lx90 with turned down edge

[kinabalu]

Hi all
I have a second hand LX90 with a turned down edge on the primary that I want to mask-off. I don't want to remove the collector and reach down the OTA to the primary mirror but want to get at it by removing the mirror cell from the bottomof the OTA. I have tried to pull it off after removing all bolts etc but there is absolutely no budging it! Any experience of this sort of thing out there or any advice?
many thanks
John

[casstony]

Couldn't you put a mask on the corrector to achieve the same thing? A temporary mask would be easily reversible if you didn't get the anticipated improvement.

Welcome to IIS John :)

[kinabalu]

Thanks for the suggestion Tony. I have had a go at that and there is no noticable difference. I think the problem is the irregular reflections from the light reaching the turned down edge of the primary itself and from what I had read, this can only be rectified by masking the edge by 1/4 or 1/2 inch on the actual mirror itself. I get good collimation on one side of the focus but dire results on the other side. I am actually contimplating removing the corrector and reaching down the OTA but, as you can imagine, that fills me with trepidation!
John

[marki]

You will not get the back off, I think they are glued??? The corrector is easy to remove if need be but I think you will find it is quite normal for a SCT to have different diffraction patterns either side of focus as the mirror has quite a bit of movement in it. Why do you think the mirror has a turned down edge? Be very unlikely on a meade.

Mark

[kinabalu]

Thanks for the comment Mark. On closer inspection I think I can see traces of glue between the OTA and the rear cell so looks like you were right!
I have been spending a lot of time working on the collimation of this scope. On a night of good seeing and after a long cool down time, I can see some pretty well-defined symmetrical diffraction rings just outside focus.
However, if I rack the focuser further in the same direction into focus and out the other side, I get a blurry, flared ,yellowish doughnut with little defined ring structure. (I have checked my eyepieces and the colouring is not due to them). From other blogs that I have read, this seems to be due to a turned down edge.I don't think this is caused by mirror slop as I am always pushing the mirror in the one direction.
Any advice or comments much appreciated.
John

[marki]

My LX200 shows similar traits in that if I use the main focuser to move the mirror, the diffraction pattern will always be slightly different inside and outside of focus which is why I don't use it to collimate the scope. When I get coarse focus using the mirror adjustment I simply lock the mirror and use the crayford to tweak the collimation. The diffraction patterns remain the same both sides of focus when the mirror is locked. A moving primary is great as it offers a large range of adjustment but the offside to this is the problem with collimating the thing, there is just too much slop in the mechanism. It is normal to see a difference if you move the mirror. If you use the mirror adjustment it is best to collimate from the inside diffraction pattern as this gives better results. You may also like to try a crayford focuser which can be purchased fairly cheaply depending on brand. I have a moonlite on mine which is fabulous but expensive compared to some. I also recently fitted a feathertouch 10:1 focuser to the coarse mirror focus and this has improved things even more. They are cheap to buy and I got mine from OPT.

http://www.optcorp.com/ProductList.aspx?uid=105-175-1041

I would resist the urge to tape the mirror as it is something that would be very difficult to undo. The best way to tell if the mirror has a turned edge is to use a ronchi which will tell you straight away.

Mark

[kinabalu]

Thanks for that Mark. I will need to look into the process for doing a visual Ronchi test and give it a go. In the meantime I'll take on board what you have said about the effect of using the main focus knob for fine collimation. I have a 1209 microfocuser fitted to the rear cell so I will look at the diffraction pattern either side of focus using that rather than moving the whole primary back and forth. I will investigate the feathertouch focuser for the coarse mirror focus as well.
Many thanks for taking the time to get back to me on this
John