Captain Scarlet

15 minutes ago, John said:

The offset on many scopes is "built in" by offsetting the position that the secondary is attached to the mounting boss. So Magnus is replicating the approach that Skywatcher and others use.

Yes just so: it keeps the spider-boss totally central, and the spider vanes and diffraction spikes perpendicular

Number-plate tape (double-sided)...

In early November 2019 I ordered, from a popular European supplier, an expensive Hubble Optics 1/10 lambda 70mm secondary mirror, to replace the OEM one on my SW300p 12” newt. I ordered it in early November to ensure its arrival in time for my Christmas visit to Ireland. Ha! It was eventually delivered in early February 2020, having I guess been made to order in the US, shipped to Germany and shipped to Ireland.

Anyway, arrive it did, and I got around to starting this little project at the start of Irish lockdown, late March 2020.

Before embarking, I had of course somehow to work out precisely how, and more importantly where, to glue the mirror-holder onto the back of the mirror. Specifically I’m talking about “the offset”, and more specifically the lateral offset across from the eyepiece: the other “up-tube” offset sorts itself out in the collimation procedure. I needed to work out where to mark 4 lines on the back of the new mirror corresponding to the two sides and two ends of the 45-degree-angled face of the secondary holder.

Obviously, the two “sides” would each be symmetrical: half the holder’s diameter away from the long centre-line of the mirror. The two “ends” though would be shifted towards one end of the long axis. I needed to work out what this shift needed to be. Purely a formula for the offset itself was not especially useful. I needed formulae for how far from each end of the mirror I needed the end-points of the holder to be, which I’ve called L1 and L2 below, so that I could mark the mirror at those points and simply (carefully) plonk it down.

These calculated distances needed to take account of where along the primary’s axis I wanted the reflection; the focal ratio of the mirror; the amount by which the secondary is oversized from the light-cone minimum, and the thickness of the adhesive layer gluing the secondary onto the holder. In particular, a typical thickness of glue makes a difference in the same order of magnitude as the total offset itself, so it’s very important to account for it. I’ve left the actual equations I used and the diagrams to the end of this post, if anyone’s interested.

Both mirrors, old and new, were nominally the same size: 70mm minor diameter, whereas the minimum for the on-axis light-cone for this scope was 66mm. Laying the two mirrors side by side the difference in quality was obvious. The blank used by SW was rather coarse and lumpy around its edge, whereas the Hubble Optics one was finer, smoother and beautifully frosted. The SW mirror itself had a straight line on one edge, presumably where it was held in the silvering process, and you can clearly see this manifesting as an extra diffraction spike on bright stars. There is no such “missed silvering” on the Hubble Optics mirror.

However, carefully measuring the various dimensions of the old mirror too, to see how well SW had placed the secondary holder, I was impressed that their mirror had been positioned spot on, including their glue-thickness, and consistent too with the oversize-factor in the offset formula. Good on them.

The SW mirror was held on to its holder with a large blob of what looked like chewing gum, giving a 2mm “glue gap”, whereas I used number-plate tape (0.7mm thick).

Cheers, Magnus

Pictures and formulae below:

Old mirror and glue-flakes

New mirror marked and holder ready to come together...

Preparatory scribblings:

Finished, ready to re-install to scope:

 Formulae:

L1 = (a – d/2 –t) . sqrt(2)                            L2 = (b – d/2 + t) . sqrt(2)

Where                a = s / (2.F/x +1) and b = s / (2.F/x – 1)

This makes the offset itself, (b – a)/2 , equal to s / (4.(F/x)^2 -1) … multiplied by sqrt(2) if measured along the face of the mirror.

L1 is the “short” distance from the rear end of the holder to the end of the mirror;

L2 is the “long” distance from the front end of the holder to the other end of the mirror;

“s” is the distance from primary mirror focal point back along its axis to the optical bounce point off the secondary: i.e. how much of the far end of the cone’s axis is bounced off sideways;

“F” is the primary mirror focal ratio;

“d” is the diameter of the secondary-holder;

“x” is the linear factor by which the secondary is oversized from that required perfectly to intercept the on-axis light cone of the primary (this increases the area of the fully illuminated region, but also increases the CO ratio). An “x” of less than 1 implies a secondary smaller than that necessary to “catch” all the primary’s converging light (the case with my 200p, actually!);

“t” is the thickness of the adhesive layer used to glue the holder to the mirror (makes a surprisingly significant difference).

19 hours ago, tingting44 said:

stunning! looks like you have some nice dark skies

Indeed lovely and dark. When the North Atlantic weather permits...

Last night, Moon notwithstanding, I tried out a new idea for collimating my 6" Intes, which turned out to be a non-starter (I spent this afternoon undoing the damage). In the mreantime, I took a few wide-angle dew-wiped images with my Canon 6D and Samyang 14mm/2.8 (10s ISO 1600 wide open). It doesn't like very bright sources, huuuge coma, but I'm quite pleased nonetheless. This was after having accidentally let a cat slip out at 1145 and spending ages tramping around fields in the Moonlit dark trying to find him. Eventually found him back in the garage waiting for us grrr.

Venus also reflected in Church Strand Bay.

Really nice

My observing session last night was cut short by everything suddenly going dead: my big Celestron double Lithium powertank had run out of juice mid-session. Oh well, I had at least got some observing done, and packed up. I'll charge it up tomorrpw, I thought.

Tomorrow was today, and could I find the charger? No. The charger lead plugs into the same socket as the output supply to the scope, except the DC input is 16V for charging, as opposed to the scope outlet at 12V.

I had an idea. I have a benchtop power supply which gives 13.8V, but is less convenient to use outside, so while I have the battery, it gathers dust. I wondered if that, 13.8V, would be enough as an input to the the battery to allow it to charge. I called a friend who works in electronics and he thought it may well work. It turns out that it did!

Magnus

Looking NW-ish towards Baltimore and Schull from Ballylynchy. Didn't bother with the Astrotrac for the starry one hence slightly streaked stars.

Just came across this

https://phys.org/news/2020-04-usgs-first-ever-comprehensive-geologic-moon.html

On 18/04/2020 at 14:41, Paul73 said:

Speaking with bitter experience of the UK weather.......

yes fair point

15 hours ago, lukebl said:

On the point regarding a subsequent star test, surely that would simply tell you whether or not you'd collimated it properly, but wouldn't help you regarding the cause? If it proved that the collimation was still out, you wouldn't be able to tell whether it was the focuser, secondary or the primary which was at fault and you're back to square one.

That's true, but you're not back to square one, after the indoor collimation you'll be much closer than square one. Even after all the indoor collimation in the world it's still going to be slightly out, and from there you use the much more sensitive star test to make the next stage of smaller adjustments. And yes you're right, you won't know which part is contributing most to the remaining misalignment. But you should be close enough that whatever you adjust, you can improve it to satisfaction. I'd probably avoid adjusting the focuser, leaving you with the choices of star-test-adjusting either the primary or the secondary, or both.

Depending on your scope type, often that choice is made for you.

If you have an SCT, you can only easily adjust the secondary "in the field". If a SW-style Mak, it's just the primary. If a Rumak-Mak (like mine) or RC, it's either or both, and here you must choose.

Whatever you choose, though, you should be able to improve it from its indoor state.

In my case, I "set" the primary (indoors) by removing the secondary and aiming the laser at the exact centre of the resulting opening. Quite an exercise. I then, much more conveniently, adjusted the secondary (having re-installed it) to reflect the laser back to its source. On subsequent star-testing, I found my diffraction-rings slightly squashed. I decided to adjust only the secondary, and got the rings symmetrical. And the view through the scope was really nice, much better than before, so I was reasonably happy.

However, back inside, I noticed that the secondary was no longer pointing back to the source, and yet the star-test was good. What was going on? The laser is a Glatter which I've tested over 25 metres in my garden. Obviously my tweak of the secondary was a final compromise for the errors remaining in the primary, and elsewhere. My next step will probably be to restore the secondary to its "pointing back to itself" state, and star-test-adjust again this time adjusting the primary only.

Anyway, the point is that, for any telescope, all components will necessarily be "out" to a lesser or greater degree. Your final configuration after a star test will be a compromise with the residual errors in the system more or less compensating for each other.

Hopefully, as you go through this process, you will inevitably be thinking about it and gaining collimation experience, and will then think "what if I try this". As you can see from the various contributors to this thread, there are many ways to skin the cat test and diagnose what's going on.

Sorry this reply a bit longer than intended,

Cheers, Magnus.

PS I use a torch pointed down the tube to look at the secondary's reflection and centre-mark

PPS I have read that RC secondary centre-marks are often not that accurately placed. My Rumak-Mak doesn't even have one!

That sounds good. The trick I think with systems where both mirrors are adjustable is to try to work out some way of aligning each mirror independently, which you now seem to have done with steps 1 and 2.

What I’ve found is that these indoor methods are a precursor (they get you into the ballpark) to the extremely more sensitive test of an actual star in good seeing, from where you can converge on perfection.

I do have one question: the step 1, can you not see where the laser strikes the secondary by looking at its 1st reflection In the primary? That’s what I do.

Hopefully you’re almost there...

M

On 16/04/2020 at 01:02, Paul73 said:

... probably by my last look for the year at Markarian’s Chain. ...

Why so? By my reckoning we still have a month to go before we start losing astro darkness, and Leo is high up all that time...

Cheers, Magnus

43 minutes ago, Ags said:

@vlaiv thanks for that - that was quick work! It makes my current collimation state look "not good".  ...

A better description would be “not bad, nearly there.”

”not good” is when the central spot is on or even outside the rings, making a tear-drop, and the at-focus view of a star is not even a splodge, it’s an actual line. I re-assembled a Mak recently and have been through all the stages of terrible collimation, and made sure to see what they all looked like.

You are nearly there...

Well done. Homing in...

On 23/01/2014 at 14:39, John said:

... I have an aperture mask with a 4" hole which is the largest I can use with my 12" dobsonian. Using this I get the effective performance of a 4" unobstructed scope with a focal length of 1590mm so F/15.5. 

As I already have a rather nice 4" ED refractor which is actually much smaller and lighter than my 12" dob, it makes more sense for me to get that scope out if the seeing conditions are not good enough for the dob to make use of it's full aperture.

John I apologize for quoting you 6.5 years on, but seeing as you have the gear, that would be a fabulous side-by-side comparison to do (please?)

M

1 hour ago, miguel87 said:

...  thinking about the primary mirror and how when we use higher magnification we must only be looking at a small area of the mirror? 

...

It must all be down to how focussing works

No. The primary mirror projects an image in the focal plane.  Every part of the mirror contributes to every part of the image*. The eyepiece is essentially a very sophisticated magnifying glass on that image.

So when you increase magnification, you’re simply zooming in further on that brightened image.

I.e. the bigger the aperture, the brighter the image you’re zooming in on becomes.

M

* ... for the fully illuminated field around the centre of the image, but that’s beside the point here 

55 minutes ago, Stu said:

Just thought that @Captain Magenta uses his AZ-EQ6 with a 12” Skywatcher newt in one side (approx 20kg plus I think) but I can’t recall which scope on the other side, possibly a Mak 180.

They are certainly very solid and capable mounts.

Yes indeed, it seems to handle the 20-odd kg 300p newt plus accessories quite readily in alt-az mode. And if another scope is being used on the other side, it’s effectively doing duty as a counterweight, so doesn’t count as payload.

Although the instructions advise against using counterweights AND second scope on the bar, that’s just to avoid a situation where you’ve not tightened the weights on enough and they slip down the bar taking the second scope off with it. Just be sure to make the weights secure.

see the pic, the second scope here is a 6” Intes Mak, but I have had my Mak 180 there too in the past, see 2nd pic.

... and I’ve added a screenshot of FLO’s spec page for the mount, which states 18kg for AP and 25kg payload for visual.

It’s not far off, and not too difficult.

With the star centered, find out which is the most relevant of the three adjusting screws by putting your hand over the edge of the front tube, seeing where it appears on the pattern, and moving it until it corresponds to the “most squashed” bit of your pattern. If you’re lucky, the “gap” will appear either next to or opposite one of the three screws.

Adjust that screw, in small increment, and see which way the pattern goes, and thereby discover which direction of turning makes the pattern better.

if your hand gap is not perfectly aligned with an adjustment screw, pick the one nearest (ie nearest to “next to or opposite”) it, get it as good as you can, then shift to another one which has now likely become closer. You should home in on perfection in this way.

hope that makes sense ... always adjust in 1/16 of a turn at most at a time.

Good luck 

M

49 minutes ago, Stu said:

I suspect @Captain Magenta can blame me for buying his LZOS 4” after looking through my Tak too! 

Yes very much so! I don't have it with me here, I must say I am missing it.

Last night the 4th clear night in a row, seeing not too bad but the wind was! Counter-to-forecast strongish NE and nowhere to easily set up to shelter from it. I’d prepared a list, but only got to item one on it and gave up: a beautifully bright Cat’s Eye Nebula was whizzing back and forth across the field of view!

Just for a bit of easy instant gratification I did look quickly at M13 and got the propeller in a patch of momentary lull.

list to be resumed, but cloud forecast for the next night or two.

Cheers, Magnus

I have destroyed a TS Optic Monorail focuser trying to do more or less what you’re doing and going, let’s say, a touch too far ... a hammer was involved. The knob turns still, but it grinds and nothing else moves...

Really nice. I initially thought both shots were actually one photo, the way they appear on a phone, with a dark fence or branch between the top half and the bottom half.

Magnus

I saw this tonight, almost by accident! I didn't star hop, I used goto and typed in NGC 2419 as that was in my mind (from this post), but somehow in my head I thought I'd asked for Tegmine. When I looked through the eyepiece and hunted around a bit, I suddenly spotted two stars in a line and a faint blob just beyond and recognized the arrangement from the sketch and realised my mistake! I did spend some time staring at it, in awe of its distance 1/10th the way to Andromeda out in the middle of nowhere. Actually noticing it wasn't difficult, I was using a 12" and my darkness tonight was 21.7.

Quite pleased, and thanks for this thread without which who knows when I'd have got around to it, Magnus

... second clear night of a few in store apparently last night, Monday was the occasion of second light for my new mirror.

although ostensibly a very clear night, even better than previously to the naked eye, in the event the Seeing was terrible. Polaris at high magnification was entirely free of diffraction rings, just a big wobbling splodge. I took in a few things in but it really wasn’t worth it and packed up quickly.

hopefully Tuesday night will be better...