Finders and hiders

[PurpleFringe]

I have a pair of 8x40 binoculars which I like very much. I enjoy tracing out constellations with them, looking for colour differences in widely spaced stars, open clusters, asterisms, and ... many other sights. They are small and light, and I haven't forgotten my sense of wonder at 'discovering' the Sword of Orion when just starting this hobby, and wondering what that cloudy patch in the middle was. Even the moon looks great in them.

They are decent quality but not extravagant achromat porro prism binoculars. They have one feature in particular I'd like to mention: on the side they report 'Field of view: 8.2 degrees'. Wonderful!

They aren't perfect though. I can't hold them steady enough for a proper long gaze. My arms get tired, and stars jiggle around despite my best efforts to hold still. After picking out enough constellations, I get a crick in the neck.

The point is: I enjoy 'finding'. It is part of my viewing experience. I want it to improve!

A finder?

I've used enough straight through 'inverted' finders (two is plenty) to know that I want my finder to be a RACI finder.

A RACI finder set on a stable scope would solve both the jiggles and the crick. But it would lose the ability to sweep across vast areas on a whim. So a finder can't do everything that binoculars can, and binoculars can't do everything a finder can. Fine - they are different creatures. In that case, since I'd like both, I'd like a bit more light grasp and power in my finder, while keeping the same exit pupil. A 10x50 RACI finder sounds ideal to me.

Unfortunately, almost all finders have a small field of view. Why? I don't know.

Let's see what should be possible.

A certain manufacturers website has a great page for calculating fields of view with their eyepieces. Suppose we use an eyepiece with biggest possible field stop from 1.25" and various objectives.

10x50: could have a 6.4 degree fov (240mm focal length)

9x50: could have a 7.2 degree fov (216mm focal length)

8x40: could have a 8.1 degree fov (192mm focal length)

6x30: could have a 10.7 degree fov (144mm focal length)

Comparison with what's available?

After looking at available RACI finders from various sites, there were three main types of RACI finder I could see:

Small budget ones, 6x30, about £40, 7.5 degree fov (vs 10.7)

Big budget ones, 9x50, about £70, 5.5 degree fov (vs 7.2)

High end ones, accepts different eyepieces, 9x50 is about £150 with 6.1 degree fov (vs 7.2)

There is also a wide fov eyepiece for the high end one. Sounds good, so I asked about it: the prism on the finder has 21mm free aperture. I guess this will limit the field of view or the brighness somewhat, probably both. Please can the wise correct me if I'm wrong.

There is another option - build your own. I've considered it, but it looks expensive. Would probably be a lovely mini-scope though.

Comparison with binoculars

I was able to find 10x50 binoculars from 5 different manufacturers, all claiming 6.5 degree field of view, some for as little as £60 and a sky at night group test winner for £150. I know these may not have full edge to edge sharpness, but here we have two 10x50 straight through correct image finders claiming the maximum possible field of view (or more!), with individual halves costing less than the price of a budget 6x30 finder.

I'm still wondering why finders have such a small field of view? I don't know. It seems so sensible that a 'finder' should have the maximum possible field of view - to help us find things. Maybe it's the porro prisms. Anyone know about any RACI porro prism finders? Or is the finder redundant, an old fashioned relic superseded by GOTO?

[baggywrinkle]

The above is a really nice summary, thanks for posting it.

The FOV info is really usefull.

I like my bino's for the wide sweep they give me. In order not to get tired I use a hig backed camping chair which give some neck support and pick a part of the sky I want to look at. By supporting my arms (elbows) on my chest my arms do not get too tired.

Even with GOTO I do not think the finder is dead. Not all objects are in the GOTO data bases (depending whose they are) and for finding asteroids, comets and other transients you still need to hop around.

[Schorhr]

I wondered too.

That's exact the problem why I don't use a finder but a rigel/telrad now. Most finders are so narrow-viewed and/or expensive.

Usualy the telrad works very, very well, but sometimes star hopping is required anyway (light pollution or otherwise no stars to use the telrad with). Plus it's nice to view everything and see DSO with low magnification.

On a larger telescope you could put a wide-field scope as finder with a nice wide angle finder...

[small telescope]

I have considered using my 76/300 telescope (available for 20€/17gbp) but it is too heavy for the telescopes I have. Perhaps in a plastic tube and mod it for 2" EPs. Still leaves the problem if you want the image the right way without increasing magnification.

WIth the 300mm focal length and a 1.25" EP this could be

25mm/67deg afov = x12 = ~5,5deg fov (nice wide angle but erfle design on f/4 has problems with outer sharpness)

40mm/45deg afov = x7.5 = ~6.5deg fov (but too large exit pupil and narrow afov, I have a 10€ Plössl like this.)

Also it could be a use to those dreaded toy/beginner-50/300 and 70/300mm telescopes for 23-40eur/20-35gbp (and up), something like the KN-SCOPE30, GX-70F300, M-30070, SV-350... Though that's about the same price as the finder I mention below.

Perhaps only a solution if you have one of those in the basement already.

[Camera lens mod]

Another alternative could be a old camera lens modified to take a eyepiece. I've see this done for larger FL to use them as a portable telescope, but I suppose it could work with a inexpensive 50mm lens as well. Camera lenses are high quality :-)

I just tried with the KIT lens (18-55mm) surprisingly crisp image using both a 8mm60deg and 40mm40deg EP. Too lazy to go get the 20mm wide angle, but you get the idea. Together with a cross-hair EP you would have a nice (zoom) finder ;-) AND older lenses for analog cameras are sometimes dirt-cheap used. Sometimes even good ones even with <f/1.8.

Has anyone done this for a finder alternative? Even new f/1.8 lenses aren't too expensive (compared to f/1.4/1.2 or good modular finder with eyepieces)

I think I now know what one of my next 3D printer-projects will be... But would work with some tube and tinkering as well.

[Telrad]

Did I mention the Telrad finder? ;-)

[6x30 finder] (eek!)

My cheap-ish 6x30 is in some box to be never used again. But I suppose there are some higher quality one. I saw some 50mm finders that can be used with eyepieces as well,

[50mm finder with EP]

http://www.teleskop-...radsichtig.html -about 200mm fl!

With a ~25mm ~65deg afov Erfle that would be 8x, over 8°deg fov, little more with the 40mm - though without stopping down very large exit pupil.

Also as kit so it's angled and not mirrored, and a crosshair EP (but not wide angle)

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p1399_TS-9x50-90--Winkelsucher---mit-1-25--Fadenkreuz-Okular---Halter.html

[diy finder with inexpensive lenses]

http://www.scopemaki...ns/smallens.htm - and lenses from surplusshed

(Eyepieces)

As for an eyepiece with those possibilities, at least 20mm Erfles are inexpensive (30eur/26gbp shipped, or while you're at it, surplusshed has cheap erfle kits).

Nothing is worse then switching from binoculars to a narrow 40 degree afov eyepiece ;-) It's half the fun, I love the spacewalk effect you get with even cheap 60/70deg eyepieces.

Ciao!

-Marcus

(I used http://www.sternfreu...de/orechner.php for the calculations though I did not bother using the exact EP specifications all the time)

[PurpleFringe]

Thanks both for your comments.

I've tried a telrad - got one currently on a dob I'm borrowing. It works beautifully with bright stars, but I struggle to find anything mag 4 or below. Was looking for Iota Cancer a few weeks ago, but never found it. The scope is 1200mm focal length and comes with 1x25mm eyepiece. Finding things is a bit of a challenge. From my location not a single star in Cancer is visible with naked eye, so the telrad is not much help.

Yes the TS finder with eyepieces was the one I asked about. I've also considered the mini borg 50. They do look like fun. Maybe one day, though not sure I'll ever figure out which components to put together for it.

[Schorhr]

Indeed, sometimes the telrad or the like is not very useful.

I bought a cheap 40mm Plössl instead of adding a second finder for the bad observing conditions, the ideal budget solution for my small scope, but it has a lower focal length ;-) On a 2" focuser there are better eyepieces available of course, though they cost the same as a decent finderscope...