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Some observations on the installation of a Telrad


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Thoroughly disgusted with the stock 6 x 30 finder scope that came standard with my Celestron OMNI XLT 102 Refractor I decided to replace it with something else.

There's plenty of choices out there in right angle finder scopes, reflex sights, laser pointers, etc... but I settled on a Telrad. Why the Telrad over anything else is a subject for another posting. For this posting I wish to provide some insights for the benefit of others in the mounting of the Telrad.

One of my disgusts with this finder scope is the height between the telescope body and the center of the eye piece. Putting a crick in your neck when using it was going to be a recurring annoyance. The centre of the objective lense for the finder scope whilst seated in it's riser is 3 3/4" (9.525 cm) from the body of the telescope. Clearly I needed to get some more height.

Researching the Telrad I learned that the distance between the telescope body and the glass viewing window of the Telrad is: 4" (10.16 cm). Hmmm... very close to the stock Celestron finder scope. Luckily Telrad has optional 2" (5.08 cm) and 4" (10.16 CM) risers available.

BE FORWARNED. If you intend to use these risers you should know that their height has been mis-stated in the sales literature. I point this out not to quibble over it but simply to make people aware of it. The stated heights of 2" (5.08 cm) and 4" (10.16 CM) represents the height of the posts between the upper and lower mounting plates of the riser. Each plate (lower and upper) is an additional 3/16" or a total additional height of 3/8". Thus the ACTUAL height of the risers are 2 3/8" (6.03 cm) and 4 3/8" (11.11 CM).

When you add the riser between the Telrad and it's mounting base the overall height from the telescope body to the centre of the looking glass would be 6 3/8" (16.19 cm) and 8 3/8" (21.27cm) respectively. On my telescope it has nearly doubled the height from the stock Celesctron finder scope.

Some people have observed that they don't like the "look" of the Telrad mounted on a refractor as it appears to ungainly. HA! The overall difference in height when mounted is negligible on a 4" telescope on a 6" or 8" even less. The relative shape is a 2 x 2 profile on a riser. Even if you were to mount it on a 2" telescope it would not look ungainly since the relative sizes are similar. If it appeared as anything --relative to a 2" telescopes tube size -- it would be as a carrying handle or the handle grip on a rifle. One couldn't describe this as ungainly or aesthetically unpleasing. Now if I had purchased a 6 x 50 right angle finder scope and mounted that on my telescope THAT would look ungainly to the point of being ridiculously outlandish. So, if anyone has thought about using a Telrad on their refractor I say give it a go. The worst that could happen is you find yourself with a Telrad that you can use on your 10"+ reflector you already have or will be purchasing in future.

Now on with the mounting of it.

Some suggested mounting it as near the end of the OTA as possible. I found through experience this past week this is a bad idea. When you are seated and looking up the length of the tube toward the Telrad's glass view finder the farther the device is from your eye the larger the concentric rings become. So much so that the middle ring (of the three reticle rings) fills the glass view finder from edge to edge both vertically and horizontally. Moving the Telrad so that it's more centered in the length of the OTA ended up having the outermost ring filling the glass view finder edge to edge. Interestingly, placing the back-end of the Telrad's base up against the plate where the stock Celestron view finder was mounted (on top of the focuser housing) produced the best results. In that position the distance between the glass view finder and where my eye would be when I looked up from the telescope's eye piece in the star diagonal ended up being 7". The three reticle rings were centered within the Telrad's glass view finder with plenty of room on all sides.

Now that I found where to mount the Telrad I had to decide how. Do I drill holes and mount the Telrad or do I use the sticky tape on the back of the Telrad's mounting base? Neither option thrilled me. The former would savage the resale value and looks should I (or the next owner) choose not to use the Telrad anymore. The latter would be a serious pain in one of two ways. In the best case scenario I would need to put new sticky tape on the Telrad's mounting bracket and the telescopes paint job would be marred. Worst case scenario, I would manage to snap the aged plastic of the Telrad's mounting base requiring the purchase of a new one and savage the paint job on the telescope. I decided to try the suggestion of another forum member to use velcro straps. The kind without the sticky-tape backing obviously. Viola! It worked like a charm. A word of caution though to those who also use this solution. The velcro will secure the Telrad to the telescope, however, if you should "bump" or "jar" the Telrad more than slightly it will move on you a bit. The movement isn't serious -- unless your "bump" is -- but it is there. It won't affect the Telrad's use to center on objects so you won't have to re-orient the Telrad's reticles in the glass view finder but it will shift it from where you had it on the telescopes body.

I've only ONE complaint with my Telrad. Two days of use and the reticles brightness control knob is either off, at 75% brightness through to 100% brightness. Anything below 75%... zilch. I have to contact the seller and arrange for a return goods exchange. Isn't that something? You finally get it sorted and BOOM the thing goes south on you. ;)

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Imformative read. I love my Telrad and couldn't be without it. You will find working with programs like CduC and Stellarium that give telrad overlays on screen it makes star hopping so much easier.

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this is a great write up. having used both this and the rigel, I prefer the telrad on my dob and the rigel on my refractor. I find the sticky tape a really good method for the mounting of the plate. to remove it just use a gentle sawing and lifting action with a plastic ruler and then clean the residue with a little lighter fluid on a cloth - maybe some fingernail persuasion too.

I thought you may find this useful now it's ready for use!

http://stargazerslounge.com/diy-astronomer/99727-telrad-frost-dew-cover.html#post1398039

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Yes the Telrad is a magic device and so easy to use.

I was reluctant to use double sided tape on my 5" refractor tube so I tied the Telrad base on temporarily with black cable ties, using two joined together to make one long one.

This gives some movement which has allowed me to experiment with positioning and also not having goo on the tube.

Regards

Nick

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I too are looking at the best place to locate the Telrad on my 10" Dob.

Is the extra piece of £22.50 plastic that raises the Telrad 2" or 4" from the scope worth the money, as I am seriously thinking of making that investment.

My Telrad has a short range as well, i.e. Full brightness move the lever .25" and you have a faint outline and touch it back towards the "Off" position for zero( nothing!!).

Would be interested in what the Telrad supplier has to say.

A mate of mine has a Telrad, I will ask him as well.

Cheers

Adrian

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My telrad also shows the short range, but I can adjust it so it is quite dark - never had a problem with finding a setting I'm happy with.

As to placement, you need to put it where you are happiest with it and that will probably be close to wherever the original finder scope sits. Basically, close to where you look through the eyepiece... On a newt, this is at the top of the 'scope.. On a refractor/SCT it will be at the bottom of the 'scope.

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arad85,

Thanks for that info, I was thinking of either removing the holder for the finderscope

(as I don't think I will use it with the Telrad) and drilling a second hole towards the front of the scope ( not overly happy drilling holes in the scope), or using the Velcro solution. My only thing against Velcro is that I do mountain treks in winter and Velcro can at times not stick in icy conditions, freezing water expands..........to form ice!!!

But, I suppose on reflection if it was that wet and icy I would be tucked up in bed.

Incidentally, have you tried the plastic raisers that lift the Telrad 2" or 4" away from the scope?

Thanks

Adrian

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Incidentally, have you tried the plastic raisers that lift the Telrad 2" or 4" away from the scope?
Nope, but then I don't have a problem looking through mine - it's on the end of a 5ft long tube so is nearly always in a reasonable position...
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  • 1 year later...

Regarding the double sided sticky pads, I use the Rigel finder, and the supplied AA batery box, and at the time was parking the scope on its side. One day while setting up I heard a thud from the scope, and on inspection found the Rigel on the floor, I think the combined weight of the finder and battery box, and the fact that when parked it was hanging horizontal to the scope made the pads give way.

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There was a recent thread on Telrads in which I told of my experience of 2 broken units - albeit over the last 20 years. The diodes on both Telrads failed after just a few years usage and in identical fashion. Working one night - dead the next.

My suspicion is that moisture accumulation inside that big empty plastic box was the culprit and that something shorted when I powered them up.

My recommendation when drying everything out after a dewy/frosty night is to open up the unit and check that there's no residual moisture inside the unit - if there is, leave it open to evaporate overnight or dry with a hairdryer.

My 10 cents from a somewhat expensive experience with these things.

Alan

Edited by Alma
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