Finder scope question

I have an 8" dob, with a 6X30 finder on. I live in the suburbs, and can barely glimpse the milky way with the naked eye. Sometimes when I’m star-hopping I have some difficulties seeing the fainter stars I use, and sometimes have to use adverted vision to see them.

My question is: Would it help to get an 8 or 9X50 mm finder scope instead?

Thanks
Peter Bruus

Yes. The larger the objective the more light you can gather. Now in pratice the differance between 30mm and 50mm (about 78% more light) I think that will astound you providing it is the same quality. Given the light polution problem you will want to get a "high contrast" scope or good quality with good coatings. Antaries Canada makes a nice scope for not to much $. TAL (Inties) Russia makes a real nice finder scope. Of course TAK is probably the best, however, big $.

Tim

Peter, I would suggest first trying a Rigel Quickfinder, it works 'just right' on my 11" dob. If you must have a finder, try the Anteres 8x50mm 90 erect/correct image, works well & low cost. I don't use finder scopes anymore, Quickfinders on the smaller scopes and Telrads on the larger ones. They don't affect the balance as much, easy to remove, reset near zero and are MUCH easier/quicker to line up. I find the reflex finders will always put the object in the fov of a low power eyepiece. Keep it simple, Ralph

Peter,

I purchased The Orion 9X50 Right Angle Correct Image(RACI) finder for my 6"F8 refractor a few month ago and I like it a lot. I find it to have a big advantage over the 5X24 and 6X30 finders on my smaller scopes from my Mag 4.5 backyard. (actually a 50mm finder collects 177.7% more light than a 30mm)

The Orion finder uses two perpendicular thumbscrews making alignment very easy. The finder comes with own mounting bracket and the tube can be rotated 360 degs in the holder. Highly recommended.

Erik D

PS. Orion also has a 9X50 straight thru finder if you prefer the more traditional reverse image view.(~$60)

Like someone else (Jason?), I upgraded to the right-angle correct-image 9x50 from Orion. I'm very happy with it, it's comfortable and intuitive to view, and the images a big step up from a 30mm finder, but I too found that initial aiming was difficult without the Rigel Quickfinder I also added - I used my straight-through finder with both eyes open, which made finding an initial reference star easy, but that didn't work with the right angle finder.

Good question - two other things to think about – the exit pupil for your finder(s) and getting the maximum field of view. Both make a huge difference in how easy the finder is to use. (And in my mind a RA finder is a must from the get-go anyway!)

The Exit Pupil of our eyes is the maximum opening the iris will go to when completely dark adapted. For each finder objective size there is a “minimum low power” (and associated size of the field of view) and if you go lower than the suggested minimum you may get wider fields but you will not use all the light transmitted through the finder. This is because the cone of light coming out the eyepiece will spill over onto you iris and does not all go into your eye.

A young person has an EP of about 5 to 9mm but our exit pupils shrink as we get older (the iris of our eyes tend to not open as far as we age) so an old guy like me (50-ish) may only have an EP of 5mm. (Actually, my eye doctor said it was 5.6mm max a few weeks ago so I happen to know).

You get the exit pupil of the finder (size of the cone of light comming out the eyepeice) by dividing the objective size by the power. Thus a 7X50 (50mm objective at 7 power) has a EP of close to 7.1mm and a 9X50 would have an EP of 5.5mm (etc.).

Generally speaking keeping the EP about 5mm is best in light polluted suburbia as it’s a compromise between age and the fact we may never actually get our eyes completely dark adapted (pupil size at the maximum) in our own backyards.

Using the 5mm EP example the 35mm finder would be best at 7 power, the 28mm at 5.6 power (etc.)

I use a very light home made 4X25 right angle on my short tube 80 (6mm exit pupil) and find it’s a good trade off between weight/bulk and seeing fainter stars and wide fields (18 deg.). I can see about 3 mag. “deeper” than with a 1X reflex (red-dot) finder while the finder does not weight a lot more than the “red-dot”. On my 8 inch SCT I use a correct image right angle 8X50 (I modified the stock Meade finder with a 45 deg. correct image diagonal) and that gives me a 6.5mm exit pupil at about 7 degrees FOV. Both of these finders were "messed around with" until I liked what I saw (but as always, your milage may differ ... )

FWIW - TM

Regarding the Sky Atlas...

If you live in the city, a 6x30 finder does not show much, an 8x50 really does a much better job.

Field of view is a good point to consider. Most of the Chinese 50 mm finders are right there about 5 degrees. Adequate but 7 degrees is much much nicer. I have two of the 5 degree jobs and one Sovietski which is far more nicely made which has a 7 degree FOV. Makes a difference.

But mostly, when the skies are Magnitude 3, 50 mm finder is nice.

jon

If you have some dough to spare, the Tak 6x30mm and 7x50mm stright through finders have got to be seen to be believed. Images are bright and you will momentarily wonder if the finders are APO too!
Ivan

For several years I've used one half of a 7x35 binocular as a finder and greatly prefer it over either a conventional or 1x finder. The erect, correct, straight thru viewing makes it easy to relate the finder view with the naked-eye view in your other eye, or with the star pattern in the atlas. The field of view is 11 degrees which is much wider than other finders. The main problem is that most binoculars don't have crosshairs. My solution was to drill a 1/16 inch hole at the focal plane and insert a tiny (#0 I think) machine screw to mark the center of the field. The screw is visible without illumination under dark skies since it is much thicker than crosshairs.

I wanted more brightness so have just bought some 10x50 binoculars which I will use for a bigger finder.

If you don't mind some tinkering, this gives you a really nice finder. I wish some manufacturer would see the light.