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Refubished the Helios frac.


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After a discussion which showed that the field of view should be the same between my Newt and my Frac the alarm bells started to ring.

I can get both M 81 and M 82 in the same frame with my 200mm Newt.

Something I had never been able to do with the Frac.

When I checked the alignment of my Helios frac I discovered to my horror it was miles out.

By placing a laser into the EP holder and firing it back down the tube it was exiting the lens cell at about 40mm off center.

Rotating the laser made no difference to the exit point.

I decided something need to be done.

I stripped off the focuser and its holder to discover the amount of play between the focuser holder and the OTA tube was huge.

The holder is held in place by 3 screws, as I undid the last screw the whole assembly flopped over to one side.

Accurate measurements to determine the difference in diameters of the tube and the holder revealed a staggering 2mm difference.

I have made up a 1mm thick collar which now resides between the holder and the tube.

After reassembling the Frac the laser now exits dead center.

To test the now aligned tube I stripped the 200mm newt off my mount and replaced it with the Frac.

After balancing it all up I swung the scope around to the now obligatory TV Ariel test target.

This Ariel is probably half a mile away so proves to be a fair test.

I then noticed a bush type thing a little closed so I thought why not.

This is the result.

As a final test I tried a new adapter with the 350 D.

This adapter steps down to 1.25 so I can use a barlow.

Back to the Ariel for a close up with the barlow.

I had to use the 3 times Barlow as I could not get enough travel to use the 5 times without an extension tube.

This is just one of the plastic brackets that hold the prongs of the Ariel.

All I need now is for the next clear night so I can test it in anger so to speak.


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Guest Tweedledum

Hi Graham,

Had a similar issue with my st 120 and when I fitted an sct 2"diag it just got worse..Had to shorten the light path, fit a std push in diag. Did the laser test, fitted a small cork washer for grip and sorted!!


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Well it is unfortunate you had a similar problem Damian but it is also nice to find out that I am not the only one.

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Hi Graham which refractor is it, I have a Helios 6". This is quite a common problem and using the laser is the best way to check, it only takes a minute. The three screws are the most common source of misalignment particularly in older scopes but scopes with crayfords can be a pain if the crayford isnt set up right. Also with R&P focusers the tension adjusters are usually all over the place on older scopes and this can cause slop.

Has your scope got a collimating lens cell?

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Hi Phil.

It is the old F8 black one.

127mm with the collimation cell.

Rack and pinion focuser.

The play was actually between the casting that holds the focuser and the tube itself.

When you tightened the 3 locating screws you could pull it all over the place.

But with a bit of precision machining I knocked up a spacer ring to fit in the gap.

Result is no more play, it is spot on with the laser and as there is no room for any play it should stay there forever..

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