8" Homemade planetary dob - The Project

[earth titan]

Just to set the scene, I have decided to build myself an 8" planetary dob from scratch. This is my first foray into such a project and the post will take many months (I have no doubt).

The project is mainly for interest sake, as I know it's not going to save me money, but mostly cos I can get the spec I would like. But mainly, and lets be honest, when the telly listing consists of 'so you think you can dance', 'my big fat gypsy wedding' or 'the only way is Essex', the alternatives generally involve self harm!

This post will therefore be my diary of what I'm doing, so others can learn from my mistakes (I'm sure there will be plenty) - thus making them wise among men. It will also contain my pleas for help and assistance, of which there have already been many so please feel free to add your two pence worth but please do not be offended if I choose to ignore all the advice given.

So here we go.

The spec:

• Planetary scope
  
• Forgiving optics
  
• 8” f7
  
• Dob mounted
  
• Good levels of contrast
  
• Reasonable light gathering
  
• Relatively portable but designed to be used at home
  
• Open design to reduce thermal problems
  
• As much homemade as possible
  

The sequence: (this will change I have no doubt)

• Build test equipment
  
• Order mirror components
  
• Grind and figure mirrors
  
• Build OTA and mirror cell
  
• Silver primary and buy secondary
  
• Fit mirrors
  

A big thanks firstly to Glasspusher (John) for words of assistance and encouragement so far and Ponytale for the Ronchi gratings. Thanks guys!

[JAO]

Sounds and excellent project. Looking forward to updates as you go along. I always enjoy ATM threads.

Adrian

[callisto]

Sounds good

[earth titan]

Test equipment:

As I'm still deciding where to buy the mirror blanks from (looks as though Galvoptics is going to be my supplier of choice) and if I should go for a glass tool or make my own, I have knocked together some test equipment.

Please bare in mind these have been made from scavenged components, therefore not pretty, thus keepng the costs down.

The main materials were:

• 3/4" mdf and a bit of fibreboard sheeting
  
• Copper plumbing pipe and pipe clips
  
• M10 stud left over from a building project now completed
  
• Toggle switch from a now scrapped work test box
  
• Old PET milk bottles cut up to provide the non slip surface
  
• Slide carriers for the knife edge and Ronchi gratings
  
• Ronchi gratings kindly donated by Ponytale
  

Need:

• DTI - currently got feelers out for a 'recycled' one
  

Welcome comments and thoughts for suitability.

Questions:

• Will the LED be bright enough? I have initially stuck a normal green 5mm 5v one in as I had some lying around. Should I go for superbright though? The difference in output is staggering, but will I need such power?
  

Foucault test

• Still confused as to how the test will work. The design I have copied(ish) involves raising and lowering the 'saddle' - which moves the light source and knife edge relative to the mirror but the two stay the same relative to each other by this movement. Is this right?
  
• Does the viewing take place at the side of the test rig and not through the window?
  

Ronchi test

• I'm presuming the LED shines through the grating during the test?
  
• I presume you look through the grating when carrying out the tests?
  

[UTMonkey]

I love seeing how these things take shape. If someone offered to do a master class I would definitely want to have a crack at it

[Glasspusher]

Can't make out the detail too well with your ronchi tester. You need two holes, close together one above the other, the lower one should hold an led light source, the upper one is used to view the returning image. The ronchi grating is placed in front of the holes covering both off them. There is no need for over engineered test gear....simple can work very well. Good luck with the project, I looking forward to updates.

John

[earth titan]

Thanks John.

The LED is shielded and shines through the grating. I can then stand behind the grating and look through it above the LED. Will this suffice?

[Glasspusher]

OK... I see, yes that will be fine.

John

[ponytale]

Hi, ET, your tester is same type as mine , i did end up using a white superbright led 10mm x 12volt and used a 12 volt power supply , i also cut off the front of the led and sanded it to get a more diffuse light source, i have just about got my diy dob to the point where i can star test my polished mirror before i get it coated, i have been modding and remakeing it in my spare time for weeks, as i have decided to go back to australia to live, i have converted my tube dob to a collapsable truss tube type that i can get in a tea chest/ packing box , the idea is to have 2 mirrors that i can swap over and adjust scope length to suit f4.7 and a f6/ ish so the length will be from 1000mm to 1375 mm minus the eyepiece to secondary mirror distance approx 200mm so far so good will update and post pics in next few days

[Adamski]

Bit bizarre coming across someone from rushden, i spent my first yrs there and again in teens. both grannies lived there, one still lives on highfield rd where the scool used to be. I've got aunt and uncles and cousins there and my old man just down the rd in bedford. I was born in wellingborough.

Adamski

[FraserClarke]

• Will the LED be bright enough? I have initially stuck a normal green 5mm 5v one in as I had some lying around. Should I go for superbright though? The difference in output is staggering, but will I need such power?
  

I have a normal LED in my tester, and it is more than bright enough for testing -- especially if you can do this in a darkened room (doesn't need to be pitch black). However, I will at some point modify the tester to include a super-bright for alignment purposes. When I'm lining things up initially, I find it very useful to have an LED bright enough to see the reflected image on the tester so that you can adjust it to the right place. I'm currently just using a hand-held LED torch for this, which works but is a bit less convenient.

Foucault test

• Still confused as to how the test will work. The design I have copied(ish) involves raising and lowering the 'saddle' - which moves the light source and knife edge relative to the mirror but the two stay the same relative to each other by this movement. Is this right?
  
• Does the viewing take place at the side of the test rig and not through the window?
  

It's basically the same as the Ronchi test, and the knife edge is a single 'line'. So you need to put your eye behind the knife edge, and as close to it as possible. I find 10-20mm works for me.

Your set-up seems fine. The knife edge needs to move perpendicular to the optical axis of the mirror so that it can cut the beam. The source and KE can move together. There is a slight difference between how you interpret the results in the final stages of testing, but that's a subtlety for now...

[earth titan]

Looks like the mirror blank request posted on the wanted forum has been a success. A forum member has sold me an old kit, with a blank which has had a little rough grinding done (certainly much less than my intended sagita) of 8.75" diameter for a very reasonable price.

Comes with a tool and a selection of abrasives and some pitch. Limited amounts of course grit included but even if I have to buy some 80 grit, some fresh pitch and some cerium oxide, it has still been worth a punt for the price.

Will post details when it arrives.

[earth titan]

I've swapped over the LED, tidied up the wiring and fitted an adjustment screw. For measurements I have bought some vernier calipers, as these are more versatile than a DTI for other measurments.

It now has a green super bright LED, with a filed flat front to diffuse the light. The only LEDs Maplins sell are 5v, so the system has a 133 ohm resistor in series to run at the right voltage.

Will post some pics later, when I have five minutes from fitting the new kitchen.

[Trull]

I'd like to offer a suggestion, as you are interested in planetary, and grinding your own mirror, why not step up to a Chiefspiegler design? That way you only grind to f/10, use no spider on your secondary mirror by tilting the primary, then reduce the height of the focusser. Imagine the lack of diffraction spikes (nothing in the light path) and getting refractor levels of contrast (no obstructions)... tempted you? Worst case scenario, you could build a folded Newtonian and still get that nice long focal length.

[earth titan]

Did consider these. Like the idea of clear unobstructed views, but without all the corrective lenses it seems thay have loads of astygmatism. Possibly beyond my first scope build - but hey - why not a future design?

[Trull]

That's fair enough, for an f/10 design you can use stock lenses from Newport Optics (which would add c. 150quid to the cost) or sometimes get a bargain from Surplus Shed. Enjoy your build - my TorcDobIV still lives (8-3/4" f7.3), and is getting converted to Chiefspiegler for TorcDobV, once I've sussed out grinding a lens.

[earth titan]

Tester all completed.

Finished the LED wiring now I have decided on a superbright (hardly rocket science) but time is precious.

I have attached a basic wiring diagram (not an ISO standard I'm sure) for mine, but please note the size of the resistor will depend on the current and voltage of the battery and LED used. Mine is a superbright green from Maplin, all powered off a 9v battery.

The DTI is on its way (simpler to mount than the vernier) which should arrive this week.

[earth titan]

Mirror kit has now arrived (pics of mirror and tool below) - a pyrex 220mm diameter which is sweet for what I need. A little bit of grinding has been done, but not enough to make much of a difference, and it came with some grits and a tool.

As the blank is Pyrex, it has some bubbles on the surface which will break through and one of them (on the surface which will probably be the mirror face), is quite large and deep. This face already has a curve on it of around 3mm (part of the moulding process) which can be seen in the attached photo. The bubble is then a further 2mm depth and around 2mm to 3mm diameter. To make this the mirror face would leave me sizeable bubble, near the middle unless I grind it out.

The other face, already flat, has a more minor defect near the edge which will possibly grind out.

My dilemna then, is this (advice welcomed):

1. Do I grind the curved face and hope to grind through the bubble. I'm going to have to flatten it anyway to get to the 2mm sagita for my F7 scope but this is a lot of grinding.

2. Grind this face flat and use as the rear of the mirror, using the other face as the mirror front.

3. Ignore the bubble and carry on.

I'm tending toward option 1, despite the extra hard work as this will make a better mirror but does the rear of the mirror need to be flat? If it doesn't then I'll probably go for the other surface as there is a lot less work.

[FraserClarke]

o. The bubble is then a further 2mm depth and around 2mm to 3mm diameter. To make this the mirror face would leave me sizeable bubble, near the middle unless I grind it out.

How near the middle? Near enough to be under the shadow of the secondary and therefore not a problem??? Probably too hopeful If it is elsewhere on the mirror, it won't actually have that much effect if you just mask it out (paint it black) after coating. During grinding though, you would have to make sure the edge doesn't chip.

Given you're only just starting though, I'd grind it out completely at this stage. Probably won't take you as long as you fear -- maybe another couple of hours of 80-grit.

If one of the other bubbles breaks through near the end of grinding, it might be worth just leaving it and masking it later...

[earth titan]

Would be nice if it was blocked by the secondary. That was my first thought.

I think I will try to grind it out. Will make a better mirror in the long term.

[earth titan]

Decisions, decisions

The blank and tool arrived earlier in the week and I have been examining them closely to see which side to use. Being pyrex, there are a number of bubbles, including one large one on the surface of one side. On close examination this bubble is around 2mm deep and wouldn't be blanked out by the central obstruction from the secondary. There are a number of other bubbles on this side as well and to further complicate matters, this face already has a 3mm concave surface from the moulding process (see first photo below). This means a lot of glass would need to come off the remove the bubble as the planned sagita is only just 2mm. Lot of glass!

Decision made then. Grind the 'bubbly' side flat (leaving the bubble where it is) and work the mirror curve on the other side which is free of bubbles.

No real guidance in the Howard book I'm using as my main reference about how to grind the back of a mirror flat, except do it on a sheet of plate glass. Don't have one of those laying around (except the windows, and she who must be obeyed would not be happy) so looked for another way.

On the advice I've compiled from a number of websites and forums (including this one) I have determined the back can be flattened using (in this case) centre through centre strokes with tool on top.

The grind begins

Started off with the roughest grit tonight and spent about 3 hours grinding. The 3 mm depression is now 1.5mm and the rear is starting to show it has been worked nearly all over (see second photo below). I've lost the 2mm bevel I put on all round the edge so I must have shifted some glass - but it is tough work with lots more to remove.

From what I can gather, on such a thick blank (6:1 ish), there are no real optical reasons to get the rear 'flat' all over and that an amount of concavity (is that a real word?) is acceptable as long as the mirror sits level on a flat surface. I'm therefore content I could leave it here but as I'm looking to get the best result I can, I will have another go tomorrow night and try to get it down a bit further.

Best order some more grit though

Questions (please jump in at this point):

Any better techniques for flattening the back?

How flat, does it need to be?

Once flat, does it need a finer grind?

[ponytale]

hi ET , i ground my mirror backs near flat on 3/8" "steel plate "in very short order , i think the only reason the back is flat is so when grinding the face and rotating the glass you don't want the mirror wobbling or changing angles as you go round, also the retaining edge clips will be reasonably even , i have a similar small area on the back of my mirror with no problem as it is not near the edges. i personally would be happy with what you have there , others may do more for cosmetic reasons only,the beveled edges are more important to avoid chips.

[Glasspusher]

The back of the mirror is ground flat to prevent astigmatism arising on the front surface when it is being worked. When the front surface of the mirror is being ground/polished the pressure applied can result in the blank flexing. If the back is flat the flexure is uniform and astigmatism is not likely to occur. Remember, astigmatism is a condition where the mirror does do have a uniform figure of revolution. The back does not have to be perfectly flat, it can be slightly concave or convex. Concave is preferable so that the blank does not rock whilst being worked.

Just how far you should go when grinding the back is a good question, and you will get different answers from different people. I have seen professionally made mirrors with their backs milled flat with a diamond cup wheel and just left, others grind the milled back to varying degrees. With a thick blank like yours grinding with 80 or 120 grit will be fine.

John

[earth titan]

Thanks Guys.

I'll grind until it is uniformly ground all over (should be completed this evening, as long as I get the PAYE done on time). I shan't worry about flat, as it is slightly concave and doesn't wobble but would like to get it slightly better - just cos I can.

More grit ordered..

[Glasspusher]

Remember, astigmatism is a condition where the mirror does do have a uniform figure of revolution.

Oops, sorry it is of course does NOT

John