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16 inch f5 with Nichol Optics.


fwm891
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Not been in this section for a quite while but I now have a project in hand. I've ordered a mirror set from John Nichol (16" f5) 411 OD x 44(ish)mm thick with a focal length of 2032mm.

Nothing to challenging in the design it's for visual use on an EQ platform, possibly some iamging but that's not the main intention.

Mirror support cell will be an 18 point structure, the optical layout and support has been worked through with "Newt for the Web" and Plop respectively. Mirror box etc have been designed around these constraints.

The drawing as it stands shows the layout (horizontally) across the top half with plan views of the mirror box and rocker box bellow. The upper cage is still a work in progress but show the intended angle of the output cone (45°). The focuser is a Baader Diamond SteelTrack NT (I have this already) this will be mounted on a plywood panel between the upper cage rings. I intend this to be a "floating" arrangement with enough movement to allow the final focal plane's position to be optimised. The secondary support will also be adjustable axially to enable it to sit directly under the focuser.

Mirror support main frame will be made from aluminium (HE30) box section 1.5" x 1" with 16swg wall thickness (1.6mm) with inserts where box sections meet to allow parts to be bolted together with gusset plates for ridgidity. Brass bushing will be used for any pivot points. The whole cell will be adjustable for collimation from the front. The mirror box arrangement can be seen in drawing 2. The mirror edge support will be a pair of pivoted arms with pads.

That's where I am at the moment. I'll order the aluminium so I can make a start on the mirror support construction. Marine grade Birch plywood will be used for all the 'timber' parts.

I intend to use 8 poles in the usual triangles at each side.

TBC

Optical-layout-16-f5_Drg-01.jpg

Optical-layout-16-f5_Drg-02.jpg

Focuser-mounting-panel.jpg

16_f5_Newt_dims.jpg

Edited by fwm891
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Well John Nichol emailed me this mornng to say the mirror is now finished and about to go off for coating with an estimate of 5 weeks for that. John has also given me a final focal length and blank thickness so I'll update drawings to suit the new data.

I've also had an aluminium delivery ready to make the support framework. I'll order some 18mm marine grade Birch plywood for the mirror and rocker boxes. I already have some 12mm for the upper cage rings and mirror cover.

Getting excited now...

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It looks like it's going to be a beauty.  16-inches is where a telescope is getting big - and you'll have some dark Herefordshire skies to exploit.

Are you making your cell or have you bought that in (and apols if I missed that in your post). 

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@MarkRadice I'll make all the parts with the exception of the mirrors and focuser (Baader Diamond SteelTrack NT)

I've been looking at the secondary mounting and come up with this based on a square aluminium box section, 4 thin spider vanes are clamped to the exterior of the box section using 3mm stainless cap head machine screws with washers and nuts.

There's a fixed plate held inside the box section with an adjustable (tip & tilt) plate. A backing plate just smaller than the mirror is fixed to the tilt plate and the mirror fixed to that with adhesive pads. Not shown: the mirror will have a safety wire(s) to prevent accidental damage should the pads fail.

 

Secondary-mounting.jpg

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Posted (edited)
1 hour ago, CraigT82 said:

What a scope this will be! Great drawings too. 72mm seems small for the secondary, is that right? 

It will be 75mm more than fully illuminates any eyepieces I'm likely to use with it. If you look at the Newt for the web screen grab above (top right panel) you'll see with the secondary at 72 mm there's no vignetting. 72mm was the figure that just gave me those results. So with mine at 75mm I have a little cushion.

Edited by fwm891
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Heres an update on the drawing of the side view. A second focuser can be seen at the end this  purely to show what's hidden behind the upper cage.

 

 

16_f5_Newt_ update 2.jpg

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Posted (edited)

I've spent the morning first printing off some of the drawings as A4 sections to build up a full scale drawing which I've taped to an odd board in the workshop. This is where I found suddenly everything is bigger than I thought !!

Then to work on roughing out some of the parts. All will probably need some fine tuning but they're pretty close. I made a decision to increase the size of the gusset plates to allow more fixing at the main frame junctions.

I used a bandsaw to rough cut pieces just over length, then used a disc sander to clean-up back to the lines. Bits get very hot when using a disc sander !!!

Images show the parts cut so far layed out on the plan.

 

Metalwork-01.jpg

Metalwork-02.jpg

Edited by fwm891
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Posted (edited)
On 30/06/2021 at 22:18, fwm891 said:

It will be 75mm more than fully illuminates any eyepieces I'm likely to use with it. If you look at the Newt for the web screen grab above (top right panel) you'll see with the secondary at 72 mm there's no vignetting. 72mm was the figure that just gave me those results. So with mine at 75mm I have a little cushion.

I don't know what kind of eyepieces you will use with this telescope, but as far as I can see I think you are constraining yourself even with a 75mm. If I remember correctly, the baader diamond focuser is quite tall (63mm?) And you will need some back focus (1/2" at least) anyway.

In my opinion the main issue with a small-ish secondary is not really the fully illuminated field (FIF), but the fact that one adds a lot of constraints to the UTA design and collimation. And if all does not work very precisely, there is a chance that the secondary mirror does not reflect the whole primary mirror, which is like having a smaller primary. All these constraints can be relaxed with a slightly larger secondary. In your case, a 75mm secondary gives an obstruction of 18%. With a 82mm secondary, you get 20% obstruction. I really doubt you will notice any difference, but it will make a substantial difference in the design. It will also give you a bit more than ~7mm FIF. Generally, one should aim for 1/2" ~ 12.7mm.

Also, in one of the images you posted, it says that you plan to have tube thickness of 3mm. That adds a lot of weight but not much stiffness. What really increases stiffness is the tube diameter, not much the tube thickness. Obsession telescopes use 1.2mm thickness (6061 alu). In the UK, I haven't found aluminium tubes 6082T of 1.2mm thickness, but you can certainly find tubes of 1.6mm thickness. The poles of my 16" f4 are 1.125" diam and 1.6mm wall thick. The struts in the UTA are 1" diameter and 1.6mm wall thickness. Both 6082T aluminium (structural aluminium). Looking at the design of your telescope, it seems to me that you will have longer poles. I would use 8 poles of 1.25" diameter and 1.6mm wall thickness.

A "+" orientation of the UTA spider / struts is stiffer than a "x". This because the poles of the telescope directly supports the UTA 4 struts, which directly support the secondary mirror. Here the rings only keep the UTA together, but don't really support extra weight apart from the focuser and finder boards. In the "x" design, which derives from the Obsession telescope design, the secondary mirror and spider are supported by the rings, and that can affect collimation at different altitudes (to minimise this, Obsession telescopes uses obsessively undersized secondary mirrors and UTA ID). The "+" orientation also allows you to place the focuser at a certain angle (e.g. 15-20 deg), rather than 0 deg.

The pole location slot in your design frees the pole when this is moved towards the edge of the mirror box. However a couple of poles form a triangle, so the weight of the UTA exerts a force in that direction. Having a pole location slot freeing the pole when this is moved inward would be safer, just in case that the hand nut loosens. Said this, for the lower truss attachments of my dob I used a positive lock.

Edited by Piero
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Posted (edited)

@Piero Thanks for the comments, I like your metalwork pieces you've posted, looks quite robust.

The only 3mm dimmension posted is within the Newt-for-the-web screen grab and that is there purely because the program required a wall thickness for it to run. As this is a truss tube dob there is no solid wall so the 3mm is essentially meaningless. The truss poles are 16 swg or 1.6mm wall thickness and 22.25mm OD which I'm sure you will feel is under sized.

The drawings I've posted have drawn for my reference and are evolving as I flesh out various details in different areas whilst keeping with the overall form of the scope. The upper cage being one area that is currently being fleshed out. I've always intended that the eyepiece will sit at at 45° angle to make life a little easier when viewing at lower angles, and also easier when viewing near the zenith by avoiding having to lean over the scope. They have been drawn that way to keep thing simple thus avoiding having to show a circle as an ellipse etc.

The upper and lower truss pole seats/anchors are based around the use of register pins to avoid location errors and slippage not shown in any of the current drawings..

 

I've updated the Newt-for-the-web dimmensions sheet to reflect the 75mm secondary changes.

I'll update the build images as things progress

 

16_f5_Newt_ update 3.jpg

16_f5_Newt_dims_75mmSec.jpg

Edited by fwm891
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Next job on the list:

The pivot bar, pivot shaft and triangles.

I've already roughed the triangles to shape. I now need to form a domed pocket to their undersides and a flat bottomed counter bore to the upper surface to allow a 2.5mm retaining screw to passs through the triangles, the domed pivot and into the pivot bar. Not shown will be a cap head screw under the innermost support pad, this will act as a retainer for an 'O' ring linking the 6 plates and keeping them aligned.

The pivot shaft will be turned from Hexagonal brass leaving a short hex section to act as a bolt head. The shaft will pass through the 1½" x ¾" x 16SWG support beam at 12mm OD, then down to 8mm OD to pass through the pivot bar. There will be a PTFE washer between the support beam and the pivot bar. Then a second PTFE washer and a cap washer and retaining machine screw ties the whole assembly together and allows side pressure to be adjusted so the pivot bar can rotate but not tilt sideways.

That should keep me busy for a while...

Triangle support assembly.jpg

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Posted (edited)

My initial thoughts on the axial adjustment of the spider to position the secondary under the focuser.

The drawings are not engineering style nore are they photos so some might find them confusing but they work for me.

The top left shows the end view into one of the 4 upper cage spacer tubes. The black fixed pressure plate is bonded to the inside of the tube (probably epoxy but may suppliment with m/c screws). The vane itself is held in the moving Vane anchor block with 4 m/c screws, there is a tension adjustment screw which passes vertically through the Vane anchor block to apply tension and radial positioning of the assembly.

The travel locking screw passes through a shaped 'nut', tightening the travel lock applies pressure via the screw to the Vane block and outwards against the tube wall. This will allow circa ±20mm to the whole assembly inside the upper cage.

Lower left is an end view of the secondary adjustment assembly giving tip/tilt adjustments via compression springs for collimation.

View on the right is my attempt to show the same from a different angle. The pale slot outline against the fixed pressure plate is the slot the Travel lock moves in.

TBC...

 

spider adjustment.jpg

Edited by fwm891
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Posted (edited)

Managed a bit more metalwork today. I've made the pivot shafts with a modification. I've made the pivot end of the shaft 10mm rather than the 8mm originally planned. Also given some of the ali parts a light clean-up with #320 W&D paper (dry).

The holes in the top of the box sections are for Allen key access.

Other mod: I've inserted a snug fitting piece of timber inside each of the aluminium Box sections: a) provide extra support where the pivot shaft goes through the ali box section b) help to deaden any vibration within the framework.

 

Metalwork-03.jpg

Metalwork-04.jpg

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Hello, nice project. I was wondering what you were using for your layout drawings? I am also using the Baader Steeltrak in my project, and could do with that dwg for my layout.

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18 minutes ago, Robert72 said:

Hello, nice project. I was wondering what you were using for your layout drawings? I am also using the Baader Steeltrak in my project, and could do with that dwg for my layout.

Hi Robert72, I've used Photoshop for all the drawings. I set scaling/resolution at 100 dpi which keeps the file sizes down. I tend to use layers a lot with these drawings and build things up by creating smaller drawings at the same resolution and transferring them to build up a full sized scope image. That way your working with smaller files making life easier all round. You can probably do the same with Gimp2 but I don't have any working knowledge of Gimp.

I hope that helps

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Coming along nicely. What are you going to mount the whiffle trees to? Was just looking at your drawings and couldn't quite tell. I was thinking they would be mounted to two points of the triangle but you have the triangle in the other orientation with one point facing down. 

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  • 2 weeks later...

Made a start on some of the woodwork for te mirror box. Back and sides are from 18mm birch plyoood, front baffle is 12mm birch plywood. There are 3 recesses cut for fans and other apertures acting as vents to allow general air movement and as exhausts when the fans are running. Corners have been dovetailed. At present everything is 'dry' fitted to allow easier access/assembly/disassembly.

Through holes have been cut for the brass collimation nuts, small recesses (3) near each collimation nut will locate compression springs. The compression springs should provide enough push to avoid needing locking screws.

More to follow...

Mirror box 1.jpg

Mirror box 2.jpg

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3 hours ago, Horwig said:

Really nice looking project, and nice looking drawings as well, what software do you use for the drawings?

Huw

Thanks Huw. I use Photoshop. I draw everything full scale but drop the resolution down to 100 dpi to keep file sizes manageable. I use multiple layers to build up individual parts or bring parts togtehr to form a complete instrument. It has a feature to auto align layers to vertical/horizontal centres or top/bottom, left/right edges then link or merge them together. The version I have is old now CS3 but for this type of project it's ideal.

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Thanks Francis, I had a feeling you were using photoshop, I'm impressed with your use of gradients etc to bring it all to life.

Looking forwards to seeing the scope come to life as well (naturally).

I had a 16 inch f3.75 from John back in 2011, and built a serrurier truss tube for it, it's quite a beast.

 

Best of luck with your build

 

Huw

 

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