Chopping the 130PDS focus tube

[edarter]

Hi,

I'm seriously considering doing this mod but have some questions regarding it:

Firstly, thanks to using a low profile Canon adaptor (1mm optical length rather than 11mm) and an extension tube for the coma corrector to keep the 55mm spacing I 'only' need to chop about 12mm off my FT rather than the usual 25mm. However, when examining it ready to do the mod I spotted that its not just the FT itself that protrudes in to the OTA, but the outer part of the focusser as well (labelled B in the attached photo). So my question is, do people just remove the end of the FT itself (A in the photo) or also trim back part B as well??

Second question is regarding preventing the FT coming off the bearings when racked out for visual work. Looking at the design, the amount the FT can be racked in or out is controlled by the length of the flat section on its outer part that engages with the shaft for the focus knob. So one option would be to cut the FT and then drill and tap a small hole in the flat section of the FT 12mm from the scope end and put a small short screw in to act as a new end stop. It would stop the FT coming off the bearings but I've then lost 12mm of outward adjustment. Would one of these get that back for me if attached at the threaded outer end of the FT for visual?
https://www.astroshop.eu/extension-tubes/ts-optics-extension-tube-m54-15mm/p,71886
Second option is to not remove the small section of FT that runs along each of the two bearings, basically leaving two fingers in place for the bearings to engage on. Advantage being its effectively no different for visual. Potential disadvantage being they may have an effect on stars. Has anyone taken this approach and can advise on imaging results afterwards?

Thanks
Ed

[Peter Drew]

I wouldn't have thought that the portion at "B" would protrude into the light path of the primary mirror enough to have any significant consequence.  Your idea to cut the focus tube enough to leave the original motion stop seems sound.     🙂

[Ratlet]

The M54 extension should work.  I ordered one from aliexpress that did the job, although it didn't thread correctly into the compression adapter I had so I ended up getting a refund. 

As an alternative I got some M48 extension tubes and a parfocal ring.  The parfocal ring acts as a stop on the m48 so I can drop it into the focuser compression ring.  Made sure the parfocal ring was flat by laying everything flat on a table before I tightened it down.  Worked out cheaper (and more useful) than an M54 extension and not had any issues.

Think I ordered these https://www.amazon.co.uk/Extension-Extending-Astronomical-Telescope-Photography-default/dp/B0BQJLTPCW/ref=sr_1_5?crid=3G431D3V5HCM6&keywords=m48+extension+rings&qid=1675009326&s=electronics&sprefix=m48+extension+rings%2Celectronics%2C118&sr=1-5

(Although I think I paid more than £22!)

[alacant]

6 hours ago, edarter said:

12mm off my FT rather than the usual 25mm.

The usual amount to cut is 10mm.

With your low profile adapter and subsequent -racked out an extra 10mm- focus position, you should be good to go without use of a saw.

Leave 'B' as it is.

Cheers and HTH

 

 

Edited January 29, 2023 by alacant

[bottletopburly]

If you use a skywatcher cc you can have it turned down a bit so it goes into the focuser deeper thus racking focuser out further , Astrobloke did it to his 130pds 

[edarter]

2 hours ago, bottletopburly said:

If you use a skywatcher cc you can have it turned down a bit so it goes into the focuser deeper thus racking focuser out further , Astrobloke did it to his 130pds 

Yeah, don't really want to have to change the cc due to the cost and the fact it changes the focal length of the scope.

[edarter]

2 hours ago, alacant said:

The usual amount to cut is 10mm.

With your low profile adapter and subsequent -racked out an extra 10mm- focus position, you should be good to go without use of a saw.

Leave 'B' as it is.

Cheers and HTH

 

 

That's really odd, the photo I attached shows the focus point with the low profile t ring used and is still 10-12mm proud, so would have been something like 23mm before. Not sure why this is? 

 

Thanks

Ed

[alacant]

11 hours ago, edarter said:

photo I attached shows the focus point with the low profile t ring

What cc are you using?

[edarter]

2 hours ago, alacant said:

What cc are you using?

The Baader MPC Mk3

[alacant]

6 hours ago, edarter said:

MPC Mk3

Yep. Unless your example came with a different focuser and judging by your image, you'll be fine.

Certainly try it first. If you are certain that your collimation is perfect and you can still discern non circular stars at one side of an image, then would be the time to operate.

Cheers 

[AstroMuni]

On 30/01/2023 at 11:31, edarter said:

The Baader MPC Mk3

I use the same and I have pushed the whole assembly into the focuser tube. That way you can make sure that focuser tube does not stick into the OTA. No need to cut the focuser tube etc.

Edited January 31, 2023 by AstroMuni

[edarter]

4 hours ago, AstroMuni said:

I use the same and I have pushed the whole assembly into the focuser tube. That way you can make sure that focuser tube does not stick into the OTA. No need to cut the focuser tube etc.

How have you done this? I'm using a 1mm dslr adaptor and extension tube for my cc so the end of the ft is literally 1mm from the camera body, yet the ft still protrudes in to the ota by 14mm.

[AstroMuni]

13 hours ago, edarter said:

How have you done this? I'm using a 1mm dslr adaptor and extension tube for my cc so the end of the ft is literally 1mm from the camera body, yet the ft still protrudes in to the ota by 14mm.

In my case I am using the CC with the ASI533 so I have around 50+mm backfocus to play with and push into the ft. I should have read that you are using a DSLR 😞

Edited February 1, 2023 by AstroMuni

[alacant]

15 hours ago, edarter said:

the ft still protrudes in to the ota by 14mm.

And still it produces distorted stars?

[edarter]

yes  have a look at the brighter stars on the right hand side of this image...

[edarter]

5 hours ago, AstroMuni said:

In my case I am using the CC with the ASI533 so I have around 50+mm backfocus to play with and push into the ft. I should have read that you are using a DSLR 😞

ahh! that explains it lol. I can't push for an astro camera right now sadly. I am sowing seeds with my wife though about the prospect of it lol.