Contrast problems with reflector

[spaceboy]

The EP collection is shaping up nicely but again I think it's force of habit to what I prefer to have in the focuser. The 6mm is basically the 10mm that used to live in my 200k but I used to regularly drop a 6mm in the 200k (with good results given seeing)which would require a 4mm in the 10" but having to manually track after being used to a tracking EQ-5 I wanted a little space of an 82' EP. I know dobs are more for DSO's than planetary but I do like globular clusters. Due to terrible seeing where I am I have only had the one chance to try the dob but turning of the tracking on my 200p I can get an idea how quick things move from view. As the dob will be for 30minute journeys out to dark skies I don't want to be like the plumber who forgot his wrench.

[sixela]

I take it regards the short EP's TMB's are the way to go regards expense

If you want long(ish) eye relief, yes.

Otherwise, try to find a cheap Chinese ortho (I like them, but Teleskop Service stopped selling them) or even a cheap Plössl (though a 4mm Plössl is pretty much glued to your eyeball, I can observe with one of these without too much trouble).

Or sell the Radian and rearrange the eyepiece collection for higher magnification around a barlow, but you've got a fairly big hole between the 9mm and the 16mm.

[spaceboy]

but you've got a fairly big hole between the 9mm and the 16mm.

Shhhhh I was hoping to forget about that

[sixela]

Well, sell Radian, buy 2x TV barlow and buy something cheap to fill the hole between 9mm and 16mm might still work and be fairly cash-neutral if you buy second hand. I like the barlowed 16mm T5 (if at least that's what you have. If it's a T2, then you need a 2" barlow and that becomes a lot less convenient), so with a barlow that Radian is less critical.

[spaceboy]

This is an idea. I have a cheap barlow at the moment but what is the difference with the powermates ? I was thinking about getting something better for my webcam so a better barlow/ powermate may work out better for my visual set up also.

Will barlowing effect the FOV at all?

[pvaz]

Yes. The apparent FOV will still be 82º on a barlowed nagler.

True FOV = Apparent FOV / Magnification so your true FOV will have half the diameter and 1 quarter of the visible sky area, when you use a 2x barlow.

[sixela]

This is an idea. I have a cheap barlow at the moment but what is the difference with the powermates ?

No need for a Powermate unless you're planning to barlow very long eyepieces (or eyepieces very prone to vignetting in regular barlows). Or unless you want to image with a webcam and are 100% determined to know the magnification factor precisely, or unless you mind the long insertion depth of some regular barlows.

Regular barlows will move the light bundles for off-axis objects more towards the outside of the eyepiece, and that makes the eye relief longer (sometimes uncomfortably long) on long eyepieces and may cause vignetting with aggressively baffled eyepieces (like TeleVue Plössls) or eyepieces that have minimally sized lenses inside them (usually when you get close to the maximum field sto in a certain size). The shorter the barlow focal length (i.e. the shorter the barlow is to yield "2x"), the more pronounced this effect is.

The magnification factor also depends on the distance from barlow to the eyepiece focal plane (but that's also an advantage, as inserting an eyepiece less deeply and refocusing gives you a moderate "zoom" function).

If none of these things apply to you, then there's no need for a telecentric telextender like a Powermate (and some others).

Will barlowing effect the FOV at all?

With some eyepieces and some barlows (especially short focal length ones) you'll see some vignetting, but the Naglers you have in e.g. a TV 2x barlow won't vignette at all and the 82° AFOV will still be there.

Of course, using a barlow is a lot more fussy than getting an extra eyepiece - that's the only drawback. But it certainly saves money on eyepieces.

[spaceboy]

Thanks again guys.