Help with increasing magnification....?

[Orange Haze]

Hi All. My scope has a 120mm lens so I think I am right in thinking that a maximum use-able magnification for me will be around x240. (F=600)

While I am not fussed about getting x240, if I wanted to get up to x150, is it best to buy something like a 3-4mm eyepiece or use a barlow with my 10mm eyepiece? Or is it much of a muchness?

A Barlow in my novice mind would seem to give me more flexibility, but any advise you guys have re gaining magnification would be great. Thanks!

[John]

The Skywatcher Startravel range are not designed for high power observing - 100x - 120x might be as much as you would want to use.

What eyepieces do you have now ?.

[Orange Haze]

Hi John. Have only what was sent with the scope, a 25mm and a 10mm. Was thinking a 2x barlow which should give me x120 using the 10mm.

[Ags]

Hi, I have an ST80, and I find it is best at very low magnification - 16 to 25x. At higher mags the chromatic and other aberations in the scope become too obvious. I imagine these aberations would be even more of a problem in an ST120.

To use higher mags on planets, you could try a green filter to eliminatethe red and blue wavelengths (and thus the CA), or you can get a minus violet filter or fringe killer that does the same but lets through a bit more light. I've never used them myself - I have a mak for high power viewing.

[Capricorn]

The problem is that it isn't as easy as picking a 5mm eyepiece, or a barlow and a 10mm.

I have an f/5 refractor, a 5mm plossl will occasionally, but not often, be OK on it, a 6mm plossl is usually OK with no problems.

A 4mm plossl I have is a complete waste of time even considering being used.

I have a 4mm TMB and a 4mm Antares W70 both are reasonable, not great or good just reasonable on the scope.

Basically just 1mm and the eyepiece design makes a difference. I have in effect about 6-8 eyepieces now at 4mm, 5mm and 6mm just to work out which is usable at what occasion. And if BST made a 4mm I would get that to try out as well. Have the 5mm BST.

[steve_bham]

I've used the 6mm vixen plossl in the scope to view Jupiter. I did stop the front down to 80mm and was a good view.

[Ags]

True - I have also found that stopping the scope down to F8 gives far better performance. CA would be much less.

[Orange Haze]

I had thought about combining a minus violet filter with the use of a higher mag eyepiece. Stopping down would seem to help as well. Want to make the most of the scope, when a clear sky appears.

[John]

Stopping down will reduce the CA a bit I guess but it also reduces the aperture with a consequent reduction in resolution and contrast. Filters are all very well but you are filtering light out of the image and therefore reducing resolution and contrast again to some extent as well.

The ideal solution is the one that AGS uses - have a maksutov for planetary / lunar and other high power work and use the fast refractor for what it's best for, ie: wide angle, low power viewing.

[Ags]

Just to add that a mak is really sharp, even my little 100mm mak shows lots of detail on mars and I can easily use 220x or more (mileage may vary). Buy a minus violet filter or save a bit more and get a second scope...?

[AlBoning]

With any eyepiece design in which the eye relief decreases with decreasing eyepiece focal length (Plossls for instance) putting a longer focal length eyepiece in a barlow will give you the higher magnification of the equivalent in magnification shorter focal length eyepiece while preserving the eye relief of the longer focal length eyepiece. Consequently you get the magnification you want with an eyepiece that is more comfortable to use.