Monster New 12" Dob (new to me anyway)

[Astrobits]

To test the mirror you can use the star test but that will need the secondary re-mounted first so that you can collimate accurately.  Alternatively, see if you can get someone from a local astro society that has tested mirrors and get their help and advice. Failing that you can bring it to me on the Somerset coast and we can test it in my workshop. PM me if you want to take this option.

Nigel

[AdeKing]

Hi Nigel,

Thank you for the information and the kind offer.  I am in the process of dismantling the secondary assembly to sort the collimation issue out, but am taking care as the secondary mirror is very well stuck to the holder and I'm not 100% sure what its stuck with.  However, its safe to say that it definitely isn't silicone sealant, its much more substantial and hard.

I may alter the design of the secondary holder slightly to make it a bit smaller before I re-attach so that I can attach a length of cord to the secondary mirror to ensure that if it parts company with the holder that it doesn't strike the primary.  

The secondary holder is currently the same size as the secondary mirror and is rather agricultural in its adjustments, and is also rather loose on the end of the supporting rod, though i don't think its in any danger of falling off as I've tried hard to pull it off and it isn't going anywhere.

I will be reattaching with three blobs of silicone sealant at 120 degree spacing and use some cocktail sticks to ensure a bit of a gap behind the secondary to make future removal somewhat easier.

I'll send a PM regarding your offer.

Ade

 

 

[N3ptune]

22 hours ago, AdeKing said:

@N3ptune thanks for your vote, the problem is that I don't have anywhere near the £650 GBP for the 14" hence wanting to fettle things together to keep costs down.

The other aim is for me to learn more about how the Newtonian works and fits together so I'm less paranoid about adjusting things on the scopes that I paid money for. 

If i'm able to do the majority of work myself the only major costs should be a new secondary (£55-114 depending on retailer) and recoating the primary (£140-160 depending on retailer) plus VAT and shipping unfortunately. With some (hopefully relatively minor) costs for materials.

Well thats the plan anyway.

I respect your choice, the project is going to make you happy at the end.

[Racey]

On 11/05/2017 at 09:58, estwing said:

14" on ABS for £650...it's a good mirror...Danny K will vouch for it

Not anymore Calvin. Thanks for the tip off 

Great thread @AdeKing  I'm envious of your resolve and ingenuity. I'm aware of my limited capabilities but I'm enjoying refurbishing this one vicariously 

[AdeKing]

I've been rather busy for the last month or so, but have some spare time coming up during which I hope to make some progress on the restoration of this Dob.

I have a replacement secondary mirror, and all of the parts to make a new spider assembly, but the first thing that I want to do is to fit a standard Skywatcher 2" crayford focuser to replace the rather basic helical focuser that is currently attached.  However, how is the best way to go about enlarging the hole in the tube for the 2" focuser?

As I'm cutting the tube, I really don't want to make a mess of it so I was wondering what the best approach is.

My plan was to carefully measure out and mark from the current hole where the new hole needs to be then carefully drill some closely spaced holes around the circumference of the new diameter and smooth off the resulting rough edges with a file.  My main worry is that the new focuser won't have enough outward travel to reach focus, but I assume that this can be corrected easily enough with extension tubes.  

Does the above sound like a sensible approach, or is there another way that is easier?

If I was drilling the hole from scratch, I thought that a hole saw attachment for a drill might be a good idea, but as I'm enlarging an existing hole I figured that drilling closely spaced holes might be a good start.

Any ideas, suggestions or pointers as to where I could go catastrophically wrong?

Ade

 

[Moonshane]

I'd do it the way you suggest. The smaller the bit, the more holes but the least filing, you choose 

the other option is to bolt some plywood to the inside of the tube and then use a hole saw as the central bit could be started and held by the ply. This requires a bit more care, control and nerves but would be a lot quicker.

yes, extension tubes would be a good solution for more out focus although ensure that you don't buy one that's too long.

[Commanderfish]

I attacked the focuser hole of my aluminium tubed VX16 with a hammer drill to enlarge the hole to fit a Feathertouch.  The resulting edge was definitely not perfectly spherical but it doesn't matter because the focuser mounting plate covers it all up - you can't see any imperfection with the focuser in place.

So in short, hack away with confidence  - just be sure to hoover up any shavings.

 

[25585]

On 11 May 2017 at 09:12, John said:

While it's worth sorting the scope out within reason, it's worth keeping in mind that you could buy something like that scope, with mirrors in good nick, for around £250-£300.

 

This is a point I am considering due to the cost of shipping and having my mirrors re-coated. 

My 12" DS is definitely the shorter length seeing AdeKing's photo. It originally had the fl written on the back of its primary in black marker pen. 

I found the bearing circles OK, just needing to little lubricating with Teflon oil spray. For balancing I bought athlete weighted wrist bands, attatched end to end and strapped round the tube base. Extra weighting I tucked underneath. Prefer McGyver to Heath Robinson

The OE finder was replaced by an 80mm finder with cross hair eyepiece and I added a Telrad sight. Focuser sent to DS was a Vixen r&p. DS provided "caps" which were small bin liners with elasticated edges. 

Tube assembly in rocker box with mirrors in place weighs 1/2 cwt, ditto mount. Steady as a rock!

And all British!

[John]

Whats the mirror like ?

Some of the Dark Star scopes had decent mirrors in them but some were not so good.

[25585]

M

11 hours ago, John said:

Whats the mirror like ?

Some of the Dark Star scopes had decent mirrors in them but some were not so good.

Mirrors were fine & bright. My eps apart from 13mm Nagler T1 were not wide field so I never noticed coma.Easy to collimate the primary as DS used wing nuts. At the edge the primary is 1.5 ins thick. 

I had sharper but dimmer images in its predecessor, Fullerscope 8in F8, wish they had made dobsonian mounts for their OTAS. 

More diffraction spikes in the DS as would be expected. Not so good for planetary detail but easier to find the fainter ones & see more moons. 

DSOs much better. M13 in whole field resolved to centre, doubles split better (but spikier) & OIII viewed nebula amazing. All I wanted. 

When I get my 10in Bresser Messier up & running (f5, 1500mm) I will be interested to see how it compares. 

[tone2012]

Hi guys, 

i recenly dug my old DS out, needs a new base as mines rotten, anyone ever made a alternative base ?