Cracked Primary Mirror in new 254mm F4

[LucasB]

Hi Everyone,

I have just purchased a Skywatcher 254mm F4 CF telescope and I have discovered there is a crack in the primary mirror. I have attached pictures to show the crack which is along the side of the mirror. I haven't pulled it out of the mirror cell as you can see.

My question : Is this a return job or does this sort of thing happen on ocassions and they glue it and all is OK?

Would appreciate your comments
Regards,
Lucas

[casstony]

Return it - a small chip would be ok, not a large crack.
Imagine trying to sell it second hand - "great optics except for 6 inch crack in primary mirror" :)

[blink138]

i fail to see how it could be ok lucas
you should already have contacted the supplier with the attached pictures
pat

[Allan]

No that's not good, it will likely affect the figure of the mirror. Just return it for a new one.

[koputai]

Are you sure it's a crack?

From here it looks like a moulding/casting mark on the outside surface of the mirror, which I've seen on blanks before.

Cheers,
Jason.

[LucasB]

Thanks for your responses everyone. How can I distinguish between a crack and a moulding/casting mark? Should I remove the clips and post some pics from certain angles? Or is it just plainly obvious to the trained eye? What should I look for?

Regards,
Lucas

[casstony]

It should be obvious if the crack is extending through the glass. It might very well be a casting mark.

[Merlin66]

Lucas,
Looks like a mould mark to me...
A crack will have sharp edges and be seen to extend inwards....
If you still have doubts take it to the supplier for evaluation....

[Shiraz]

it's almost certainly a quite benign casting mark - mine looks much the same. Not a problem.

A few issues to be aware of:
- the black paint that they use inside the tube will dissolve in water and runs if you have heavy dewing inside the OTA
- if you unscrew the top ring of the focuser, don't also loosen the tensioner screws - there is nothing to stop the draw tube from falling into the OTA - mine hit the secondary on the way in (luckily with only minor damage).
- the main mirror clamp screws have some sort of gooey stuff on the threads to stop them coming undone after you loosen them. during the last summer, my scope was stored in it's plastic bag in a very hot room and some of the gunk migrated onto the mirror, forming a sticky film that attracted dust and lint - took a lot of messing around to clean it off.

Still a remarkably good telescope though.

[raymo]

I think Ray is on the money; it looks like a classic casting mark. You
should be able to feel the small step with a finger nail, or if it is very
slight, maybe a razor blade or some other fine object.
I'm curious to know why you thought it necessary to expose the mirror in your new scope.
raymo

[Satchmo]

Its definitely a molding artifact - higher end mirror blanks would have the edge machined with a diamond edger.

[Shiraz]

these have machined edges too, but only to shape up the blank so that it fits the holder - the machining does not extend over all of the edge, just where it is proud of the desired shape - in some places, the moulding marks are still visible, as here. The mirror edge that is sitting below the lip of the holder will be machined - it has to be precisely shaped because these mirrors float free with only a few thou clearance between the mirror edge and the side support pads on the holder walls.

[LucasB]

Thanks everyone for your responses. It has been very informative. I should have probably titled the thread "Is this a crack in my primary mirror" as it turns out it is a casting artifact.

Ray, Thanks for the extra tips regarding the OTA. I have also noticed the light leaks in from behind the primary and around the focuser tube also which can easily be baffled.

Raymo, the reason I removed the tube to expose the mirror is that I am looking at flocking the tube above the primary and was also checking the mirror clips.

Regards,
Lucas