Lukehurst-Nichol classic dobsonian modifications

[Piero]

The new mirror cell was built based on the Kriege design. There are more modern designs, but this is simple and effective. It is a 6-point mirror cell. A 6-point mirror cell gives the same performance of a 9-point mirror cell but is much simpler, therefore it is easier to get it right. Also, it can be upgraded to 18 points any time.

All metal is stainless steel 304. I did not find too difficult to work with.

 

After drilling the holes, I cut (yes, that's correct) the bottom of the original mirror box, so that this could be used as a frame for welding the new cell.

 

 

 

 

 

 

 

The cell was tack-welded in the frame first. Then the welding was completed and the mirror cell was cleaned / sanded:

 

 

Finally, it was sprayed with grey primer and then flat black:

 

A real Glatter's sling was installed. This is the assembled mirror before cutting the cable to the correct length:

 

 

 

[Piero]

MIRROR BOX

This was cut for a total of 7 inches from the bottom (below is the second round of cutting). Then the mirror cell was installed.

 

 

 

 

 

 

Here is the new mirror box on the old rocker box:

 

Edited June 3, 2022 by Piero

[Piero]

ROCKER BOX

Now the time for the rocker box! 

 

 

 

 

 

 

[Piero]

Here is the shorter MB on the shorter RB.

 

 

 

Edited May 30, 2022 by Piero

[Piero]

Feet and ground board

The original feet always reminded me of wood clogs heels.. These were shortened as all that length was not needed at all. I also added a fourth pad.

I am not a great fan of this ground board to be honest. This structure is in direct contact with the ground and humidity... it's basically the place in which one should limit the use of screws.. None of those screws are stainless steel either! 

Anyway, for the time being I will keep it as it is. In the future, I might ditch it.

 

 

[Piero]

MIRROR AND ROCKER BOXES AGAIN

One of the issues of the original structure is that it is so heavily built with wood panels that the primary mirror not only struggles to cool down properly (the fan at the back is nearly useless as too far away from the back of the mirror), but also the air boundary layer above the mirror surface can be a serious. 

The new cell and the shallower MB allow the mirror to cool down and remain at ambient temperature more efficiently, but they do not tackle the air boundary layer issue. Here are the changes to improve this.

 

 

 

 

 

On the other hand I also wanted to reduce the weight of the rocker box.

[Piero]

UTA

I decided to get rid of that weird focuser / finder panel and the magnifying focuser itself. Not shown here, but I will only use a Quick Rigel finder. The Moonlite CR2 will be sold at some point this summer. This was replaced with an Antares helical FOCH focuser. With this telescope I decided to use only 24 Pan and Nagler T6 eps which are very light, powerful and parfocal with each other. 

The original focuser panel is 18mm thick (yeah, no kidding ! the one on my 16" is 12mm thick which is the norm..). The photos below do not show this, but I actually removed a lot of wood from the back of the focuser panel to make it lighter. I also removed a couple of plies from the front.

Oh, another curiosity... when playing with the UTA, I found out that the focuser holder is made of plastic (either nylon or ABS). A really unique design.......!

For the time being I will keep this UTA. In the future, I might ditch it.

 

 

[Piero]

VARNISH

The original telescope was pseudo- lacquered. I write pseudo because certain parts were not lacquered at all. As this made me incredibly nervous, I did lacquer it (see page 10, I think). Nevertheless, I never liked lacquer. It gives a pale / dead tone. After trying CPES + clear matt polyurethane on my astro chair (see my profile pic), I decided to apply this combination to the telescope. 

 

 

 

 

 

 

 

Now the black paint. 

 

[Piero]

Installed fan grills to the MB side holes.

 

 

 

 

 

[faulksy]

that looks ace @Piero should of tig welded the cell instead of migged.

hope your cell is in the right place, due to balance and focal length.

what paint did you use looks very black

ace job mate

[Piero]

TRUSSES

Despite what physics laws say, there is a belief in the EU/UK that one can use small diameter long poles whilst retaining adequate stiffness. The original poles of this dobson were 7/8" SWG 16. Personally, I never felt they were adequately stiff, despite the fact that they were rather short given the depth of the original mirror box (22"). In addition to choosing slim poles, the original builder also built the tube connectors too. Guess what.. the pole inserts were made out of plastic and the pole ends connected using a non-stainless steel eyebolt which were subjected to bending due to the lack of stiffness. The pole inserts were held in place by a mini bolt. It was so mini that was not even capable of holding 3 of the 8 inserts in place. I had to replace them.

Anyway, here I replaced those poles with longer 1 1/8" SWG 16 poles, like those in my 16" f4. I used star nuts and 1/4" aluminium angle brackets as pole connectors. These brackets were made by me. They are the same as those used in my 16". The lower truss attachments of the original telescope were retained instead.

For the top ends, the poles were squashed. The design for these was copied from Highe's Portable Newtonian Telescope (amazing book BTW).

Like in Highe's book, I cut a channel connecting the holes in the squashed plate, so that the UTA can be slotted in. That's much more comfortable than the original design.

The new structure is very stiff.

 

 

Edited May 30, 2022 by Piero

[Piero]

Here is the final telescope next to my 16". They look like too buddies who wish to go out!  

The red bag between the two contains my 4" Tak refractor.

 

 

 

 

 

Edited May 30, 2022 by Piero

[Piero]

And finally, here is a summary of the main changes (side by side). I am pleased with these changes now. I wanted to spend as little as possible as this is not my main telescope, but the new design is considerably easier to handle. 

Oh, forgot to say... I could not find a more appropriate name to this re-born telescope than PHOENIX!   

 

Edited May 30, 2022 by Piero

[Piero]

11 minutes ago, faulksy said:

that looks ace @Piero should of tig welded the cell instead of migged.

hope your cell is in the right place, due to balance and focal length.

what paint did you use looks very black

ace job mate

Thank you!  

The cell was not welded by me, but by a local welder here in Cambridge. It's not a super elegant job, but it works. 

Good point regarding the balance. I built a spreadsheet for the COG equation and other things. The telescope was built with the idea of adding some counterweight at the base. The reason is that I prefer to have some movable weights but a lighter / more portable telescope than the other way around. As you can see, I am not going to use wheelbarrow handles here. At the horizon, the telescope needs 3kg counterweights, but I don't plan to use this telescope below 45-30 deg. My plan is to take it around for DSOs, high in the sky. For altitudes from 90 to 30 degs, only 0.5-2kg are required. I installed a kind of hook at the bottom back, which can be used to hang ankle weights or a bungee cord. For the time being, I will use detachable ankle weights.

I also ordered pole clips (like in Highe's book) to pair the poles so that they align for assembling the UTA.

Regarding the paint, I used Rustins BLAB1000 1L Quick Dry Blackboard Paint - Black: https://www.amazon.co.uk/dp/B001OX9YAE/ref=pe_27063361_487055811_TE_3p_dp_1 (many coats).

[Highburymark]

Incredible Piero - as a refractor man most of the above is beyond me, but it looks like you did a beautiful job

[Piero]

Just now, Highburymark said:

Incredible Piero - as a refractor man most of the above is beyond me, but it looks like you did a beautiful job

Thanks Mark,

weather permitting, I will do a star test during the platinum jubilee.  

[faulksy]

awesome piero. you could use that tak as a counter weight 🤣

[Stu]

Great job @Piero, the scope looks really nice now. Hopefully the results will be as good, I’m sure they will after all your hard work.

I have a 16” scope which is built to be quite light and require quite a lot of counter balance weight. It’s actually a really good approach, because it is much more manageable to move around.

[Piero]

12 minutes ago, faulksy said:

awesome piero. you could use that tak as a counter weight 🤣

It would be a very expensive one! 

[faulksy]

my old 18" obsession was very light to move which ment counter weight to the mirror box. it is a good idea. it came with tubes you filled with sand 😃

[Stu]

7 minutes ago, faulksy said:

my old 18" obsession was very light to move which ment counter weight to the mirror box. it is a good idea. it came with tubes you filled with sand 😃

Mine has a mix of stainless steel tubes and some filled with metal (lead?) shot. They clip under the mirror cell very easily.

[Piero]

7 minutes ago, Stu said:

Great job @Piero, the scope looks really nice now. Hopefully the results will be as good, I’m sure they will after all your hard work.

I have a 16” scope which is built to be quite light and require quite a lot of counter balance weight. It’s actually a really good approach, because it is much more manageable to move around.

Thanks @Stu 🙂

In my opinion, the best way to sort out balancing issues is to use larger trunnions. However, depending on the telescope, one might have to compromise. Therefore there isn't a single solution here.

I did consider redesigning the trunnions as the current ones are a bit small (15" diameter, whereas 18"-20" would be more appropriate even for a classic design). Having said this, larger trunnions also increase the weight.. my current ones are 440g each!

How much weight do you need to add to balance the telescope?

[Piero]

For the time being I hang them like this:

I never observe at the zenith so, the weights never touch the ground and always stay behind the MC (so less weight to use).

I might replace the weights with a bungee cord at some point in the future. For the time being this solution works fine. 🙂

[Piero]

Last night the redesigned telescope had its first light!

I observed from 11.30PM to 2AM. The sky was still quite bright due to the long daylight in these days.

The telescope is considerably more manageable due to the more shallow shape and reduced weight.

The views exceeded my expectations. The amount of spherical aberration on stars that I used to see, was reduced to a negligible level (e.g. it is similar to my 4" Tak or 16" dob). Despite the bright-ish skies, it was possible to see that kind of `star dust` background when scanning Cygnus and in general many faint stars were detectable. I never had views like that with this telescope before these changes. The quality of star shapes and the ability of detecting faint stars is now in line with my other telescopes. Considering the major issues of the original mirror cell (e.g. incorrect lateral supports, non-isometric triangles, COG of triangles not where they were supposed to be, and much more... - see summary: https://stargazerslounge.com/topic/337091-lukehurst-nichol-classic-dobsonian-modifications/?do=findComment&comment=4249464 )The secondary mirror does not have issues, but it was glued to the secondary holder by the original maker. Gluing secondary mirrors to holders can cause issues (e.g. astigmatism). There was no trace of issues in the views, but it is too early to say this as the secondary mirror was glued in May-June 2019 when the temperatures were similar to those in these days. In the winter the cold temperatures will shrink the glass of the secondary and the silicon. Therefore, there is a chance that the secondary mirror figure will warp. If that happens, I might have to replace the holder.

 

Mechanically, the telescope was a joy to play with. The axial movements were fine in the original telescopes and for this reason I did not change them. The general feeling was to handle something light but sturdy. 

The new Antares FOCH helical focuser worked well. I am not a great fan of helical focusers, but my plan was / is to use this telescope with parfocal TeleVue eyepieces (24 Pan and Nagler T6). Because of this, the helical focuser works very well.

2 kg detachable counterweights at the back balance the telescope at all altitudes.

Here are some photos:

 

 

 

 

 

Edited June 18, 2022 by Piero

[Captain Scarlet]

Very nice and the problem-solving must be satisfying!

To truly earn the name Phoenix of course, the original needs to have been set on fire 🙂.

I will be gratefully drawing on your experience when I, very soon now I hope, start building my own 20” dob. I’ve had the mirrors (Nichol) for a year now, and now my house is sold I have the time and the finances!

Magnus