New project

[FLO]

Fascinating stuff; keep the pix coming CC

[Kaptain Klevtsov]

I've been poking about looking at sled type focusers and I think I might be going that route for rough focusing with the conventional focuser for fine tuning (as in no collimation during focusing required). For the uninitiated this is where the secondary and focuser (if any) slide up and down the 'scope to get focus. The question is therefore:- how is this a bad thing, being fairly unusual? I get the bit about the collimation going off during focusing so I can deal with a per-session collimation if I have a target in mind and I like the idea of a three speed 'scope. I could set up for imaging with a 0.5x focal reducer and snap away during one evening and the next night go for broke with Barlows and stuff to get in close. At the moment I can't see a big issue with this idea but as its not well represented on t'internet I could easily be missing something big.

Did I miss something here?

Captain Chaos

[GordonCopestake]

Theres a couple of threads on sleds on cloudy nights atm forum at the moment (i'm a poet and i dont know it). The idea is sound and the only reason they are not used more often is cost. As there are more moving parts the mass production cost is higher. There are issues to be solved with stability of the runners but a handy man like yourself can always sort those out

Take a look here

[Kaptain Klevtsov]

Thanks Gordon, now I'll be late for work

Captain Chaos

[MartinB]

Keep the posts coming cc. This is getting interesting

[Kaptain Klevtsov]

Looks great CC. Have you got the mirror sorted? I look forward to seeing it mounted on your new pier

Well here it is on the pier. The pier is obviously too high for it.

More to come when I get going on the sled focuser / secondary carrier.

Captain Chaos

[geppetto]

Great project CC

We're all sat waiting for a first light report so no pressure

[Kaptain Klevtsov]

Check out the sky behind the 'scope. No chance anytime soon because of the clouds. I still have to get the focus sled sorted (as in make the sled, find the materials for the curved spider vanes and then build the thing) and then it's just a matter of waiting for dark with no clouds.

Captain Chaos

PS Anybody got an idea of a target? I was thinking M51 (again).

[GordonCopestake]

Given the height of your pier i'de advise against M51 unless you have a good step ladder (maybe Arthur can borrow you one hehe)

Joking aside, it looks pretty good CC, very impressive. A shame even ATM scopes suffer from the weather curse!

That cross bracing looks effective, how solid is the scope on the pier? As for the sled focuser you could always cop out and buy a crayford or even a R&P focuser....

[Astroman]

I'd recommend M23, if you can see it from your location, CC. It'd be low on the sky, so a ladder wouldn't be necessary, it's a bright open cluster, so a star test could easily follow, and it's just a heck of a nice cluster!

[Kaptain Klevtsov]

Gordon, Arthur has kindly offered me one already which I have gratefully accepted. The sled bit is to get enough travel on the focus, so its in addition, not instead of.

Astro, I'll check out that M23, its over the wall in front of the van.

Tonight I got somewhere with the secondary collimation assembly. A plug from the bottom of a gas pressure regulator does duty as the boss, or whatever its called.

Shame the 'phone can't do macro mode.

Anyway here's the base for the sled bit

This is supposed to be the sideplate off a gas burner, but I found a better use for it. It needs cutting to length either side of the 2 1/4 inch hole when the design is sorted, then rubbing strips down each side and a clamp thingy.

more to follow as always.

Captain Chaos

[Ambermile]

Hmm - not sure about letting you near the Ambermile workshop CC, there'l be b****r all left when you leave... :shock:

Arthur

[GordonCopestake]

Hehehe. Looks great CC you're going great guns! There might even be some dark soon for you to test it all! You shouldn't need much travel on the focus and with the sled once your close you will find you wont move it at all and let the crayford do all the work. Unless you wanted to use bino's or something....

That secondary looks good, waiting for your spider to appear!

[Kaptain Klevtsov]

Question time then.

I need some shims to go either side of the sled, some kind of rubbing strips. The easy option is wood, oak or pine as I've got some. Any reason not to stick some baby planks in there?

The secondary mirror being eliptical, should I centre one of the foci in front of the focusser, or is it something else?

Is there any point in trying to prevent rotation of the secondary holder to keep the ellipse in the correct orientation? I could turn it sideways on somewhat and still collimate it, given enough screw travel and I know that that would be bad.

According to what I've researched so far, the sled needs to move along the optical axis of the 'scope. It seems to me that I can position the secondary mirror such that the optical axis of the 'scope is defined by the sled axis so that I can adjust things to just work. All I need to do now is make a sled that allows the secondary to be postioned in two axes which are at 90 degrees to the optical axis of the 'scope.

I need to get the spider vane thickness right as well. I'm going for something rigid and thin, so probably stainless steel is the first guess, so how big a difference does the thickness make? Doing the sums I get very little difference in area, percentage wise WRT aperture, between 0.5mm and 1.5mm. What I don't know is the difference in diffraction. As I understand it (poorly) the diffraction comes from edges, and I'll have as many of those with a thin vane as with a thicker vane.

Lots to ponder on,

Captain Chaos

[geppetto]

CC

I.m going to show my ignorance now :oops:

Cant get my head round this sled thingy.

Is the focuser and the secondary attached to it to move up and down the

OTA in unison or as usual, am I on the wrong track :?

[Kaptain Klevtsov]

If you look at the link that Gordon posted above it will become clear.

The basic idea is that the focuser and secondary mirror are mounted onto a slide which can move along the telescope. In Gordon's link the guy uses it for focussing. In my plan the sled will be used to compensate when the focuser runs out of travel. With my current skywatcher Newt. I can't use a focal reducer as these is insufficient in travel of the focuser. I moved the primary mirror forward using the collimation screws as far as I dare (just before I run out of screw) but it still won't work. This coupled to the fact that I might want a colour filter wheel between the camera and focuser means that I want loads of in travel, but still retain the "normal" position of the focuser for conventional use. Consequently I have decided to have both coarse and fine adjustment of the focus using both methods. A conventional Newt. focuser will sit over the hole in the sled and look down at the secondary mirror.

Hope this makes it clearer.

Captain Chaos

[GordonCopestake]

WRT spiders, thinner is better IMO. WRT the wooden shims, as long as they work there are no problems i can see. Go for it

[geppetto]

Thanks CC

Tis now clear 8)

[Astroman]

Question time then.

I need some shims to go either side of the sled, some kind of rubbing strips. The easy option is wood, oak or pine as I've got some. Any reason not to stick some baby planks in there?

Given the oak/pine option, I'd go with oak. It's harder and cleaner. It's been used for centuries as drawer slides in furniture, too. Must be a reason.

The secondary mirror being eliptical, should I centre one of the foci in front of the focusser, or is it something else?

The secondary should appear circular through the focuser. There should be an offset away from the focuser to account for the difference between the forward leaning edge and backward leaning edge of the 45º secondary mirror. This shouldn't affect the optical axis or the movement of the sled along the axis.

Is there any point in trying to prevent rotation of the secondary holder to keep the ellipse in the correct orientation? I could turn it sideways on somewhat and still collimate it, given enough screw travel and I know that that would be bad.

You want to keep it as stable and steady as possible with respect to the optical axis. The sled MUST move absolutely parallel to the axis to remain collimated.

According to what I've researched so far, the sled needs to move along the optical axis of the 'scope. It seems to me that I can position the secondary mirror such that the optical axis of the 'scope is defined by the sled axis so that I can adjust things to just work. All I need to do now is make a sled that allows the secondary to be postioned in two axes which are at 90 degrees to the optical axis of the 'scope.

Exactly right, CC.

I need to get the spider vane thickness right as well. I'm going for something rigid and thin, so probably stainless steel is the first guess, so how big a difference does the thickness make?

Stainless steel is not the best choice, as it acts as a spring and oscillates when struck. Aluminum is better, lighter and easier to work. The thickness isn't that important because the act of using a curved spider should cancel most of the affect of a thickness issue. That in mind, you still need to make it as thin as possible. One-eighth inch is good. Rigid, still light but not too thick. It's also the thickest you want to go. 3/32" or 1/16" would work, too.

Doing the sums I get very little difference in area, percentage wise WRT aperture, between 0.5mm and 1.5mm. What I don't know is the difference in diffraction. As I understand it (poorly) the diffraction comes from edges, and I'll have as many of those with a thin vane as with a thicker vane.

Also correct, but the shape of the edges makes a difference too, and is the basis behind the curved spider. A straight line through a circular wavefront introduces an interference pattern easily recognized as spikes on the stars. A circular spider still introduces an interference pattern, but is less easily detected by the eye as it diffuses differently. It's tough to explain, but if you think of patterns you've seen in wave tanks or wind tunnels you may get the idea. If you have lots of time on your hands, or are an insomniac, I recommend Harold Suiter's book, Star Testing Astronomical Telescopes. He describes in depth the effect aperture and flaws in the optical path have on incoming wavefronts. Really dry, but a great theoretical description nonetheless.

[Kaptain Klevtsov]

Thanks Astroman, wise words indeed. I have, however progressed somewhat today and I have the thing at least collimatable (no focusser yet).

To comment on your points WRT my design decisions if I may:-

Sliders are oak  - easier to machine accurately

Secondary appears circular through the hole where the focusser will go, I was looking for a mathematical / measurement method of defining it's position so that it didn't get involved in the measuring and could stay safely in it's drawer. it got involved and got out unscathed thankfully.

The sled appears to move on the optical axis as the red dot stays exactly in the centre of the primary (YAY!)

The sled axis IS the optical axis of the 'scope

Stainless steel for the vanes as the aluminium distorted if I turned the assembly the other way up. The stainless steel did not. I'll have to remember not to strike it while imaging 

The vanes are curved, so hopefully that will help

Thanks for your time, it has given me confidence that the thing will work one day soon.

[Kaptain Klevtsov]

First light this evening!

First daylight bash at it as it didn't get dark, in fact its still not very dark. I got the Nebuliser (need a better name as that is poor) on some stands and checked the collimation system. Secondary collimation works OK, dot in the centre of the primary O, and it stays there as I slide the sled! First go, no fiddling! Talk about impressed!

Primary collimation also works. The laser dot went back into the laser collimater after some fiddling and after I got it centred I rechecked the sled. Moving the sled along the 'scope axis moved the dot off centre by about 2mm as the lower end of the sled was unguided at that time (since fixed). Putting the lower end of the sled where it should be put the dot right back in the centre. Having a really good day so far.

Sky clearing so off to put the Mak. out to cool (seems pointless as its out anyway, just in a van) then back to business.

Stuck a 2" 40mm EP in the hole where the focuser will live - no joy with the focusing. Pulled the EP out about 35mm and acheived focus on some buildings a couple of miles away. Focusser will be this high or more so no worries.

Played with the primary collimation adjustment and got the primary mirror back about 10mm and now focus is acheivable with less pull out of the EP (well Duhhhh!).

It works!

This shows the curved vanes.

This shows the carp welding that needs sanding down, and not much more.

The new Alt. Az. mount is clearly shown here.

And you can clearly see the back of my 'phone in this one.

Put the Mak. away at about 10:50 as Jupiter had been boiling furiously, obviously pointless getting the webcam on it, and then the clouds came back.

Fun night again.

Captain Chaos

[Astroman]

Looks great, CC! I think the biggest issue is to ensure the sled travel plane is the same as the optical plane. Nice work!

[geppetto]

Excellent stuff CC 8) 8)

[Kaptain Klevtsov]

Update on this.

Total weight with mirrors but no focusser is 32 lbs, so slightly too much for the HEQ5. Also the balance is off as I underguessed the weight of the top end (wasn't thinking of a sled originally, so another brace to weld in.

I nicked the dovetail off my Frac. to mount it so I need to get another one of those, any ideas where to get one?

Really need to get to Arthur's place to pick up the EQ6 and focuser now, but finding the time is a pain, work comes first as I have to pay for these toys.

Does the EQ6 use the same size dovetail as the HEQ5?

Oh and anyone know where to get a sheet of black flocked card / heavy paper that I can form into a tube and slide inside the frame?

Captain Chaos

[GordonCopestake]

As far as i know the EQ6 is physically identical to the HEQ5 (apart from being twice as heavy). You can buy dovetails from scopes'n'skies. Must be a beast to be too heavy for the HEQ5!