Fitting a Telrad RDF

[Moonshed]

I own  a 200mm Celestron SCT, built circa 1985 and owned and deeply loved by me since 1990.  Until a few years ago I was still using the original  finder but it was a bit of a pain to use and I eventually replaced it with a cheap RDF, designed I believe for use on firarms. It made life so much easier I regret not doing it years earlier. However, it required constant re-aligning using an Allan key and became very irritating. So  I looked around for advice and found this excellent site and read many comments on RDF's. I settled on the Telrad which I ordered and it arrived today, it looks sturdy, well designed and easy to adjust. Just two questions.

1) I placed it on my scope and it appears that the self adhesive pads only make part contact on the tube on their outside edges, as if it was designed to fit a different size tube. Is this okay and does it provide enough grip to stay put?

2) I am a little concerned about getting it correctly in line with the tube. How accurate does it need to be? I assume there is some margin for error that can be corrected with the adjustment knobs?

Any advice would be very welcome.

As this is my first post I would just like to say hi.  

 

 

[Dave In Vermont]

I'm strictly a RACI-man myself, however you do win this:

Greetings and welcome to SGL - it's nice to have you aboard!

We love finding answers to any and all astronomy-based questions - so don't be shy!

Clear and dark skies,

Dave

[Demonperformer]

Hi, and welcome.

The telrad is an excellent choice. I added one a couple of years ago and it is orders of magnitude better than a standard rdf - even works under my LP'd skies (which I could never get the rdf to do.

(1) IIRC, the base is fairly flexible when attaching, so firm but gentle pressure of the base against the OTA will give good contact (mine is certainly fixed all across the width of the tape).

(2) I had the same worry. Don't know the exact construction of your OTA, but mine has a raised portion around the primary mirror cell (it's the 8SE version) I just butted the rear edge of the telrad mount against this on the basis that this would probably get it quite close - it did.

HTH

[baggywrinkle]

There are three adjusters on the Telrad which providing you have it at perpendicular to the back plane to make a final alignment.

[joncrawf]

I'm a relative noob but the advice that's freely available here from people much more knowledgeable than I is a really valuable resource. Use it well and welcome to the forum!

One of the more valuable bits of advice that I picked up here was to get a Telrad. I did just that and since then I don't think I've used my 9x50 optical finder at all. My advice would be to choose carefully where you position it. I have a SkyWatcher 200p Newtonian and I've just decided to buy another Telrad basepalte so I can have another mounting option. Perhaps try attaching the telrad with bubber bands in a few different positions before mounting permanently. 

[Skipper Billy]

The tape that fastens the base unit to your OTA is 3m double sided tape.

I use this product to hold various items of radio control gear to the frames of my radio control helicopters - the most critical bit being the 3 axis gyro - if it came unstuck the heli would immediately plummet into the ground.

It is subject to massive vibration from the 26cc strimmer engine, oil and grease from the exhaust and up to 9g in various hard 3D moves - when its time to remove the gyro even with a razor blade to slice through the sticky pad it is still almost impossible to remove with bare hands (this is a piece about 25mm square).

Would I trust it to hold my Telrad on even if its not fully seated ??  Yup !!

[Uplooker]

31 minutes ago, Skipper Billy said:

The tape that fastens the base unit to your OTA is 3m double sided tape.

I use this product to hold various items of radio control gear to the frames of my radio control helicopters - the most critical bit being the 3 axis gyro - if it came unstuck the heli would immediately plummet into the ground.

It is subject to massive vibration from the 26cc strimmer engine, oil and grease from the exhaust and up to 9g in various hard 3D moves - when its time to remove the gyro even with a razor blade to slice through the sticky pad it is still almost impossible to remove with bare hands (this is a piece about 25mm square).

Would I trust it to hold my Telrad on even if its not fully seated ??  Yup !!

I hear what your saying Billy, but disagree I fitted a Telrad to my 300PDS. It was fine for a couple of months, then every other week I would walk into my garage and find the Telrad a bit more bruised and battered having fallen off the tube. I finally saw red and drilled holes in the tube and bolted it in position - not fallen off since

[SacRiker]

It sounds like you've already slapped the Telrad on -- don't worry about perfect alignment with the tube, I've found that the adjustment screws can point this thing in all sorts of directions. You will be able to get it aligned without too much of a problem.

[Moonshed]

Thanks for all the advice. I stuck it on this afternoon and as advised I butted it up against the raised part around the primary mirror cell, giving what appears to be more or less spot on alignment. Found it very easy to align on the target I had centred in the eyepiece. Seems to be stuck on very securely even though not a perfect match to the curve of the tube, but has around 2/3rds of the tape in direct contact. I know I am going to be very pleased with this purchase. Many thanks to all those that recommended the Telrad and for the fitting advice.

Keith

[skywatcher58gb]

Hi Moonshed, I have had a telrad fitted to my Meade LX90 8" SCT for about 3 years now with no problem. I do remove the telrad when the scope is stored and only screw it into position when observing. Maybe if the telrad were left on permanently it might have an effect on the self adhesive pads due to the additional weight. I would advise removing when not observing.

[Charic]

I cut the 3M pad length-ways, down its middle, and attached each piece to the edges of the Telrad's base unit, leaving nothing along the middle of the base unit.
The base was aligned to the OTA (Optical Tube Assembly) and additionally two extended tie-wraps are fitted, just my own preference.

This is my second Telrad? The first got very little use so I sold it on, then felt the scope looked bare without one, and on occasion when I could not see the reticule on the standard finder, maybe the Telrad could help out, but on reflection, I prefer the magnified 9x50 straight through finder scope, locating with BOTH eyes open!