SCT losing focus reasons ?

[powerlord]

Hi chaps,

so.. my C8 is behaving weird. I thought I collimated it. took some shots which were ok. A few weeks later was getting terrible pictures - checked collimation and it was terrible. fitted bobs knobs and roughly collimated it again. and seemed sharp.

2 days later, just tried and its blurry again.. this is during the day.. even weirder - the focus seems to change almost as I watch.

something is up. I'm wondering if the secondary is actually loose inside ? I'm hoping its that end rather than the other...

So my next step was going to be to try and get the glass/secondary off the front following this guide:

https://astromart.com/reviews/telescopes/show/disassembly-of-an-sct-childs-play

am I on the right track, or wasting my time ? I don't really want to shake it and rattle is around.. but a very small shoogle doesn't suggest something is lose, but it doesn't make sense otherwise.

stu

Edited July 27, 2021 by powerlord

[newbie alert]

Focus and collimation are different things, but sct do suffer with mirror shift  as the mirror rides up to come to focus so final focus should go against gravity 

With Bob's knobs they're easier to use collimation wise but don't sit as tight as the original screws, so collimation needs to be done more frequently.. that's the reason I didn't go with them

[Shimrod]

23 hours ago, powerlord said:

Hi chaps,

so.. my C8 is behaving weird. I thought I collimated it. took some shots which were ok. A few weeks later was getting terrible pictures - checked collimation and it was terrible. fitted bobs knobs and roughly collimated it again. and seemed sharp.

2 days later, just tried and its blurry again.. this is during the day.. even weirder - the focus seems to change almost as I watch.

something is up. I'm wondering if the secondary is actually loose inside ? I'm hoping its that end rather than the other...

So my next step was going to be to try and get the glass/secondary off the front following this guide:

https://astromart.com/reviews/telescopes/show/disassembly-of-an-sct-childs-play

am I on the right track, or wasting my time ? I don't really want to shake it and rattle is around.. but a very small shoogle doesn't suggest something is lose, but it doesn't make sense otherwise.

stu

My SCT is 'parked' until autumn, but could it be the hot weather we have had recently - especially if it has been out in the sun causing a slight expansion of parts?

I have also read that some users have issues with Bob's Knobs - while they make collimation easier, the scope has also become more prone to losing collimation. Maybe you are one of those unfortunate users.

[RT65CB-SWL]

Just a thought… mirror slop. Normally affects the primary mirror where the locking/retaining ring that slides along the baffle tube becomes loose. I had this issue with my ETX05.

[powerlord]

i took it apart. well.. took the glass and secondary off. it was dusty inside anyway. I cleaned it and poked the edges of the primary a bit - seems secure. seconday secure also.

so put it all back together. and none the wiser.

 

tonight/tomorrow morning forecase is cloud free so I will be trying it then. First I'll try it on a few stars and see how the collimation is. adjust if required. Then the plan is to try to capture my first planets at 2.30am or so with my asi224 and 2" ED barlow.

stu

[newbie alert]

4 hours ago, powerlord said:

i took it apart. well.. took the glass and secondary off. it was dusty inside anyway. I cleaned it and poked the edges of the primary a bit - seems secure. seconday secure also.

so put it all back together. and none the wiser.

 

tonight/tomorrow morning forecase is cloud free so I will be trying it then. First I'll try it on a few stars and see how the collimation is. adjust if required. Then the plan is to try to capture my first planets at 2.30am or so with my asi224 and 2" ED barlow.

stu

Well the very best of luck, with Saturn in opposition early August and Jupiter at the end it be some good practice 

[John]

Is it an older C8 ?

I had an older one and it suffered from mirror flop. When the scope was pointed at a certain angle, the weight of the mirror would cause it to shift a little throwing the collimation out.

It was very annoying !

I tried running through the full focusing range a few times to spread the lubricant out more evenly and that helped a bit but didn't make it go away. I believe the ultimate cure is to lock the mirror and to use a 3rd party focuser on the back of the scope.