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Sanity check my modification plans to my 10" newt to accommodate new secondary and focuser?


kbrown
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Hi,

Just wondering if someone could validate my ponderings before I start drilling new holes on my Skywatcher 250PX. Here are the current key scope parameters:

  • Primary mirror: 10" / 254mm
  • Focal length: 1200mm
  • Focal ratio: 4.72
  • Secondary mirror minor axis: 58mm
  • Tube inside diameter: 286mm
  • Tube wall thickness: 1mm
  • Focuser to front of tube: 128mm (ish)
  • Mirror face to back of tube: 72mm (ish)
  • Focuser minimum height: 76mm (**1)
  • Spare focuser in travel: 16mm (**1)
  • Additional height for camera: 55.667mm (**2)
  • Focuser inside diameter: 51mm

**1) The 76mm focuser height is the height from the top of the focuser to the outside of the tube when the scope is roughly in focus with my imaging train. The 16mm spare in travel is how much I can focus in from this position.

**2) The distance from the top of the focuser to the sensor + 1/3 thickness of a filter.

Using the Newt for the Web calculator it tells me the secondary is too small to admit 100% ray. The new focuser is a Baader Diamond Steeltrack with the optional ClickLock S58 clamp. Unfortunately this combination is higher than the 76mm mentioned above. Furthermore at minimum height the focuser tube would be in front of the primary mirror. So with these in mind I'm planning to switch the secondary mirror to a bigger one and move the focuser closer to the primary to "push out" the focal plane further outside the tube. The new modified parameters would be:

  • Secondary mirror minor axis: 75mm
  • Focuser minimum height: 100mm (**3)
  • Spare focuser in travel: 27mm (**3)
  • Focuser inside diameter: 55.4mm

**3) The 100mm focuser height is the height from the top of the focuser to the outside of the tube when when it's racked out enough for the focuser tube not to be in front of the primary. The 27mm spare in travel is how much I can focus in from this position.

By entering these new values to the calculator it tells me the Mirror Face to Focuser Hole distance should be 35mm shorter. It's also flagging some vignetting but this is due to the slightly over sized secondary as I wanted to prioritise better illumination of a larger sensor. This scope isn't meant for visual observations really.

I've never done a mod like this before so I would appreciate if someone could take a look at these values / calculations and let me know if there's anything fundamentally wrong before I start drilling holes on the tube.

I drew a template in LibreCAD to see what the new holes would look like. To be able to bring the focuser and the secondary spider vanes down 35mm I'd have to turn everything inside the tube 60 degrees:

IMG_20210818_155752.thumb.jpg.c3fafc983e0a745cdacafd8f317028c4.jpg

Edited by kbrown
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  • 2 weeks later...

I've used Newt for the Web when I was building my 12 inch truss Dob.  Since it's just for visual use the design goals were a bit different, to use a 
smaller secondary to get higher contrast for planetary observing and using higher magnifications/lower exit pupil.  I didn't need to maximize the 100% illuminated diameter like 
for AP.  But I understand the basic concepts.  Newt for the Web is a good tool that's easy to use and the results matched closely with what I finally built.

I plugged the original numbers you gave in Newt and got the same result as you -  secondary is too small to admit 100% ray.  

For your new design I didn't know where 100mm minimum focuser height was coming from exactly so I had to study up on the new focuser. I found the manual 
https://agenaastro.com/downloads/manuals/baader-planetarium-steeltrack-focuser-manual.pdf very helpful.  It had drawings and dimensions so I could understand it's
geometry.  That's a nice focuser.  Better than the MoonLite that I used for my scope.  

After looking, I think you may have only 20mm for the spare focuser in travel.  You don't obscure the mirror but the focuser tube goes into the light cone if you 
don't rack it out 20mm.  

The of the tube inside radius = 143mm 
Tube wall thickness is 1mm
Primary mirror D = 10"/254mm  - Radius = 127mm
Focal Length F = 1200mm
Focal Ratio = 4.72

From the book "The Dobsonian Telescope - A Practical Manual for Building Large Apertuire Telescopes" by David Krieg and Richard Berry the incoming light cone will be little beyond the radius of the mirror.  They gave a formula T ~ D + sin(angular field of view) x F  which they simplified to  T = D + F/100

For this case T = 254 + 1200/100 = 266mm is diameter of light cone  133mm is radius
From the diagram in the user manual, The focuser tube extends 31mm below when fully racked in.  


Inside tube radius + thickness = 144mm

Radius to racked in focuser tube = 144 - 31 = 113mm
Light cone radius = 133mm

So the focuser has to be racked out 20mm to get out of light cone.

The ClickLock S58 clamp is 16mm? I'm not sure how the focuser minimum height adds up to 100mm so I didn't put the numbers in Newt for the Web.  

Is it feasible to slide up your mirror cell up 35mm and enlarging the current focuser hole for the Baader focuser?  That might be easier than cutting a new 
focuser hole.  

 

Phil

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  • 2 weeks later...

Thank you Phil! Really appreciate you took the time to look into this! My new secondary mirror hasn't arrived yet so I have still time to study what you've just said. Haven't thought about moving the primary mirror. Initial thought is it's not possible but I will think about it...

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On 28/08/2021 at 06:19, pjsmith_6198 said:

The ClickLock S58 clamp is 16mm? I'm not sure how the focuser minimum height adds up to 100mm so I didn't put the numbers in Newt for the Web.  

Is it feasible to slide up your mirror cell up 35mm and enlarging the current focuser hole for the Baader focuser?  That might be easier than cutting a new 
focuser hole.  

 

Phil

I just re-read what you wrote and now have a better understanding. The 100mm was basically a guesstimate to put the focuser somewhere in the middle of the travel when the system is in focus plus ensuring the tube isn't in the light cone. That would leave me a little bit of in and out travel should I ever need it for anything. That was the thinking anyway. Not sure it this makes sense though?

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I think drilling a new focuser hole would be possible but I wouldn't advise it, as you would be weakening the tube at a fairly critical section... Under the weight of the imaging train you may get a lot of flex in the OTA. The 1mm sheet steel of the SW tubes is not exactly known for it's great stiffness anyway so putting another big hole there is not a great idea in my opinion. 

Better to do as Phil suggested and move the mirror up the tube and utilise a larger secondary with the original focuser hole.

This can be done (I think) by simply removing the mirror cell from the tube, chopping off 35mm or whatever you need coming off (jigsaw with parallel guide should do), drilling new screw holes in the tube and reattaching the cell?

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I fitted a steeltrack to my 200pds using existing hole but just drilled new mount holes to fix so it should be ok to move the mirror up by cutting the tube as mentioned

i have the steel track on my VX14 now its a great peice of kit and very solid also like the dovetail attachment for the focus motor.

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  • 3 weeks later...

Finally got around to do something about this. I decided to cut 35mm off the tube length as suggested above. I did it using a Dremel tool with a cutting disc. I designed and 3D printed a flange guide that allowed me set the cutting depth and to run it along the inside of the tube. Took me about 20-30 mins to do the cut and I had to change the disc twice as it wore down quite quick. Anyway, I think it's accurate enough. Had to sand down the rough edges and then vacuum the inside of the tube afterwards as it got a bit messy (especially the velvety flocking material). Still need to drill new holes for the mirror cell etc etc...

IMG_20211001_112009.jpg.7652cd944382e3f110293b39d9e0cec0.jpg

IMG_20211001_120046.jpg.a9bcdf1ac4a8c2832464435b6237c238.jpg

IMG_20211001_121557.jpg.31f25d7b7a3b085dda06dc73fdfbe280.jpg

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Posted (edited)

Got the mirror cell holes done. Using the old holes I measured the distance between them along the circumference and printed matching templates then cut and taped them together:

IMG_20211001_152026.jpg.4d18244a6039a136936247b6ae28da22.jpg

Then I simply taped the whole strip onto the tube then punched and drilled the holes. I drilled them slightly (0.5mm) over size to allow some wiggle room.

IMG_20211001_154307.jpg.83ac1b115dd721834194776da56d66eb.jpg

Got them close enough. With a bit of gentle persuasion the screws went in nice and tight. In case you're wondering I've replaced the stock M4 screws with M5 socket head ones and also installed V-Coil thread inserts on the mirror cell which makes it a lot more durable than just screwing into the cast aluminium.

IMG_20211001_162001.jpg.07fe6bb59ace1fad90ee9eba9f6ba756.jpg

Edited by kbrown
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Posted (edited)

New focuser in place finally. Used a printed a template again to aid drilling the holes in the right place.

IMG_20211004_145827.jpg.63a5fecf2cd5328e6ca4992b4f100d7b.jpg

IMG_20211004_161353.jpg.4886607e180b20c061e96e85a52f4544.jpg

IMG_20211004_161416.jpg.6a2059d2cb53068f50e808462be4b303.jpg

Edited by kbrown
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57 minutes ago, CraigT82 said:

Illuminating from the inside of the tube like that to line up the template is an excellent idea... I'll steal that next time I upgrade focuser... if you don't mind! 

Of course. I just had to come up with something as I couldn't  follow Baader's instructions to the letter and use the old screw holes as the guide. Figured I'd make use of the template I printed in the initial post like this. :)

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On 05/10/2021 at 19:24, pjsmith_6198 said:

Excellent work.    Upgrading a focuser can be tricky, especially if you have to perform surgery.   I'm glad it worked out so well.

 

Phil

Thank you! I've now put the rest of it together as well and have been able to test it. Good news is that I'm able to reach focus with plenty of spare inward travel and the focuser tube well outside the light cone. The bad news is that it seems like I have managed to pinch the secondary mirror with the 3M VHB tape I used to attach it to the stalk. I spent quite a lot of time diagnosing this and really was hoping it wasn't the secondary but I think it is. I did what I could with the primary mirror including loosening the clamps as much as I dared and rotating the cell in the OTA but it just didn't improve the situation. The stars just are not round. I did my best to centre the secondary with OTA and the focuser and collimated everything with a laser collimator and a cheshire eyepiece and re-collimated after each change.

So I'm going to have to re-attach the secondary. I'm hoping some gentle heat will soften the tape enough I can remove the secondary without too much trouble. I'll then try again with some RTV silicone instead.

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