Skywatcher 150 PDS /OAG/ coma corrector

What is the required backfocus from the coma corrector?  You didn't list which one you're using.  That's what I would verify first.

So it's a PDS it's designed for astrophotography. It comes with a low profile focuser. I'm using the skywatcher 0.9x coma corrector which needs 55 mm of back focus.

I've been using a ZWO OAG with my 150pds with standard focuser both with and without CC and achieved focus without getting close to maximum travel of the focuser. I think your OAG is 19,5mm thick (thats only 3mm more than the ZWO one).

As the OAG doesn't do anything for your backfokus I should recommend you to first find focus without it and se if you hace 19,5 mm travel left on the focuser. If not I should start with trying to get the sensor closer and secondly move the primary up into the tube.

Alan Hancox:
Hello

I'm struggling achieving focus with my newly acquired 150 PDS and 0.9 coma corrector.

I'm trying to use my Askar M54 OAG (I assume you can use it with this scope?)

My focuser runs out (just) of inward travel. I've packed away and come in defeated 😞. I'm pretty sure my measurements are correct. 18 mm back focus (camera), 18 mm spacer and 19.5 mm for the OAG. I'm using a QHY 168C for reference.

Can anyone offer any advice??

As others have said above Alan, I also use the 150PDS with a 0.9x coma corrector.  The only item I've changed on the focurer tube is replacing the screw bit adapter ring with a compression fit.  These seemed to be about the same length.  Focus is reached with around 1cm of outward travel.

I have ZWO camera, ZWO mini EFW and spacers - loads of room to get more in there I reckon.

Alan Hancox:
Hi Lee

It should work. That's the issue. I've calculated the distances and they seem to work ok. Maybe I'm wrong. Maybe I should remove the OAG and go back to a guide scope....

As you say - I'm not sure how replacing the OAG with an equivalent spacer will make any difference as the same backfocus is being used - very strange!

Was the 150PDS modified in any way before you got it?

Alan Hancox:
I rang Rother valley optics this morning who recommended I add an extension to the coma corrector to bring it further out. I always get confused when talking about inwards and outwards travel...

I'm struggling to understand this recommendation.  If you add an extension above the coma corrector, you're eating into the existing, forced, 55mm backfocus.  If you somehow force the coma corrector further out from the focuser then surely it would then need to be racked in even more.

As mentioned by someone above, does the focus tube have the standard adapter on the end - does it look a little like my image above?

Sounds like the suggestion is to add a spacer between the focuser and the coma corrector, moving the coma corrector away from the secondary.  This would be a good suggestion if you need more outward travel to reach focus.  From this post it appeared you wanted more inward travel.  If you think outward is what you need, you could test this without a spacer by simply not pushing the coma corrector all the way into the focuser, leaving some space.  Then try to reach focus.

And you feel you need to have the sensor move further into the focuser to reach focus?  If yes, why?  Do you see out of focus stars starting to get into focus but run out of inward travel?

Alan Hancox:
Yes that's my problem...

Then you'll need to move the primary mirror or get a focuser with a lower profile.  Perhaps contact whoever you bought it from to verify it's the right scope because a PDS shouldn't have this challenge.

Well I've managed to get the camera to focus without the OAG using the coma corrector. Strangely it's plate solving at 698 mm not 675 which is what I thought it would. Anyway at least it's working now. I just need to invest in a guidescope….oh no more money 🙄

The very edges are a little bit elongated. I think my measurements must be out a little but the main thing is it focuses…