My 130 P-DS and a hacksaw?

[Whistlin Bob]

A few weeks back I had my old DSLR modified for Astrophotography and the law of sod being what it is there has been very little chance to try it out since. There was a promising evening shortly after I received the camera back, but just as I got the scope set up the clouds rolled in and I was trying to time shots to match breaks in the cloud. Altogether I managed 10 reasonable 1 min subs over the next couple of hours, but noticed 2 things- the focus position for the camera is now a bit further into the tube, and (irritatingly, at the end of the session) I didn't actually have it perfectly focused.

Fast forward to processing, and whilst it's not the greatest picture in the world, I'm very pleased with the effect of the modification. What's not so great is that the star shapes now seem to be paying homage to a certain 1980's video game. My guess is that this is down to the drawtube for the focuser now protruding too far into the light-path of my 130P-DS. Now, obviously the sensible thing to do here is wait for another clear sky evening and check that collimation and focusing are all bang on before jumping to any rash conclusions, but in the absence of the weather being kind enough to allow me to do this are there any other 130P-DS users out there who can offer me a second opinion on my diagnosis? And if I am right, and surgery is required to shorten the tube, any tips or things to watch out for (obviously I want to minimise the amount I remove, so the plan was to set it up, find focus with the new camera, measure the protrusion into the light path and remove this amount).

 

[Peter Drew]

I think your diagnosis is correct and shortening the drawtube is what I would do to eliminate the problem.  

[alacant]

9 hours ago, Whistlin Bob said:

surgery is required to shorten the tube

Hi. I took 15mm. Remove the focuser, tape a line and cut. Blutak the nuts to the tube for easy replacement of the focuser. 20 minutes:) HTH.

 

[Whistlin Bob]

Thanks for the feedback guys- I can feel some DIY coming on...?

[RolandKol]

There is no need to remove Focuser in full as Its quite tricky to fit it back due to the spider vanes which blocks the hand.

Simply slightly loosen, one by one, 4 corner screws which are on the plate under the Focuser's handwheel (of course, loosen the Lock screw first)

and it will allow you to remove drawtube only, all screws and the holding plate will stay in place!  easy

The plastic part from drawtube is also removable, simply unscrew it before "circumcision ceremony".

10min job. from start to finish.

P.S. mark the drawtube while you in focus with your Camera, cut a bit less initially. I started from 1cm, with Canon 1300D, quite happy, but not enough.

(Altair via drawtube circumcised by 1cm, - Canon 1300D, and Skywatcher Coma Corrector / 0.9 reducer)

P.S.

Looking at your Orion... 
I do not think these V form stars are formed by the drawtube intrusion... they look just like my stars while I was struggling with guiding.

Once we have these clouds off, try long exposure on a  bright star.
The circle around the star with one flattened side and sometimes even with one shorter spike will indicate the intrusion problem.

I have not noticed such a problem on your picture, however, if you will start using Coma Corrector, - you will Most Probably have it.
 

[Pompey Monkey]

I, quite literally, approached the problem from the other direction and chopped the primary end of my 150 PDS down by 10 mm. Same result, although I guess that the secondary is probably 2% too small for the light cone now...

 

[glowingturnip]

out of interest, does every sub show the same shape ?  If I'd seen that in my subs I'd have assumed a tracking issue, but then the shapes would be different from sub to sub.

Also, tbh, I suspect a lack of focus is exacerbating the problem