Installation of RC12 optics into new carbon truss

[Paul Haese]

Today I decided to install the optics from the RC12 into the new truss that I have taken possession recently. I drove down the observatory and picked up the scope and brought it home for the disassembly and reassembly.

When you look at the truss you will see the rear of the scope has 4 truss blocks screwed onto back plate. Unscrew the screws as shown in the first image. Remember the tension of the screws as you loosen them. (pic 1)

Next unscrew the bolts that hold the dove tails onto the back plate. There are 4 bolts to remove. (pic 2)

Once you have done that the rear assembly including the mirror assembly come away from the truss. (pic 3)

If you intend on using a complete mirror cell from another scope remove the mirror cell by unscrewing the bolts that hold the mirror in place. Those 3 bolts are located in the middle of the mirror back and are used to collimate the primary. The large screws are the ones you want to unscrew. Not the smaller locking screws. (pic 4)

Now remove the old mirror cell from the ali tube of the RC12. (pic 5)

Pull the whole assembly out and be careful of the baffle which is really long. (pic 6)

Remove the mirror cell from the outer casing by undoing the bolts that are used for collimation on that scope. (pic 7)

Sit the new back assembly on a flat surface and then sit the mirror cell with mirror and baffle on top of the new back assembly. (pic 8)

[Paul Haese]

Now make sure to install not only the bolts but the springs that provide tension to make the adjustments to the primary. (pic 9)

Install all three bolts and three springs. Now the primary mirror cell is installed onto the rear assembly. (pic 10)

Now install the out casing (black painted shield). Make sure it fits into the slotted ring in the rear plate. (pic 11)

Next fit the truss back onto the rear assembly and shield. Look out for the slotted ring on the middle support plate. The shield fits into that. (pic 12) Screw all the bolts that hold the truss blocks and the dovetails back into position.

Now remove the secondary assembly from the old scope. Undo the collimation bolts first and then remove the centre bolts with a Phillips screw driver. (pic 13)

Install the secondary onto the truss in the reverse fashion to the removal from the old scope. (pic 14)

Now it is time to collimate the scope. You will need a takahashi collimation scope to perform this task. If you own one of these scopes it is best to have a collimation scope anyway. (pic 15). The trick with this is to roughly centre the primary by using the large screws. Then work on moving the secondary so that the centre spot is centred around the tak spot. Then go back to the primary and get that centred. This needs to be repeated several times until you have it all concentric. It should take you about 15-20 minutes at most. I recommend you take a look at Ken Crawfords site for a video explanation of the procedure.

Pic 16 shows a view through the tak scope of the secondary getting close to being centred.

[Paul Haese]

Once you are totally happy with the collimation with the tak scope (take your time with this, the better you get this the easy the final collimation via star test will be) if you have a laser collimator check the collimation with that. (pic 17). If you have done the first lot of collimation right you will get concentric circles. If not go back to the tak scope and keep going.


Finally, I decided to fit a dew heater to the secondary. Inside secondary assembly there is enough space to fit the dew heater made by Kendrick. I drilled a hole in the holding plate and then ran the wiring through that. Some tape of silicone over the hole holds the wiring in place. Make sure you silicone stick the heater to the secondary mirror itself. (pic 18)

Then tape or fix the wiring neatly along the truss. (pic 19)

So there you go, that is how to remove one set of mirrors and install those into a carbon truss.

Feel free to ask me any questions you may have.

[allan gould]

Nice set of instructions Paul. Very concise and informative and I hope the scope now lives up to your expectations.
allan

[strongmanmike]

Yeah good luck with it Paul, certainly looks the part :thumbsup:

Mike

[Paul Haese]

Thanks guys. It looks like a real scope now. Very sexy, to which my wife looked at me and shook her head last night. I think she is beginning to wonder. :lol:

[SkyViking]

Nice, looking forward to first light! :D

[peter_4059]

Looks good - where did the truss kit come from?

[Paul Haese]

Hi Peter I bought it through Bintel which was organised with GSO.

[Paul Haese]

Scope installed in the dome. Man that looks sweet. I checked collimation with the Tak scope after an 88km drive and it was still perfect.

Star testing now.

[Paul Haese]

Yes this will be interesting Rolf. I am hoping that I can finalise any further glitches in the coming weeks and then off imaging each clear night.

I noted that the scope was slightly heavier at the front end than before. I was surprised at this as I had expected it to be heavier at the back end. The scope had to be moved further down the versa plate to compensate.

[RickS]

That does look very cool, Paul!

[Paul Haese]

Wouldn't you know it, we had 40mm in Adelaide today which is a new record for February and tomorrow is supposed to be somewhere between 60-100mm. Yep astronomers curse. :rofl::rofl: Drought averted. Haese buys new truss and heavens open up.

[White Rabbit]

They do look very sweet, I wonder if they have a 10" truss planned?

[allan gould]

They have 10", 12", 16" and 20" truss scopes planned
Allan

[rat156]

My wife stopped wondering ages ago, she's now convinced I'm crazy.

New scope is looking good Paul, can't wait for first light.

Cheers
Stuart

[Marke]

Paul excellent write up what was the final weight like compared to the original ?

[Paul Haese]

Hi Mark, the final weight is 23.7kg for OTA alone.

[Logieberra]

Beautiful. Looks like an iTelescope installation :)

[Satchmo]

So it is heavier than the steel tube version ?

[alan meehan]

Nice scope Paul ,good write up as well Bintel now has these in stock I
have been looking at the 10" ,also Andrews has these coming in march will be interested to see some shots taken when you get rid of the rain .

[Paul Haese]

It's got to behave like one soon. :P Sorting focus again is a pain.

[dpastern]

Paul - do you think a 10" RC truss would be usable with a EQ6 Pro? I expect not due to weight/stress, but thought I'd ask. Not that I'm in a position to buy anything now. Hell, I haven't used my current setup in 3 years due to bad back and laziness and a lack of a permanent observatory and also poor skllls on my part.

Dave

[Star Hunter]

testing scopes is good for the soul..

[Paul Haese]

Just a further update-

Luckily over the last couple of months I had to buy three adapters from the camera to the FLI Atlas focusor. One was 21mm, one was 27mm and one was 31.5mm. Now to remind others, I had to buy these as the ray trace diagram that was sent to me from GSO is not right. It says the back focus on the old tube is 281mm, when in fact it was not near that. It turned out to be something like 290mm. So I had to guess for the first two and then the final adapter got it right.

Now funnily enough the second adapter worked out right with the smaller adapters (mirror back to focusor) that GSO supply on their scopes.

Now for the sake of others not having to do the same stupid experiment I can say that it appears that the back focus on the new truss is 270-272mm. This assumes that the primary and secondary are the same distance apart (I used my mirrors from a Tubed RC12 from GSO) and that the primary adjustment screws are half way in. On my scope it is 271.84mm from the mirror back to the sensor.

Once I could collimate the scope I could collimate it too. See attached image. It needs a little tweek but it is pretty close to the mark. I will do some test images to see how sharp this collimation is compared to the previous one where I shot the meat hook.



It looks that way Mark.



I would not like to say. Although there is not going to be much swing arm due to the truss. My opinion is maybe. Best bet though is to ask Michael from Bintel and see what he says.




Yes, probably but I want to get on with imaging Jim. ;)

[Satchmo]

I wondered whether you are getting some vignetting of your primary aperture there Paul - your defocussed star image suggests you have a 65% obstruction .

[Paul Haese]

I am not sure. In the Tak scope I can see the mirror clips on the side of the primary. And; the secondary baffle does not appear to be causing a huge obstruction in the Tak scope. Mathematically the secondary is 5mm further away in this scope too. So if anything it should present less of an obstruction really??

On another topic; star shapes in the top left of the full image are not right too. There appears to be some tilt with elongated stars and the appearance of coma (yes I know RC's don't have coma, but do have field curvature). Star shapes else where seem consistent with this design of scope. So I might have a tilt problem or some other issue going on. I will do some more testing over the coming nights to see what results I get.

Now that the scope is collimated I have done a pointing run on the scope. I did a Tpoint model of 252 points and ended up with the results in the image attached. Near perfect pointing. I did several test shots after that and each time the selected object was dead centre of the cross hairs or just vaguely off.

[Satchmo]

If that is so then the only other thing that could cause an abnormally large secondary shadow is a correction error. You mention a 5mm change of mirror spacing in the new incarnation. I don't know how tight these spacings need to be held to remain with good correction. A star image racked the same amount in mm the other side of focus would show if the abnormally large secondary shadow is a correction error or a vignetting issue. If it is a vignetting issue the appearance will stay the same on the other side of focus.

[bratislav]

RC's don't have coma, but only if secondary's vertex is exactly on primary's axis. Miss that by only a little, other aberrations creep in, including high order coma.
Problem with those RCs is that you have no way of confirming that; all your collimation devices rely on the focuser - none are aware where the PRIMARY points at.
One way to get this in check is to remove secondary and then define focuser's axis by laser collimator. Place a small ball bearing at ROC of primary (they works well when hit by a laser as a light source - but be careful with scattered laser light! use low power lasers). White piece of paper behind the small steel ball will help a lot with finding the reflection. Tilt the primary via its push pull screws until source's image centers on itself (making sure primary is perfectly colinear with laser beam). After that, lock the primary's screws and finish the collimation only by working on secondary.
Needless to say, focuser (laser) axis must go directly through the center hole of the secondary holder at step 1. If it doesn't, align the focuser first to match (or secondary's lateral position via spider vanes).
Also, needles to say laser MUST be perfectly collimated. Spin it in the chuck or on V blocks to be sure it has no tilt at all.

That is why I love those CDKs with spherical secondary (and that is why am making one of those rather than an RC).

BTW, those RC's really need a dedicated flattener to work with large chips.

BTW2 - the BFL depends HUGELY on secondary's distance. Every millimeter there gets multiplied by 7 to 10 times (not sure what is secondary magnification in those RCs).

Good luck,
Bratislav

[multiweb]

The HOTECH ACT (http://www.hotechusa.com/category-s/23.htm)will allow you to get the inititial Primary tilt in respect to the mechanical axis of the tube/truss then you can center your secondary accordingly and confirm with the howie glatter at the back.

[Paul Haese]

I will check the infocus view tonight if it is clear. That should tell me what is going on. One image at infocus and one at out of focus should give a good view of the secondary. Using the same star will provide accurate results. I will get back here. I have not seen this with this problem with this mirror set before though. When it was in the ali tube the secondary shadow appeared ok to me.

[bratislav]

They surely will; but that is a lot of money for essentially one-off use.


< edit: just watched the video, they have packed into that ACT a lot more than I initially thought. Well designed and top marks for explaining video!
Still expensive though >

[Paul Haese]

Don't know if you missed this but the mirror set came out of an an Ali tube RC from GSO. It had performed well in the past. I made sure I put the mirrors back in the same orientation as they came out. I marked the orientation prior to coming out of the old tube.

I don't think it is really much of a collimation error but there could still be some present. The primary could well be out a little, I did use a Tak scope to set it properly though so it was square to the optical line. Mind you I was using a feather touch focusor rather than my Atlas as I don't have an appropriate adapter for the Atlas. That would give better results but I doubt there is much difference between both focusors. Thought I admit there could be some error there.

Your tip on primary centering is a good one. I will investigate this further. I have a couple of steel ball bearings here somewhere.

On this mirror set the STXL11K field of view shows field curvature near the very outer reaches of the image. It is problematic with GSO not to have a dedicated flattener. Having a dedicated flattener would make all this easier to work on too.

The Atlas focusor does not appear to have any capacity to adjust any tilt. Though it does have a zero tilt control system on the actual connections and all other components such as the rotator are on v flange with multiple screw points. I will check these carefully with a set of feeler gauges next time I am down.

[bratislav]

Indeed, shame they don't provide flatteners. Those are simple to design and make (and being all BK7 should be cheap too!).
I only have data for 10 inch RC (from Rohr's measurements) and scope can barely cover 25mm circle (after that things go south very fast). A simple two lens flattener gives you a diffraction limited circle of nearly 60mm across (black circle is Airy disc).

[clive milne]

At the risk of flogging a dead horse... to reiterate my previous comments; the structural & geometric compromise required to employ a carbon fibre truss (in an ota) largely (or completely) offsets any weight savings that one might hope to achieve compared to a properly engineered steel truss but at significantly higher cost. Another advantage often attributed to the combination of carbon fibre and zero expansion optics as their sole domain (albeit erroneously) is stability of focus. You can actually achieve the same result with plate glass optics and OTA's incorporating highly expansive materials (like aluminium) if you design the system such that the errors induced by the collective cancel out to zero.... have a look at the cal poly 18" as an example of an intelligent telescope design employing basic materials on the whole and using exotics where there is a performance/$ advantage.




Yes, and it might also be instructive to others to qualify (or quantify) that statement: Coma induced by virtue of the secondary apex being non-coincident with the optical axis of the primary is not (meaningfully) influenced by the field angle. In layman's terms that means in such a situation the coma will be uniform in magnitude to all intents and purposes across the entire field. Perhaps a useful factoid to keep in mind for those using CCD images in an attempt to analyse mechanical errors.



To the best of my recollection it would set a precedent to see me disagree with you concerning optical theory.... and today is not that day. However, whilst agreeing with you in principle there is a caveat that must also be included in the above statement to be fair. The centration tolerance for the optics in CDK's is even less forgiving than that of a functionally equivalent RC. The distinction here is that wrt the secondary, it is the (strong) aspheric component figured onto to the surface of the mirror which MUST be axially aligned with the primary optical axis. The DK secondary is rotationally symmetric so it can be mechanically displaced 10mm from the optical axis and have zero impact on the PSF so long as it is realigned with the tilt tip adjustments typical in secondary mirror holders.

Anyway, minor concerns in the scheme of things.
I do wish you would post more often. Your wisdom, insight and knowledge are sorely missed by those who remember your contributions on the old ATM list.
best,
c.

[SkyViking]

You make an excellent point. Carbon trusses are everywhere these days but I don't really see a need for them in typical amateur instruments. I think they are more of a gimmick because they look nice.
When I did my 12.5" f/4 Serrurier truss I did consider using carbon truss tubes but could not justify the extra expense for no apparent gain. It would also have made my OTA heavier because I would have had to add proper fittings to each tube end.

Instead I built the OTA from birch plywood and aluminium tubes. And I can confirm what you say, there is literally zero focus shift throughout the night. I carefully inspect each subframe afterwards, so can say that with certainty.
If that still holds for, say, a 20" made of the same materials I don't know. But at this scale any expansion is evidently within the focus tolerance and not measurable.
The materials for my OTA (excl. optics and focuser) cost around $300 so that's decent value for money :)

[clive milne]

Rolf, your rendition of 5128 is proof of the pudding.
That image btw, has been used as my desk top image since your last repro.
Imho, you set a new standard right there.

[Paul Haese]

Well focus last night was maintained the best I have had with these optics. I focused twice during the night and there was little if any Shift in focus. So in this instance the carbon truss does hold focus better.

Now in terms of it being heavier, I am only going on the specs as shown on Bintels Site. It was certainly easier to lift it into position than the tube but I cannot say with absolute clarity it was lighter in weight.

The use of a truss also is pretty obvious too. Tube currents are the bane of most imaging systems and this allows good egress for discharged energy to escape. The old tube would take forever to settle down.

I seem to have collimation better overall, images taken last night looks sharper than previous images taken.

[multiweb]

Regarding CF vs. aluminium I've always had less focus shift with temperature drop with CF tubes. I'd assume the longer the length of aluminium the worse the focus shift would be.

[strongmanmike]

That's very interesting (and good :thumbsup:) to hear Rolf. I need to tweak focus less with the AG12 (CF Tube) than I did with the Starfire (Al Tube)...night before last I did a 2hr image run up to the zenith (object in the perfect spot = didn't have to move the dome slit :cool:!!!) with no noticeable change in focus ~ 1120mmFL at F3.8, subs just got slightly sharper as the object climbed toward the zenith and the seeing improved :thumbsup:.

Mike

[SkyViking]

My experience is identical Mike, the FWHM decreases steadily as the object rises towards the Zenith. :thumbsup: At least we can say any focus shift is so little that FWHM doesn't increase :lol:

When it comes to truss versus tube OTA's I believe trusses are a lot more rigid. I would think even a CF tube sags a little bit under gravity, and in that case that's perhaps what causes focus shift rather than any temperature induced changes?

Of course a truss OTA will also sag to a degree, but I think it is less because of its stiffness. And in particular a Serrurier truss is designed with two truss segments to cancel out any sagging. While my OTA is not optimised with different truss tube thicknesses for each segment, I imagine it still benefits from this principle to some extent. Hence why it appears to need no focus adjustment throughout an imaging session.
But when that is said, even a fast Newtonian is more forgiving than the more complex optical designs. I don't know how this homemade OTA would fare if it housed a Cassegrain type system.

[Logieberra]

Paul, re the secondary mirror and housing on the 12" GSOs, would you please confirm how the mirror is attached. Is it fixed, stuck flat up against the housing? If so, that might explain your tip that the secondary collimation on these should be 'finger tight'. Cheers

[Logieberra]

All good. Took it apart. The 3x secondary colli screws are well isolated from the secondary unit and mirror itself. Good to know. Tighten away :)

[Paul Haese]

I have never removed the secondary glass from its holder. I have removed the secondary from the secondary assembly via the threaded support before to install the dew heater into the secondary assembly. So I would not have been much help.