Barlows and Focal Reducers

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Barlows and Focal Reducers

Postby aztecastromcj » Wed Jun 17, 2009 11:13 pm

at min i hav used my barlow now and then but prefer just to use lower ep instead (i think having another lens or lenses in way must affect quality)
although i did find it more useful on brighter things like moon and planets and its doubling ur no of eyepeices magnifications obviously is main benefit
so what about a 0.5 focal reducer i have seen an opticstar one for £25 would this be gud investment in similar way to barlow
i understand it will do opposite but would i be able to use it with my 7mm say to get 14mm and lower mag ?
my scope has factor ratio of 11.8 so when using barlow it is 23.6 and if using 0.5 focal reducer it would be 5.9 factor ratio optical system
would this be ok ?
when using my 25mm i get x60 mag and x120 with barlow, so would i get 30xmag with 0.5 focal reducer ?
also my 25mm has afov of 50 degrees giving be 0.8 of a degree view in eyepiece
i gather x2 barlow halves this ? so what effect would 0.5 focal reducer have ?

i'm going to get 32mm ep anyway for wider field of view to help with viewing galaxies
at min not doing any imaging just viewing and like to know how useful a focal reducer would be in viewing galaxies and maybe helping me to use my 7mm more
thanks again for advice
aztecastromcj
 
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RE: Barlows and Focal Reducers

Postby ronin » Thu Jun 18, 2009 8:20 pm

My advice would be to stop and slow down a bit.
Buying a reducer to be able to use a barlow seems, well, odd.

A 32mm would give you a better field of view, however with the focal length you have it will still be fairly narrow, better for locating things with if they are bright enough. I have one and use it for the initial locating. MAgnification on yours would be ~37x

After that I usually step down to a 20mm, possibly 15mm.

After that with it centred I may move down to the smaller one - My scope if a much smaller focal length then yours.

The 7mm is I would say too much for your scope, a 10mm is probably better. But keep the 7mm as on a good night it may be useful.

I would go for the 32mm as you want, for FoV, then later get a 20mm and one in the 10-12mm range. Or the other way round if you want the magnification.

That should cover you for a fair range of conditions and magnifications. You may like to look at eyepieces that are classed as wide angle. Also it depends on the price you are willing to pay for an eyepiece, range can be £20 up to £300.
ronin
 
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RE: Barlows and Focal Reducers

Postby aztecastromcj » Fri Jun 19, 2009 12:13 am

hi thanks for advice i didnt mean i wanted to use focal reducer with barlow
i just wondered if using 0.5 focal reducer alone with 7mm say would give me 14mm eyepiece view
(or would it only be useful with 25mm 32mm to give us wider view)
like 25mm with barlow gives me 12.5mm view
been having a look at eyepiece prices and wouldn't spend over £40 at min
seems i have 2 choices for decent 32mm at this price celestron omni or skywatcher both seem same 4 element plossyl designs
so prob stick 2 skywatcher tho not decided yet

also 32mm divided by 1500mm = 47x mag
aztecastromcj
 
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RE: Barlows and Focal Reducers

Postby sftonkin » Fri Jun 19, 2009 10:46 am

Some old "rules of thumb" for eyepieces:

[*] A few good eyepieces will be more useful than lots of cheap ones: get the best you can afford.
[*] Your first eyepieces should be a low power and a medium power.
[*] Low power: focal length about 4x the focal ratio of the telescope (but AFoV x FL should not exceed 1600 in a 1.25in focuser).
[*] Medium power: FL about = FR of scope (note that this will give an image that is fully resolved on your retina; any further magnification compensates for lack of visual acuity and will also serve to darken sky background and extended objects).
[*] High power: FL just over half the FR of the scope. This will be your most rarely useful eyepiece.

This will give you approx 32mm (assuming 50* AFoV), 12mm and 7mm -- i.e. almost exactly what Ronin suggested. I would suggest something like a decent 32mm Plossl and a 12.5mm Orthoscopic to start with if you are on a budget. (Orthos are not "popular" at the moment, but a decent Ortho is a superb contrasty eyepiece for solar system objects.)

Instead of the 7mm, you could use a [b]decent[/b] (i.e. apochromatic) Barlow. For more info on Barlows, see [url=http://astunit.com/tutorials/barlow.htm]here[/url].
sftonkin
 
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