Filters & eye-pieces?

Hi all,

Im looking for some advice on filters for my 8" Orion..i know bout lunar filters but do i need filters for say other planets i.e Mars, Saturn? What would be the best for novice like myself as in a set or just individual filters
I use a 5.7mm eyepiece for looking at mars,saturn is that the right type or should i use a barlow with a 20mm for example?

Tony --

It's probably a controversial position, but I don't favor filters much for planetary viewing from Ireland. Here's why:

Our seeing isn't the greatest. In order to get the most detail, one usually has to sit at the eyepiece for 30 minutes to get 4 or 5 episodes, each lasting maybe 1/2 second to several seconds, during which the seeing settles and you can see much more detail than the rest of the time.

If you're constantly switching filters in and out, then you're usually not waiting long enough to get those few settled moments -- and it's usually those moments which will show you much more than a filter will.

Now, if one is willing to view Mars or Saturn for, say 3 hours straight, then you could probably make use of several filters (with 45 min or so on each) and eek out a bit more detail. But I'd be frozen stiff after 3 hours, so I usually just skip the filters and give the planet a good 30 - 45 min unfiltered.

(Note that even 30 min is a lot longer than one might think. I've seen a lot of people view for less than 1 minute at a go -- which is unlikely to reveal much detail from under our skies.)

I imagine your Orion is f/5 or there-abouts? If so, your 5.7mm is giving you in the region of 175x, which is a bit low. If you put the 5.7 in the Barlow (is it a 2x?) then you'd get about 350x, which would work great on a couple of nights a year, but would be too much on most nights. A 20mm in the Barlow will only give 100x, which is definitely too low. Something around an 8mm in the Barlow (or a 4mm w/out Barlow) would be about perfect for planetary viewing most nights (250x, again, assuming your scope is f/5).

-- Jeff.

Hi jeff,

I aint shur what type it is theres no sticker onit stating type...orion is on it alrite no type do?? you kno any way of finding that out

Tony --

Hmm... does it list a "focal length" anywhere? (The focal ratio, or f-number, is the focal length divided by the aperture -- or size -- when both are in mm. So an 8" -- 200mm -- scope which is f/5 would have a focal length of 1000mm, while an 8" scope which is f/4 would have a focal length of 800mm.)

Once you know the focal length, then the magnification can be determined by dividing the focal length of the telescope by the focal length of the eyepiece (which is the number printed on the eyepiece). So a 1000mm focal length telescope with a 20mm eyepiece would give you 50x, while a 5.7mm eyepiece would give you 175x.

To make things even more confusing, there are two "Orion" telescope makers. One is "Orion Optics" in the UK, and the other is "Orion Telescope" in the USA. Do you know which yours is -- if so we can probably look up the focal length on the net....

-- Jeff.

In general, I don't use filters either Jeff.

Hi Jeff,

Its orion optics....aint shur of the focal length i must check it out thanks