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Imaging with the 130pds


Russe

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Hi All,

Happy with my M45 Pleiades from last week.

130PDS, EQ5, 350d unmodified

20 * 180s guided

DSS + PS (referencing Steves wonderful new book....Dark arts or magic bullet)

All comments and advice gratefully received

Vern

Pleiades M45.jpg

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TBH I am sure my PA is out, focus  is out, I probably have cone error and balance isses on top of this I am sure I need to re-tune the worm gears again. In fact I really need to do a hypertune. You just know the minute you start stripping things down the cloud clears.

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Hi, apologies as have only read the first 17 pages ! of this amazing thread . I have a 130 pds , with the skywatcher CC and am looking to shorten the focusing tube  - at focus so much of it protrudes inside the tube that images of bright stars really show a lop sided effect. It is mentioned a few times up to page 17 but no details are given. Is it discussed in detail anywhere? Are there any problems/pitfalls/special precautions needed/disadvantages in doing this. Anyone got a pic of their shortened focuser? How much was considered safe to cut off? Many thanks in anticipation  :icon_biggrin::help:

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2 hours ago, pushrod said:

Hi, apologies as have only read the first 17 pages ! of this amazing thread . I have a 130 pds , with the skywatcher CC and am looking to shorten the focusing tube  - at focus so much of it protrudes inside the tube that images of bright stars really show a lop sided effect. It is mentioned a few times up to page 17 but no details are given. Is it discussed in detail anywhere? Are there any problems/pitfalls/special precautions needed/disadvantages in doing this. Anyone got a pic of their shortened focuser? How much was considered safe to cut off? Many thanks in anticipation  :icon_biggrin::help:

I think there were some pages that resolved this issue on this thread, can't exactly remember where though. I personally have not cut it.

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I cut about 1/2" off mine.

Which end did you cut it off, the inner end or the eyepiece end?  I am not at all able to do this myself, but need to be able to give my engineering friend instructions if I ask him to do it for me.  

If you take it off the eyepiece end, is it easy to remove the bit that had the compression ring on and put it back on again?

If you take it off the inner end, I presume you have to unscrew the whole focusser assembly to get it out, so is there some sort of "stop" mechanism to take into consideration that might get cut off (i.e. that stops you from winding the focusser all the way out.

Any pointers and links would be appreciated.

Carole 

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12 minutes ago, carastro said:

need to be able to give my engineering friend instructions

Hi. No need for engineering or dismantling, just a hacksaw. If you wind it out too far then just push it back in again. HTH.

 

Edited by alacant
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47 minutes ago, carastro said:

Which end did you cut it off, the inner end or the eyepiece end?  I am not at all able to do this myself, but need to be able to give my engineering friend instructions if I ask him to do it for me.  

If you take it off the eyepiece end, is it easy to remove the bit that had the compression ring on and put it back on again?

If you take it off the inner end, I presume you have to unscrew the whole focusser assembly to get it out, so is there some sort of "stop" mechanism to take into consideration that might get cut off (i.e. that stops you from winding the focusser all the way out.

Any pointers and links would be appreciated.

Carole 

Inner end, then I stuck a bit of circuit board on the flat to stop it winding out too far.

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Took the focuser apart today, small hands are definitely useful when reassembling so you can hold the 2 furthest back  internal nuts  in place without having to dis-assemble the spider. 

Took the chance to inspect the Crayford mechanism and regrease the ball bearings in the focuser. Looks a very straightforward job to shorten the tube and the only tools needed to take it all apart are a good set of fine metric allen/hex keys and a phillips screwdriver. A junior hacksaw should be more than capable of cutting the tube. Looks like 3cm could easily come off the internal side of the focuser tube without any affect at all on the mechanism.

I have reassembled it and will mark the protruding tube with a felt pen when focused, (with CC in place) with my camera and a variety of lenses so that i don't cut too much off! If you are ever going to use it without a coma corrector i suspect you will need to leave more tube but then without a CC the projecting tube is less of an issue anyway.

When i do it, hopefully in a day or two, i will take the opportunity paint the remaining tube black as its present silver colour could contribute to stray reflections.

This single cropped image of Alnitak in Orion shows the issue i am trying to avoid in about the 9 o'clock position on the star :) 

 

IMG_7871_edited-1a.jpg

Edited by pushrod
adding an image
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6 hours ago, pushrod said:

Looks like 3cm could easily come off the internal side of the focuser tube without any affect at all on the mechanism.

Careful! just be aware that the maximum amount you can remove is determined by the requirement for the lower bearings to have some tube to bear on :icon_biggrin:

I think it would be possible to file a 'scoop' in the end to follow the tube radius.

Don't forget to black the cut end of the tube so it doesn't scatter light from a bright off-axis star.

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7 hours ago, pushrod said:

like 3cm could easily come off

Ooo, careful; you'd be dangerously close to the rod which bears against the flat on the tube allowing only a few -if any- mm of outward movement.

2cm leaves you a more useful cerca-15mm of outward movement. HTH.

saw.jpg.1f3f01bd833123f1f1ffafdb243c959b.jpg

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15 hours ago, carastro said:

I presume you have to unscrew the whole focusser assembly to get it out

Hi @carastro, you only need to take out the screws that hold the focus adjuster plate in place. You can then just pull the draw tube out. You can just unscrew the compression ring at the eyepiece to remove it, if it's needed.

I would ask him to use a nice flat sharpening stone over the flat part on the tube, This will grind it flat and make better contact with the part that move the tube up and down.

There is a youtube vid somewhere.

Found it.

 

Edited by spillage
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Hi

Is this the same mod as explained in the article in Astronomy Now, March 2017 page 98.  They have shortened the tube of a 130 PDS but left two 'tongues' protruding presumably where the 'guide/support' mechanism is on the focuser.  I have the 200PDS so have been considering the same mod.

Sunnie

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Thanks Spillage, though I can't see how the remaining protruding parts of the tube in that link wouldn;t still cause a problem.  

If you guys think you have problems with star shapes and the focusser just take a look at mine (this is a crop):

 

SW 130PDS star shapes.png

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34 minutes ago, carastro said:

Thanks Spillage, though I can't see how the remaining protruding parts of the tube in that link wouldn;t still cause a problem.  

If you guys think you have problems with star shapes and the focusser just take a look at mine (this is a crop):

 

SW 130PDS star shapes.png

You seem to have four dark segments for each bright star. Faint ones at 1, 5 and 9 o'clock and a strong one at 3 o'clock.

My modded 130P-DS hstill shows some of these segments (see below) which I assume are caused by the mirror retention clips?

So my guess is that doing the mod on your scope will remove the darkest segment at 3 o'clock, but not the three fainter ones.

Pleiades.thumb.png.6d96803c9bd711aea66183f70f29f60d.png

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