130PDS focussing issue

[halli]

Hi All

I have a 130PDS and use a Canon 1100D for imaging.  I have found that in order to achieve focus when the camera is attached,  the focusser  tube  intrudes into the OTA  too far and causes a chunk to be missing from a star when the disc is out of focus.  When the star is in focus it doesn't seem to be visible. However I was wondering if this is normal and if I can do anything about it.

Any help appreciated

Thanks in anticipation.

[toxic]

it is normal for the 200 pds not sure for the 130 pds.i cut 25mm of the length of my focus tube to stop this , i used a fine marker to mark where it was when focused on the inside of the scope then took it out and added 5mm back onto the length then cut it of as i only use this for imaging. i found even on long exposures part of the stars had a chunk missing.they dont have any chunks missing now.

[Epicycle]

I have found this in my 130pds also and it is quite noticeable. For large sensors this also produces a darkening that has to be carefully removed with flats. At the moment I just live with it. Uranium 235 has other solutions besides cutting for this which involve threaded connections and the corrector sitting quite deep in the draw tube, but the modifications are not trivial,

Cheers

E.

[SW130p]

I have the standard 130p and had to modify my whole focuser to prime focus. Had the same problem with the end of the tube, so I tested it to get focus the cut off the excess. But now it so much better. Need an extension tube now for observing though but not a problem.

Sent from my iPhone using Tapatalk

[halli]

Thanks very much for your input  - I now know its not just me !  I thought the PDS range were meant to be aimed at astrophotography !

I guess its the balance between imaging and observing that the design struggles with.   Anyhow cutting the tube and using extenders for observing seems the best option although seems fairly brutal.  

I dont use a coma corrector currently but probably will get one at some point - wonder if this affects the focal point at all ?

[Uranium235]

As mentioned above, its completely normal. But it can be overcome by either of the methods mentioned: 1) Shortening the drawtube, or 2) Sinking the coma corrector further inside the drawtube (method 2 is more suited to CCD cameras).

Unfortunately, there is no third option of a replacement focuser since none are available for a 5" tube.

[halli]

ok thanks - reality dawns !

[PorkyB]

I had the same problem, so yesterday I chopped 15mm off the focusser tube.  Whilst the scope was in bits I took the opportunity to flock it as well and then collimated it very carefully.  It is now a whole lot better than it was before.

[nightfisher]

Okay, i have the 150p ds and dont have this problem when i conct my 1000d, i unscrew the 1.25 ep holder and fit the T ring direct in place

[Uranium235]

nightfisher, on 10 Nov 2014 - 06:22 AM, said:nightfisher, on 10 Nov 2014 - 06:22 AM, said:

Okay, i have the 150p ds and dont have this problem when i conct my 1000d, i unscrew the 1.25 ep holder and fit the T ring direct in place

Might be worth looking down the OTA when your camera is connected and in focus to see if the focuser tube is clipping the primary. The "problem" (I call it that because its not really a problem, just a cosmetic issue on some bright stars) is more apparent when you are using a coma corrector since it shifts the focus point inwards slightly (even moreso if using the SW 0.9x corrector).

Edit: I just need to add a note to all that chopping off the focuser tube will mean that if you ever want to use it for visual, then you will need to use a longer 2" extension to get into focus.

[tingting44]

i image with the 130-pds and canon 7d with SW coma corrector and baader neo filter, i have no problems at all.....

[Uranium235]

i image with the 130-pds and canon 7d with SW coma corrector and baader neo filter, i have no problems at all.....

Exactly, its only a problem if youre being hyper-critical on very brght stars. Once the stack is processed (fully) it shouldnt be even an issue.

[Epicycle]

At the moment, stars bother me less, it is the darkening that you get in larger sensors that is a bit annoying.

It is more noticeable in narrowband and it can be removed with flats, but everytime I get a sub I feel that obstruction should not be there!

E.