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ChargeNut

Celestron Red Dot Finder Alignment Issues - Fixed!

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I recently got a Celestron Astromaster 130EQ Telescope for my birthday and have been having loads of fun looking at and taking photos of the Moon over the last few days. One issue I had with the scope was that I couldn't angle the Red Dot Finder down enough to align it with the object I could see in the Telescope. I have been using it by taking note of the distance the red dot was out and trying to direct the telescope taking this difference into account. It occurred to me (before I looked at this forum) that if I could only raise the tail of the red dot finder higher (I could see that to bring the dot down the LED at the back was raised), I could get the alignment accurate. I took the mounting bracket apart and looked at the design of the plates. I then designed some new ones in Sketchup widened the back dimension and, as I have a 3D printer, printed the new ones out. The new bracket plates are 2mm wider at the back hence raising the back of the finder higher by 2mm, which in turn lowers the red dot down to it's correct alignment point.

I thought I'd post on here to say how I resolved this issue as I notice that others had also suffered from this problem.

I have posted the STL (design) files for the 3D parts up onto Thingiverse should anyone want to download and print some for themselves, I will also put out an offer to print some for others should they not have access to a 3D printer (just PM me).

Here is a picture of the new mount plates in place.

Terry. 

2017-04-04 22.31.33.jpg

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Good job. The Astromaster series used to have a custom RDF that didn't get great reviews so it looks like they've made a change to fit a standard RDF instead. Shame that the new Celestron bracket isn't quite right, perhaps you should forward what you have had to do to Celestron so that they can make further adjustments for the next batch. 

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I wonder if this sort of modification could be useful to non RDF finders too.  I have a RA Optical finder on my telescope and always notice that to get it aligned I am literally right on the limit of one of the adjustment screws - I always think that its a good job that just it just about gets plumb on as I would have no adjustment left and I think that a bit more starting angle under the back of the finder would be useful.  I am still trying to work out how to align my new Skywatcher red dot finder as when you move your head the position of the red dot varies - I guess the notion is that I should get the red dot bang in the middle of the finder, but I can't work out how the adjustment on the RDF functions at the moment.   It looks identical to the Celestron one above, but is on a Skywatcher bracket so possibly sits just a bit higher off the tube.

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That is very clever what U have done there.:thumbsup: Problem solved. /baader-40mm-super-sky-surfer

This what I have just bought. Same alignment problem. Bit more adjustment on this though.

 

Edited by Grotemobile

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1 hour ago, JOC said:

I wonder if this sort of modification could be useful to non RDF finders too.  I have a RA Optical finder on my telescope and always notice that to get it aligned I am literally right on the limit of one of the adjustment screws - I always think that its a good job that just it just about gets plumb on as I would have no adjustment left and I think that a bit more starting angle under the back of the finder would be useful.  I am still trying to work out how to align my new Skywatcher red dot finder as when you move your head the position of the red dot varies - I guess the notion is that I should get the red dot bang in the middle of the finder, but I can't work out how the adjustment on the RDF functions at the moment.   It looks identical to the Celestron one above, but is on a Skywatcher bracket so possibly sits just a bit higher off the tube.

It's not a case of a "Skywatcher" bracket and a "Celestron" one, rather that you have the standard Synta bracket and the one pictured above is unique to the Astromaster range. The RDFs look identical because they are identical. Anyway, the red dot always points to the centre of the finder window. If you see it anywhere else then your head must be in the wrong place. When you turn the adjustment knobs the entire finder moves up/down/left/right, not the red dot.

With regards to the optical finder only just having enough movement "vertically" the standard thing that people do is to place a shim under the finder shoe to angle it a bit. If it needs to be raised at the back loosen off the screws/nuts holding the shoe to the tube, push your shim in from behind (a piece of card will probably do) and then tighten up the screws/nuts again. You have to be careful doing this of course because the nuts are not captive and there is the risk that they fall off and you might need to use a tool on the inside of the tube (near your secondary!) to do the nuts back up again. The other cause could be that your secondary mirror is not quite centred properly. If so then the primary will be tilted to compensate and the optical axis of the telescope will not coincide with the mechanical axis of the tube. In other words, check your collimation before you try adjusting the finder bracket.

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44 minutes ago, Ricochet said:

If so then the primary will be tilted to compensate and the optical axis of the telescope will not coincide with the mechanical axis of the tube. In other words, check your collimation before you try adjusting the finder bracket.

Many thanks for this, I think I had experienced a stray thought sort of in this direction and dismissed it.  I collimated the telescope for the first time successfully the other week, since doing so it has been out of action having a squeak fixed and I have not had a chance to see if the collimation has achieved anything.  However, this comment about the finderscope is usefully made as I have not tried the optical finderscope since doing the collimation.  If it could make a difference then I will be sure to try it before I unscrew anything. :icon_biggrin:

47 minutes ago, Ricochet said:

When you turn the adjustment knobs the entire finder moves up/down/left/right,

So it does!  It's very subtle isn't it?  I have been twiddling those knobs, but couldn't see what was changing - I've just spotted where the movement occurs on both axis.  Before spotting the movement I did wonder if the RDF aligned in a similar way to the optical finder (which I've always had the hang of doing), but then wondered if the fact that it sat so close to the end of the tube meant that it perhaps wasn't as critical.  Now it seems likely that I should be able to find a distant object in the scope and twiddle the knobs on the RDF until this is also aligned with it.  Talk about learning as you go LOL  SGL threads are wonderful! 

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4 hours ago, JOC said:

 when you move your head the position of the red dot varies - I guess the notion is that I should get the red dot bang in the middle of the finder,

Talk about learning as you go LOL  SGL threads are wonderful! 

:thumbsup: they sure are :)

I may be speaking out of turn here cos I've not used my RDF, (not fitted it,  I squint along the ota and that is near enough to get my object in a low power field, or close enough ! ) and if I've got it wrong someone will be along in a min. :) but  : -

I think the RDF is meant to give a dot which does not move   relative to the stars  , ie. when you move your head the dot will move in the circle of the housing but so do the stars, so when you plonk the dot ( sorry got a bit technical there ) on a star it will stay there as you move your head. It does not have to be in the centre of the housing, nor need your head be.

No parallax because the dot is an image at infinity in the same plane as the stars.

EDIT I've just unpackaged my rdf ( 'specially for you :) ) and tried it on a tree at the other side of the garden and yep, the dot remains coincident with the branch as I shift them both within the frame as I move my head. within the limits of the parameters of the experiment :happy7:

Edited by SilverAstro
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44 minutes ago, SilverAstro said:

in the same plane as the stars

You know what I hadn't considered it from that perspective, but yes, given the logic of the argument you must be right :hello2: i.e. my head, the plane the red dot is superimposed upon and the star will all line up no matter which angle I view it from.  So providing I can see the red dot on the screen it should be lined up with a star and if the finder is properly aligned with the telescope (which I will look into doing the next time I play) the telescope should then be aligned with the star.  Yep, put like that it makes perfect sense.

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Lots of interesting thoughts here... I too am new to all this (first scope as of the 24th March), I've already started to invest some cash into the hobby as I received today a Skywatcher 4mm UWA Eyepiece and just tried it out (literally just now). Great magnification on the moon and made even better with the addition of a moon filter.

BTW when I said I would be happy to print and send these brackets to people. I wasn't joking, I ran a batch last night on my printer, so seriously if anyone wants to try them out, please let me know! (Now available in 1mm, 2mm and 3mm variants). If I send a set out, I will include all three sizes for the person to experiment with until they get what they need in the way of alignment.

 

2017-04-05 23.11.09.jpg

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15 hours ago, SilverAstro said:

[...] I think the RDF is meant to give a dot which does not move   relative to the stars  , ie. when you move your head the dot will move in the circle of the housing but so do the stars, so when you plonk the dot ( sorry got a bit technical there ) on a star it will stay there as you move your head. It does not have to be in the centre of the housing, nor need your head be.[...]

That's certainly my experience with a Baader Skysurfer III RDF. I find that I can keep both eyes open and just look along the scope. The red dot appears on the star field and although the body of the RDF moves relative to the stars when I move my head, the red dot is stationary.

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2 hours ago, Geoff_L said:

That's certainly my experience with a Baader Skysurfer III RDF. I find that I can keep both eyes open and just look along the scope. The red dot appears on the star field and although the body of the RDF moves relative to the stars when I move my head, the red dot is stationary.

That is the trick to using these RDF and illuminated reticule type finders - look at the background sky rather than into the finder. Also keep the dot / circles / cross at the lowest brightness setting that enables you to see them against the stars. Many of these finders have brightness settings which are just too bright for deep sky searching in my experience. The exceptions are the Telrad and the Rigel Quickfinder whose brightness levels seem to be much better optimised for astronomy.

Edited by John

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[ Side note:

I started over in the Welcome thread with an off-topic query — and, as was suggested there, this discussion completely took care of my question (about reflex sights).

Thanks, all. ]

 

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On 05/04/2017 at 23:22, ChargeNut said:

if anyone wants to try them out, please let me know!

That's so generous of you! I'm new to SGL so I don't know if there's a special place to discuss 3D printed astro gubbins but there should be. At the moment buying a 3d printer is off the cards for me...maybe someone would want to set up a service for 3d printing things for astronomers! Sure they could make a good few pennies...

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I had the same problem with my celestron evolution 6 

so I put a bit of folded playing card at the back

 

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I bought a astromaster 130 eq this spring. I got it at costco and they included the  accessories kit with eyepieces and filters, so a really good deal ( I save 100$ compareew to buying the same item on amazon!). I like the scope. Of course the mount is not super stable and has coupe of cheap thing but for less than 300$ CND it's great. But I the fact that celestron couldn't make the redo dot findert to actually Work was beyond me. It totally useless. I wwas looking for good finder scope. I was looking around  the intertubes to find a solution and I find this post! This will save me money and lot of frustration. Lucky for me I work IT at a university and the engineering department has 3D printers, and we can make them print us stuff! I'll will use those files. 
Thank you very much.  

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