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I've been looking for a cheapo solution to attach the cheap and ubiquitous red-dot finder to my Celestron 20x80 but didn't like the official clip thing that Celestron sells. Bad reviews complaining of it easily snapping, and to me, overpriced.
After much research and counting of pennies, I went for this all steel, no-snap solution, costing a whopping £6.90 (with free shipping). From the top:
1 x 20mm Dovetail to 11mm Rail adapter. £2.69 with free shipping.
1 x Picatinny/Weaver 20mm Rail Base Adapter (used to attach scopes to rifle barrels) . £5.11 with free shipping
1 x bit of thick plastic to act as a shim. Anything will do.
Bit of background info first.
Having problems auto aligning my Meade 8" LX90GPS. Works at home Liverpool, but not at my site in Hadnall. Doesn't do LNT, not level and never finds north, just spins around like a dalek! Thought LNT module was duff or damaged in transport. Tried everything, then noticed my phone was off north too, tried two other phones, none of the three ever showed same north at same time, even after calibration.
Then to my amazement one of the phones suddenly flashed up High Magnetic Anomalies. Moved 20 metres away and ok.
So upshot is that under the ground below where I can view from is either a metal dump or electric cables or some other magnetic force. Can't dig it up, decked over. Can't move further away it's beyond my fence line!!!!!
So now to finders.. Can't get on with the red dot LNT module for alignment, too flimsy and to big a dot and useless for auto align as above.
So want a decent optical finder, to sight and alugn manually so criteria are
About £100 all in. Not straight through back can't contort any more....
So need RA but is RACI really better than just RA for the extra cost.
6x30 is it too small mag, maybe 8x50, 9x50 or look for used 10x60 or is that too big.
Any ideas or what else do I need to consider, some say EPs are fixed, is this ok or not. Looked at DIY, but the mounts are soooooo... expensive too.
Hope this makes sense and look forward to your help.
OK, I am hoping that for myself and anyone else, the collected SGL can help clarify the signifigant differences which all the tags above cover, & any I leave out.
There are many diagonals on the market. Some costing a large amount, some for not much at all. Most look very similar to each other, moreso than eyeypieces even.
By Ben the Ignorant
Don't blame me for the silly pun, Tele Vue actually used it in their ads back in the days when they were written on paper. Now that you are enlightened by this piece of trivia in the history of advertising, here's the topic. My urban observing spots are surrounded by public lamps so I need complete blackening in my finders as well as my scopes, or arcs of light and various shapeless flares will show when I don't aim high. Little stars in a 30mm scope can't compete, star-hopping is made difficult.
This is how I do the blackening. I start with the amici prism.
Cleaning it with alcohol proved necessary, some grease was on the exposed faces.
Next I paint the rough surfaces with a sharpie.
And the rough edges, too.
When all non-polished places are black, funny, it's actually possible to make the prism look all black from a certain angle.
Then its housing was not cleanly put out of its mold. I don't like finding uneven stuff, so I rectified it even it if was not important for the finder's function.
40-grit did the job in a couple minutes.
The plate side is rough from the 40-grit paper but is now planed, the plate will screw onto it without leaving gaps.
From its usable angles the prism now looks like that: a clear window with black sides that will absorb stray light. Larger amici prisms for full-size scopes might require the same treatment; practicing on cheaper stuff makes it less intimidating.
The housing is garnished with blackboard paint. See how the inside and the barrel are darker than the surface with the screw holes. This paint is water-based, doesn't smell, dries in minutes, and can be removed from places it covers by accident, just scrape it and rub with a wet towel, not a trace will remain. But it sticks hard enough to not chip over time. Have you seen you school's blackboard chip?
The sharpie also cures the objective lenses' rough edges disease. Another funny effect, when the side is barely half-blackened, the untreated edge already looks gray, as if black could reflect on other things; this is promising for the final effect!
The promise is kept! In main scopes or finders, this black ring will kill off nearly all the light that touches it, I can guarantee it from experience!
The dewshield (made in the proper length by Sky-Watcher, by the way, congrats!) is also painted. See how the bare anodized aluminum ring at the rear is shiny. I don't paint that area or the doublet won't enter. Its own layer of sharpie paint plus the blackboard paint would be too much. Where are all those white dust specks coming from?
The retaining ring is a treacherous spot in telescopes because the total area is large even if the thing is narrow, can't leave it shiny, especially at those grazing angles! See the difference with the threaded outside of it. Not an essential job in a finder but done it a minute, so why not?
The eyepiece lenses were white on the side, too, Before the sharpie touch, the objective was that white and bright, but you'd be surprised how quickly the non-yet-painted part turns dark gray when you start painting the rest. The inside of these retaining rings will receive the blackboard touch.
Now that's how things should look! Both the lenses and their bevels are coated in black. The bevels seem a bit shiny from this angle but their absorption of bad light is vastly better. The eyepiece is a simple Plössl, only two cemented doublets with rounder bellies facing each other, mounting them right is foolproof, unlike other optical designs. You can improve the contrast in unexpensive and simple eyepieces with a good blackening. Costs nothing, proportionate to the thing's price. The eyepiece is fully-multi-coated, by the way, more congrats to Sky-Watcher for taking accessories seriously.
Another improvement: the tiny original screws are replaced by homemade larger screws. Those white plugs are used in the assembly of furniture, but only one tool store had them, and only once. I bought the two packs they had, can't find them anymore, anonymous packaging. They include a piece of threaded 6mm rod of the right length, just had to plane the tip. I superglue a stainless washer at the top, and fill the space with O-rings. Only those at the outside need to be glued, the others are pinched between them. Just seeing the screws makes the advantages obvious, compare with the two original plastic screws. Hard to grab with gloves, or even without gloves for that matter.
Now the flocking. The foam side of adhesive velcro is ideal for small areas. I don't glue it to the tube because that would make replacement messy. Instead, I glue it on a strip of paper.
Then I fold it into a loop, and tape it. It's not round now but it will when it's forced inside the tube. That might be a useful trick when flocking larger tubes; glueing directly onto the tube allows no mistake, and can force you to leave a poorly applied flocking if it sticks too hard. Can't remove that thing but I need to adjust it! GRRRRR!!
The finder is so short, only two rings did the job. Do not put that too close to the objective or it will enter the light cone. So, a few millimeters are not flocked but that's okay because the tube was already painted flat black, and all the rest of the finder is treated. Sky-Watcher put a sensible baffle in the back.
There, the light cone is not eaten up by the foam, we have a clear view of the optics' edges from objective to eyepiece. The criteria are the same for bigger refractors.
Before any flocking and blackening was done, the inside of the eyepiece was that shiny, the reflection on the side is very bright! Pic taken through the completely assembled finder.
It was very tough getting a pic at the same angle, the flat camera objective is hard to position as accurately as the eye with its round cornea, but it's clear the lateral reflections are much dimmer. Again, picture taken through the complete finder even if it might look like the eyepiece was removed from the tube.
Before the black-ops job.
After the ninjas came. Sorry if the shot is blurry but the brightness comparison still stands. The area around the pupil is darker, and even the inside of the eyecup is darker since I applied a little blackboard paint there, too. It's shiny on the top picture but matte here.
And if the difference does not impress you, see how the tests shots were made: with this setup, flashlight at an angle, and only one inch from the dewshield. Note how a few extra O-rings between the objective cell and the finder bracket keep it from playing. Another set of O-rings between the prism housing and the bracket complete the task. Finders moving fore and aft, and allowed to rotate lose alignment. Thanks to the firm push of these rubber rings, the tube is held tight but free to be adjusted.
I had to buy a few O-rings for something non-astro, but of course you have to take the whole box. Not liking to leave tools unused, I looked for ways to make these rings profitable. One of the useful tasks was as loosened screw safety. This finder won't fall off to the ground.
She's not missing anymore.