piece can be like driving down the motorway in hazy or foggy conditions.

There are several other factors like elevation of object (the closer to the horizon the thicker the atmosphere), wether the scope is cooled to ambient temperature (to eliminate warm air currents), location (looking over warm houses can cause a shimmer to be seen), darkness and light pollution (the darker the better), just to mention a few.

HTH

[Moonshane]

a couple of things not mentioned are :

• the shorter the focal length (eg a 5mm eyepiece has a shorter focal length than a 9mm eyepiece) usually means the shorter the eye relief - this means that you have to get your eye very close to the glass with eg a 5mm or even a 3mm eyepiece. eye relief is not changed with a barlow so eg your 9mm plossl (eye relief 7.2mm) would still have the same ER when barlowed but a 4.5mm plossl (if one existed) would have ER of 3.6mm.
  
• field of view - yes with more magnification, things race across the view more rapidly. wider field eyepieces help but not much with a driven scope.
  
• the moon - although the stated maximums are quite right for most targets, the moon and double stars can often take a bit more. and even approach the 50x per inch you mention. with my 12" I can manually track at 500x on the moon but it's hard work and the view is not much (if any) better than at 250-300x so I don't usually push it that much. it's only in places like Hawaii where the air is really steady they can get really high mags.
  
• my most used higher power range is between 120x and 200x. there are not many occasions (except with the moon) when you can exceed this.
  
• 200x in your scope would be a 3.75mm eyepiece. a 7mm barlowed would be more comfortable and more used than a 3mm or even a 4mm I suspect.
  

cheers and hope this helps a bit.

Shane

[Moonshane]

ps. to answer your question (and actually against my comments above ) I tend to prefer a simpler arrangement with just an eyepiece and just buy more eyepieces over time, hence my stupid collection below.

[Kevdan]

Thanks guys

All I need do is get a better quality barlow, I got a 2xGSO but it is quite poor condition. And I may try and push by getting a decent 6mm eyepiece

Kev.

[swamp thing]

Hi Kev

Good call.

IMO a good Barlow is always a good investment.

I would definitely try a few eyepieces around 6mm before you buy some have quite short eye relief that some people find a bit uncomfortable.

Regards Steve

[Moonshane]

one use for a barlow that I have just recalled (in fact two) is/are that you can test what a possible new eyepiece will look like (broadly). a 2x barlow gives 2x increase in magnification (effectively) but you can unscrew the lens cell and screw this into your eyepiece like a filter. this creates a 1.5x barlow (usually) therefore with a 9mm plossl you could get an idea about what a 4.5mm or a 6mm eyepiece would be like (in terms of magnification and field of view at least).

the other is that you can use a barlow in conjunction with a laser collimator to effect good primary mirror collimation. google 'barlowed laser' for more.