secondary holder - Skywatcher 150p

[bambuko]

There are few things that didn't impress me when I had a closer look at my new telescope (Skywatcher 150p).

One of things that really caused me some grief was the way Skywatcher designed secondary mirror holder.

Three adjustment grub screws bite into aluminium mirror holder and the indentations created make it difficult to adjust things:

I have seen other people complaining about it - here is my attempt at trying to make it better:

Instead of adjustment screws bearing directly on mirror holder I am proposing to introduce intermediate washer, like so:

Three indentations are used to locate it on the grub screws and the mirror holder can be adjusted left/right, up/down and twisted any which way, all the time sliding smoothly on the large flat washer face, without screws biting into it:

I have made it (hopefully) thin enough not to push secondary mirror out of the centre of the focuser.

I will put it all back together and try adjusting/collimating it again - hopefully adjusting of the secondary will be easier.

I will report the outcome.

[bambuko]

Total success. Adjusting/collimation of secondary is now smooth and precise as well as easy.

I guess that since secondary is adjusted so seldom, few will be interested, but I am very pleased with it.

[John]

This is a common issue with secondary holders. I put a large steel washer between mine and the 3 adjusting screws which has improved things a lot but even better, apparently, is to put two plastic disks cut from a plastic milk carton. I've yet to try the milk carton washer approach but from all accounts it works even better than the hard washer.

Here is a thread on this:

http://stargazerslounge.com/topic/193369-collimation-tip-bottle-washers/

[bambuko]

Thanks for the links John!

Didn't see them before but it's good to see that I and "my" solution is not unique

It never ceases to amaze me how easily can the likes of Skywatcher spoil otherwise excellent product by trying to save few pennies.

[John]

If they sorted all the issue they would be quite a few £'s more expensive in a very competitively priced market and would loose sales.

Orion Optics scopes cost a lot more than Skywatcher and still have some issues !

Your solution seems elegant though so thanks for posting the details 

[bambuko]

Just in case anyone was ever going to copy the washer dimensions (doubt it given popularity of plastic washer solution, but you never know...) - the washer as originally made at 2.5mm thick was too thick for space available.

I ended up turning it down to 1.7mm and now is just perfect - more space for adjustment as well.

[Moonshane]

this is always an issue and your solution clearly works. as John said above the softer more flexible material of the plastic milk bottle washer mod provides finer adjustment for that last tweak and is also 'free'/more environmentally friendly.

I have thought that one day I might do the adjusters the other way around - tap holes in the face of the secondary holder and add nuts at the top end with springs providing tension - more like the primary adjustment in other words as this provides a much easier process. yet another 'one day' job.

[bambuko]

I still remeber (now with amusement) the first two nights I have spent trying to adjust secondary and getting increasingly frustrated  

Today after finishing the mod to the washer it took me genuinely about two minutes to collimate the whole scope including the secondary.

BTW - contrary to popular view, there is quite a lot of flex in metal washer solution. For a starters you are able to use spring loading on a central screw to a full advantage with metal washer sliding smootly against seconary holder and finally, providing you tighten the three collimating screws progressively and judiciously and in very small steps (now easily possible) there is a lot of "spring" in the threads before they finally go "tight".

With plastic washers you get creep rather than spring and my main concern/objection is that this creep might continue, requiring further adjustments in the future. I can't see myself having to adjust secondary for the foreseeable future now that it is set to my satisfaction.

Having said all this I do realise the ease of plastic washer for a vast majority of people without machine shop and I am sure it works fine (and thousand times better than anything that Skywatcher and the like offer).