150Pds Focuser Modification Required?

[Steve789]

So I've had my new 150Pds for less than a week now and I'm very happy with it apart from one thing that's best explained by the two images below.

If you look at the brighter stars in both images you can see they have bite marks taken out of them on the left caused by the focuser draw tube protruding in front of the mirror. I was aware this could happen after reading a few reviews before buying the scope but I didn't think it would bother me. After just two imaging sessions I now find it does annoy me and must be rectified.

Having just bought the scope I'm not interested in buying a new focuser but wondered if any other 150pds owners out there have done any modifications to either shorten the existing focuser draw tube (I guess by hacksawing the end off) or by replacing the draw tube with a shorter one. I'd rather not take a hacksaw to my lovely new scope so would prefer the second option, keeping the original part for future resale value.

I guess my question really is, where can I buy or how can I make a replacement draw tube for my 150Pds focuser?

Maybe this should go in the DIY section?

[Thalestris24]

Hi Steve

I have a 150pds and a 130pds but haven't had that problem with either scope though only the 130pds has a dslr attached. I have coma correctors with both scopes - a Baader MPCC on the 150pds and a TS/GSO on the 130pds. I'm not sure how much difference having a cc makes. Fwiw they both have about 40-41mm of the focus tube protruding externally. They both have about 5-6mm protruding on the inside. If you need to, you can quite easily undo the focuser to release the focus tube and I guess you could cut a bit off the end...

Louise

[jimjam11]

I noticed this problem with mine as well, that is with a coma corrector in place.

[laser_jock99]

Move the main mirror up the tube slightly and thus force the focus position backwards and out.

[Starlight 1]

I can see in all the Stars, may be the secondary mirror niping in the clips.Edit make that primary  mirror oppssss.

[Steve789]

Thanks for the replies. My focuser tube protrudes about 25mm inside the ota with only about 10mm of backfocus available. This is roughly where the focus position is with my DSLR and coma corrector attached.

I'll try moving the mirror further up the tube by unwinding the collimation screws, the springs looked quite compressed and the locking screws are currently wound half way in so there's some adjustment there. I'm guessing this will mean I'll need to collimate more often.

Starlight 1 when you say nipping in the clips are you suggesting the mirror clips maybe too tight?

[Starlight 1]

Thanks for the replies. My focuser tube protrudes about 25mm inside the ota with only about 10mm of backfocus available. This is roughly where the focus position is with my DSLR and coma corrector attached.

I'll try moving the mirror further up the tube by unwinding the collimation screws, the springs looked quite compressed and the locking screws are currently wound half way in so there's some adjustment there. I'm guessing this will mean I'll need to collimate more often.

Starlight 1 when you say nipping in the clips are you suggesting the mirror clips maybe too tight?

yes that right only take 5 mins to have a look , and are collimating just off center.

[Steve789]

Great, thanks Starlight 1, I'll add that to the check list.

[Thalestris24]

Thanks for the replies. My focuser tube protrudes about 25mm inside the ota with only about 10mm of backfocus available. This is roughly where the focus position is with my DSLR and coma corrector attached.

I'll try moving the mirror further up the tube by unwinding the collimation screws, the springs looked quite compressed and the locking screws are currently wound half way in so there's some adjustment there. I'm guessing this will mean I'll need to collimate more often.

Starlight 1 when you say nipping in the clips are you suggesting the mirror clips maybe too tight?

Hi

Is it a reducing coma corrector? That would explain the shifted focus position. Maybe someone else on here has had the same problem and can advise.

Louise

[Steve789]

Louise, its the Skywatcher coma corrector which is also a 0.9x reducer I think. It would explain why my focus position is different to yours. Seeing that the 150pds is specifically sold as being optimised for imaging and that the coma corrector is sold as being "optimised for all Sky-Watcher fast-ratio f/5 Newtonian Reflectors" that the two would be a perfect match and that these little problems wouldn't be encountered but I guess that is asking too much.

[Thalestris24]

Louise, its the Skywatcher coma corrector which is also a 0.9x reducer I think. It would explain why my focus position is different to yours. Seeing that the 150pds is specifically sold as being optimised for imaging and that the coma corrector is sold as being "optimised for all Sky-Watcher fast-ratio f/5 Newtonian Reflectors" that the two would be a perfect match and that these little problems wouldn't be encountered but I guess that is asking too much.

Hi

I agree with you! If you bought it from FLO I'd have a word with them....

Louise

[Steve789]

I bought the coma corrector from FLO but the ota I bought elsewhere. I normally try to stay loyal to UK companies but on this occasion I found a much better price in the Eurozone.

I can't really complain because I was aware this could happen I just didn't realize it would bug me so early on. If winding the primary further up the tube doesn't work I'll either look at finding a secondhand focuser I can butcher (there's one on Astro Buy&Sell) or maybe replacing the collimation bolts and springs. Until then I'm just happy to have coma free images

[Thalestris24]

You could saw a bit off the focus tube What's protruding inside seems pretty much useless...

Louise

[jimjam11]

You could saw a bit off the focus tube What's protruding inside seems pretty much useless...

Louise

That is where I was getting to.

Slightly eggy stars from my previous eq3 masked this problem!

[toxic]

hi steve 

i had the same problem with my 200 pds and it was the draw tube going to far into the ota i cut 18mm of the end of my draw tube and now dont get the bite marks. BUT BE AWARE once you have done this you will need to ad spacers to use it visually .

just incase you are interested once your camera ans scope is in  focus then mark the draw tube inside the scope where it protrudes then when you take it out add 5 mm to the mark you made so that you are cutting of less but be carefull that it is what you need to do as you cant stick it back on. good luck in what ever you decide.