Adventures with an old SW 300p

[CraigT82]

Having just taken ownership of an old blue tube skywatcher 300mm f/5 newt I'm starting this thread to document the modifications and upgrades I'm making to it. This scope is going to be put to use for lunar and planetary imaging pretty much exclusively.

The primary mirror has a rather odd graze in the coating which is easily visible and was disclosed by the seller so I knew that it was there before I bought it. I could see from pics that the defect was likely to be in the shadow of the secondary and so I wasn't too fussed. What is surprising is that on removing the mirror from the scope it seems like whoever had made the scrape had tried to remove it with something, most likely a scourer! Luckily all the scratches are within 30mm of the mirrors centre and so all are withing the shadow of the 70mm secondary.

[CraigT82]

Having a good look over the primary mirror cell I found a couple of issues that needed attention. 

The cell is a 9 point floating design with 3 nylon studs for edge support. It was immediately obvious that the triangles were solid and most definitely not 'floating'. This was fixed by loosening the centre screws and removing the small silver coloured pins  (one for each triangle) which were supposed to prevent rotation of the triangles but pretty much prevented them from moving at all. Three strips of tape were added linking the triangles to prevent then from rotating too much. 

The edge support comprises three m6 grub screws with nylon tips. This is actually quite handy as the clearance between the edge support studs and the mirror can be fine tuned.

In order to check the studs were contacting the CoG of the mirror I removed the mirror and marked on  CoG lines (calculated in Crux). Putting the mirror back in the cell it was clear that the studs were too low, missing the CoG by about 6mm.

Edited May 3, 2021 by CraigT82

[CraigT82]

I marked out new locations for the edge support studs and drilled/tapped new holes, so that each stud now contacts the mirror right on the CoG pencil line I made earlier. 

Cruxis suggests that three points at 120 degrees is perfectly sufficient for this mirror, however Cruxis is assuming that the mirror is in an Alt az mounted scope, whereas this being an EQ mounted scope I'm not sure that three points is good enough (edit: seems OOUK think 3 point edge support is OK for EQ mounted newts, as seen on their mirror cells, whereas Aurora in the US make cells for EQ Newts with 6 point edge rollers for a 12" mirror- who to believe?!).

There's not much I can do about that as the rim of the cell between the existing three studs is not high enough to drill and tap holes for another three studs and have them at the right height, although I could simply glue some nylon studs in the right place but they wold not be adjustable and great care would need to be taken to ensure all 6 points contact the mirror uniformly. For now I will resort to turning the OTA in the rings so that the mirror is resting on two studs as equally as possible during an imaging session, and will make a slip ring to facilitate this.

This isn't too bad a situation to be in for now as the majority of my hi-res imaging I'm only going to be looking at a relatively small patch of sky about 10 degrees east and west of the southern meridian where my targets will be at their highest. 

I also modified the mirror clips as there is no need for them to be so large and clunky! 

Edited May 4, 2021 by CraigT82

[Swoop1]

An intriguing thread Craig. I am an inveterate dismantler- I love to disassemble things and put them back together to see how they work (nearly got me into loads of trouble in the army but, that is another story) but, I haven't got the cojones to start messing with my scopes like you are with this. I shall follow with interest! 

[CraigT82]

I don't get on well with the standard collimation arrangement of these cells: three adjustment screws and three locking screws. The adjustment screws have coils springs but these are pretty feeble and so I have fitted much stronger springs to enable me to get rid of the locking screws altogether. 

I bought a length of 15mm OD compression spring with 2mm wire and cut this into the required lengths. I finished the ends of each of the three springa by heating and manipulating the ends and then grinding a flat. These springs are very strong with a 130N/mm compression strength. 

Edited May 3, 2021 by CraigT82

[CraigT82]

A big newt like this will require some kind of thermal management in order to perform well. 

I have a pretty decent 140mm Noctua fan taken off my Fullerscope, which will work nicely. I made a baffle from 6mm plywood to close up the rear of the scope, which will allow the fan to work most efficiently. 

The fan is working to suck air out of the tube, and has a good strong airflow which should hopefully be enough to completely suck away any warm air rising off the primary.

The baffle is attached to the cell using velcro and the fan is also attached to the baffle using velcro. The velcro provides some mechanical isolation and using a mechanic's stethoscope against the cell and OTA I can't detect any vibration on either with the fan running.

[CraigT82]

23 minutes ago, Swoop1 said:

An intriguing thread Craig. I am an inveterate dismantler- I love to disassemble things and put them back together to see how they work (nearly got me into loads of trouble in the army but, that is another story) but, I haven't got the cojones to start messing with my scopes like you are with this. I shall follow with interest! 

I never 100% sure any of these mods will actually help to improve the performance of the scope but I do like to tinker!  Good way to learn how it all works though. 

[kirkster501]

That scratch will make no difference at all to the views other than a minute amount of contrast.  I saw this scope when it was for sale and thought it would be a great scope for someone.  Had it been a Dob I'd have bought it myself to leave in Cornwall.

Glad to see you are making some mods to it

[CraigT82]

45 minutes ago, kirkster501 said:

That scratch will make no difference at all to the views other than a minute amount of contrast.  I saw this scope when it was for sale and thought it would be a great scope for someone.  Had it been a Dob I'd have bought it myself to leave in Cornwall.

Glad to see you are making some mods to it

Yeah it is an awful lot of scope for the money, I'm excited to see what it can do along with my new QHY462c... Jupiter's apparition isn't far off now and will be a bit higher than last year. 

[CraigT82]

Continuing the upgrades I'm fitting a Baader Diamond Steeltrack to the 300p.

I was planning on fitting my motorised green moonlite CR2 to this OTA but couldn't source the necessary curved base for the 12" tube, and so that focuser will be staying on the 200p f/6 which, when I get it mounted on a Dob base, will become a visual scope. 

The existing 2" single speed Crayford that is the OE focuser on the 300p is going to be out in my Fullerscope as that is currently stood focuserless in the corner. It will become a dedicated solar WL scope with a full aperture film. This old focuser should work well with this scope as being f/7.5 it's not fussy on collimation. 

After removing the OE focuser I used cotton thread to create cross hairs in the tube hole and the focuser base. This allowed me to accurately centre the steeltrack base over the existing hole whilst I marked it up for drilling.

 

[Saganite]

On 03/05/2021 at 13:15, Swoop1 said:

An intriguing thread Craig. I am an inveterate dismantler- I love to disassemble things and put them back together to see how they work 

I have a Father of the same ilk, who at 91 years old still living at home, on his own, disassembled his vacuum cleaner, ordered online a new clutch and fixed it rather than throw it and buy a new one , as is so often done these days. I have inherited that, but not even close to the same degree...

[Saganite]

Excellent work Craig !

I see in that image an old black EQ6.  I had one of those for my 5" refractor but traded it for an NEQ6 , the worst decision I have ever made in Astro I think.

[CraigT82]

Thanks Steve, yes for an old beast it does work well. It's had a long life before me, mounted in the obsy of a gentleman who sadly couldn't do any astro anymore due to advancing age, and I bought it from him for a pittance really (as it's a bit tatty). With a bit of work it's been brought back to full working order with the addition of goto motors and motherboard plus new worm bearings, Baader #1 grease and new Teflon shims of the correct thickness.  It might yet give another 10-15 years service with regular care. 

I did some DSO imaging with it recently with my 200p f/6 dob OTA on board and it performed well beyond my expectations, guiding it at under 0.5" RMS when seeing was good. 

It's struggling a bit with the 300p and all the extra C/W on board but I don't need it to guide well, just point it where I want it and then roughly track and I'll do the rest! 

Edited June 26, 2021 by CraigT82

[CraigT82]

Steeltrack is fitted... tomorrow's job will be to fit my lakeside motor to it and then finish wrapping the tube in radiator foil - to help stop the tube metal cooling below ambient and creating temp. deltas inside the tube. 

[Saganite]

16 minutes ago, CraigT82 said:

Thanks Steve, yes for an old beast it does work well. It's had a long life before me, mounted in the obsy of a gentleman who sadly couldn't do any astro anymore due to advancing age, and I bought it from him for a pittance really (as it's a bit tatty). With a bit of work it's been brought back to full working order with the addition of goto motors and motherboard plus new worm bearings, proper grease and new Teflon shims of the correct thickness.  It might yet give another 10-15 years service with regular care. 

I did some DSO imaging with it recently with my 200p f/6 dob OTA on board and it performed well beyond my expectations, guiding it at under 0.5" RMS when seeing was good. 

It's struggling a bit with the 300p and all the extra C/W on board but I don't need it to guide well, just point it where I want it and then roughly track and I'll do the rest! 

I bought mine from a trusted friend who said " all it does is track, but solidly and accurately", and indeed it did for the few years that I owned it.  I was lured by the prospect of GOTO and it was , frankly, a disaster.  I had very few enjoyable evenings with that mount, always a fraught experience.  Never part with it Craig, it owes you nothing.

Edited June 16, 2021 by Saganite

[CraigT82]

Eventually got round to fitting the lakeside motor to the steeltrack. Had a but of an issue withthe bracket not fitting the focuser at all (despite being sold as a steeltrack specific bracket) but got there in the end. 

Also finished wrapping the tube to help keep tube currents at bay. Bit of a mess but doesn't need to look pretty.

 

[Swoop1]

It looks more like Hubble every day!

You now need to get a hinged end cap to add to the aura.

[CraigT82]

55 minutes ago, Swoop1 said:

It looks more like Hubble every day!

You now need to get a hinged end cap to add to the aura.

Haha yes it does! Got some Nasa worm logo stickers on order and tempted to add some fake solar panels but they won't do much but catch the wind! 

[markse68]

Looks great Craig- what's the x-y platform thingy for?

Mark

[CraigT82]

8 hours ago, markse68 said:

Looks great Craig- what's the x-y platform thingy for?

Mark

Thanks Mark. How are you getting on with the old f/4? I saw you were having some chip alignment issues with the cam?

That is a bit of an experimental feature at the minute, it is to be used with the ADC.

When the ADC is adjusted the target moves, quite often right off the camera sensor which then requires you to move the scope to get it back on. The X-Y adjuster allows me to move the camera itself rather than the scope to get the target back. The theory being that the target remains exactly on-axis and in the sharpest part of the FoV. 

Not used it in anger yet so don't know if it works! 

It is also very handy for making very fine collimation adjustments whilst looking through the cheshire, which are often needed once I've plugged the imaging train into the focuser (having colimated the scope first without the imaging train) as the extra weight and length of the train causes the collimation to shift very slightly (due to the tube flexing I think). 

Edited June 24, 2021 by CraigT82

[markse68]

that sounds ingenious Craig! I know my focuser flexes under the weight of my imaging equipment as the hot spot of the vignetting from the CC shifts around if I re-orient the camera in the focuser tube. Too many other issues to consider before that though. My scope's coming along ok thanks- the alignment is an issue because my Astronomik filters are hard vignetting in certain positions. So much to learn what with all the equipment and the processing side- fun though  

Mark

 

[CraigT82]

23 hours ago, markse68 said:

that sounds ingenious Craig! I know my focuser flexes under the weight of my imaging equipment as the hot spot of the vignetting from the CC shifts around if I re-orient the camera in the focuser tube. Too many other issues to consider before that though. My scope's coming along ok thanks- the alignment is an issue because my Astronomik filters are hard vignetting in certain positions. So much to learn what with all the equipment and the processing side- fun though  

Mark

 

Ive seen a few people reinforcing the upper tubes with sheet aluminium riveted/screwed on the inside or outside of the OTA, to make it all stiffer up at that end.

I wonder if you could 3D print a section of tube, with an OD just less than the ID of your OTA and a wall thickness of a couple of mill, to slide down inside the OTA beneath the focuser and reinforce that area? 

Could even glue it in place instead of screws or rivets so it would be an invisible reinforcement? 

[Captain Scarlet]

I have both a 300p just like that and a 200p. They both suffer from flexure, the 300p not too bad but the 200p absolutely terrible. So much so that I resorted to putting an aircon hose-ring , which has helped but not fully cured:

 

The 300p tube I’ve now ditched and replaced with a Klaus Helmerichs carbon tube.

[CraigT82]

I often find myself on Klaus' website drooling over the CF tubes! 

How do you find the performance of the tube over the standard SW one? 

I have got a vague 5 year plan for a self build newt which involves a 12" Nichol mirror in and Aurora cell all wrapped up in one of Klaus' carbon honeycomb tubes with matching carbon rings. 

[Captain Scarlet]

I haven’t applied the tube yet. I’m in the process of moving to Ireland, and the carbon tube was delivered to there just after I last left 3 months ago. So it’s still in its packaging. I’ll be back in 2 weeks though and will install it then. Also I swapped out the SW mirror for an OO one, so it’s a bit Triggers Broom really 🤣.  I didn’t actually need to lose the SW mirror, I think they are rather good, but I got seduced by the OO website. Only the cell and the spider remain SW, and I’ll be building my own cell quite soon.

After I ordered the Klaus tube he got quite ill I think, and everything was seriously delayed. He wrote later that he had suspended taking new orders until maybe late summer, I’m not sure how things stand at the moment.