Setting circles- remember them?

[markse68]

I've been playing with my new scope tonight trying to figure out the setting circles.

It has "hour angle" and declination circles. In principle this seems a really great system as in theory there should be no need for polar alignment- I just look up the coordinates in sky safari for a known star, set the circles to those values and then just rotate the whole head on the azimuth pole until the star is in the centre of the viewfinder. Then lock the azimuth axis and from there look up any objects coords and easily find it- cool!

However, the circles were a bit loose when I got the scope and I'm not sure of their precise alignment- it's different to using RA and DEC where you (I think) polar align then point at a star and adjust the dials to that stars coordinates then move from there- could be wrong on that though as I never managed to get it to work when I tried before.

So my question is what's the best way to adjust the circles to get this system more accurate? I tried setting the coords for Polaris and spinning it to point at it but it was a bit off. Presumably there can be only one correct set of angles assuming I set the angle of the polar axis correctly to my latitude? How do I tell which dial needs tweaking? Is there a system or order of adjustment to get it right?

It's all a bit of a mind bender for me but I can see this opening up a whole new world of discovery- an analogue goto system that will really help finding those fainter objects I have had so much difficulty finding in my very light polluted skies

Edited May 4, 2020 by markse68

[andrew s]

You don't say what system you have but  cone error will put you out.

Regards Andrew 

[markse68]

System? It's got an hour angle dial and a declination dial- is that what you mean Andrew?

I'll look up cone error.

I got it as close as I could on Polaris by tweaking the declination dial then set for Tegmine and it was within my widest eps view which I thought was pretty good but I'd like to get it more centred if possible

Edit- looked up cone error and as I understand it thats error caused by the axis of the scope not being perpendicular to the declination axis? By some inaccuracy in the dovetail mount? I guess there maybe some error there but it looks quite precisely machined by Zeiss so hopefully it's ok

Edited May 4, 2020 by markse68

[andrew s]

Cone error is when the telescope  is not exactly parallel to the dec axis. So if your aligned on  a star close to the meridian on one side a meridian flip will not centre the star again. 

By system I intended scope and mount but the picture answers the question.

Regards Andrew 

 

[markse68]

Looking at the mount I guess the setting of the latitude angle is never going to be terribly accurate- it's a very small scale. The mount does have a bubble level though so the azimuth axis should be fairly vertical

[andrew s]

Google "Setting an equatorial  head Baryebrook Observatory " that seems to be what you need.

Regards Andrew 

[markse68]

I shall have a read of that in depth thanks Andrew- Id really like to get a better understanding of it all.

From a quick glance it struck me that the easiest axis to get right is probably the declination so I checked mine with my most accurate bubble:

Check level of tripod and thus verticalness of the azimuth post

Put head in alt-az mode and check level  verticalness of azimuth axis

put scope tube perfectly horizontal

check dial- it was perfect

so now if I set it equatorial and point to Polaris using the setting circles, I know if it’s not spot on the error must be in the angle of the polar axis or the hour angle setting circle. Getting there

 

 

 

 

Edited May 4, 2020 by markse68

[markse68]

Tweaked the hour angle circle and the polar axis angle and now it's much better!

This is a very easy system to use in combination with a smartphone that can give you the hour angles in real time- it's fantastic! I guess the RA/DEC system is better if working from paper star charts and you want to go totally off grid

Edited May 4, 2020 by markse68

[Merlin66]

Mark,

your setting circles look large enough to be reasonably effective.

I used setting circles for the best part of twenty years. I did many years SN hunting in faint galaxies with 12” reflector using the circles.

you do need good polar alignment.

I made a Perspex wedge index marker illuminated by an LED which also lit up the markings on the circles - much easier in the dark!

 

[markse68]

That sounds a great idea Merlin- I think I’ll just use my red head torch though- keep it simple- and finally a use for it 😉

I can’t believe how excited this has made me- finally I’ll be able to find things- maybe even those elusive comets. Love this scope more and more each day!

[Merlin66]

An image of the 12" f5 on the disk fork mount, circa 1985. The yellow setting circles were screen printed on  an 8" aluminium pot blank (??!!)

[markse68]

Nice! That’s a serious looking scope! Did you build it? Rotating upper vf part? Did it hold collimation? Love the colour! What’s the intricate looking counterbalance weights on the upper part for?

[Merlin66]

Yes,

It was built back in 1975 when I was in South Africa.

Full thickness 12 1/2" Coulter perforated mirror (>1/10 wave), 18 point mirror cell (Novak) and rolled aluminium tube.

The mount had a 24" x 25mm solid disk and 75mm HD steam pipe arms. The polar axis used an 8" plummer block bearing from a sugar cane conveyor!

The drive gear was a custom cut steel worm/brass gear wheel with 359 teeth.

The rotating top ran on some 400 3mm ball bearings and worked very well. If I did something similar again I'd just use teflon sliding pads...

The counterweights were partly for the guide scope (see the empty ring on the lower section) and to balance the eyepiece holder (on the far side) on the rotating head.

The original design was to have an f5/ f20 Newtonian/ Cassegrain with a flip secondary.....but that's another story.

It's name was "Canopus 320"

 

[markse68]

Had a disastrous night of it last night- such a shame as it was so clear but I couldn’t get enough accuracy and with the slo-mo controls not working it got really frustrating really quick and it was really cold! To make matters worse I couldn’t get skysafari to display hour angles in 24hr format and subtracting their displayed times post meridian flip from 12 was really awkward and frustrating- though maybe good for the brain 🤔 Luckily I’ve since discovered Stellarium does so will use that in future

[DaveL59]

can't recall the name of it, but doesn't one of the apps allow you to lay the phone on the OTA and it'll tell you what your pointing at? Might help to make aiming a bit easier till you can get the slo-mo controls functional again.

[markse68]

Hopefully it won’t be too long till the slo-mo is working 🤞

[bomberbaz]

I have a similar plan for my new scope once it arrives, I like your planning though, hell of a learning curve none the less.

 

[markse68]

Well @bomberbaz, after a really enjoyable 2 1/2hrs last night I can thoroughly recommend persevering with this technique! So much better than the previous session. Ok the weather played nice- mild and only light breeze, and fixing the slo-mos meant the mount was at least functional! But there’s something enormously satisfying about dialling in those numbers then finding your object bang there in the middle (ish) of the eyepiece! Last night especially so as I could only see the brightest stars through the light high altitude clouds so little chance of finding things by eye! Yes it’s time consuming compared to a computerised goto or swinging a dob round to a well known target but as it becomes more familiar I’m finding it very rewarding indeed! 

I figured out how to use SkySafari’s 12hr E and W hour angles on my 24hr scale and it’s actually pretty straight forward- any E value requires a meridian flip and then I just have to count the hours backwards from the 12o’clock marking. Sounds a bit complicated but it became easy with use. This is great as I’m not fond of the stellarium user interface and last night it seemed to be giving wrong values anyway! 

I had to tweak my hour circle and polar angle a little again but have arrived at being able to put any target E or W of the meridian within the 25mm ep fov- this is really great. One thing I noticed that i now keep an eye on is with the tripod on grass or other non firm surface like decking, swinging the scope around or on decking just standing near it will affect its level as seen in the bubble and if that’s off then everything is off! I had to re-level several times but that’s quite easy to do.

In praise of the telementor T-mount, it kinda looks like a piece of surveying equipment and it really feels like a precision instrument- it was a joy to use in this way now it’s fully functional.

[markse68]

Update: I’ve been using this mount with the telementor for a couple of good long sessions now and am still having frustrating issues. I can get it to quite good accuracy in the Western half of the sky but when I meridian flip for the Eastern half things are not where they should be.

I’d  previously adjusted the declination scale to pretty good accuracy but couldn’t figure out how to check the hour angle scale. Then the other night it hit me! The hour angle scale references the meridian! Not just any meridian but your own personal meridian. That is the plane that intersects the north south polar axis and your zenith. It can only be that as otherwise you’d have to adjust the hour angle scale depending on your longitude and that kinda defeats the point of it! So with the mount being adjustable to alt-az mode this turned out to be rather easy. 

Set the tripod and level it with built in bubble- this time on solid ground so it doesn’t keep shifting as you walk around it 🤦‍♂️ I double checked the verticalness of the spigot this time with a not so accurate bubble and it looked ok.

Adjust mount to alt-az and as before fine adjust the az axis to be perfectly vertical by using precise bubble to get alt axis perfectly horizontal

now set azimuth (Dec) scale to zero to align dovetail clamp perpendicularly to alt axis.

using precision bubble adjust alt axis to get dovetail clamp perfectly horizontal. 

Adjust hour-angle scale to read zero locking with 3 grubscrews (well Zero is straight up so here it's set to 6 o'clock  )

Now to set it back to equatorial mode and get the polar axis angle as precise as possible. I used my super accurate Russian optical vernier angle-o-meter for this- a very beautiful and useful tool- set to exactly 51.5 degrees- my latitude. Couldn’t take pic holding it and the precision bubble at same time but I held the bubble to rest on the top of the angle-o-meter and with a bit of brute force moved the polar axis till the bubble levelled and then tightened it up very tightly.

so now all the axes should be pretty accurate. There’s a little play in the vertical mounting spigot bearing surface and a tiny bit in the declination bearing which I might try tightening. And the tripod seems to mysteriously lengthen and shorten it’s legs all the time 🤷‍♂️ These will throw things off a bit but hopefully this will work much better this time. If it doesn’t I’m at a loss what else I can do. Except use a wider aov ep! 

I’m  lucky to have access to some nice tools but the principal should be possible with any bubble provided you reverse it back and forth to average out any inaccuracy. Rather than the angle-o-meter, Russian or otherwise, the polar axis angle could be set after the other axes just by entering Polaris coordinates and then adjusting polar axis and spigot rotation angle to centre it in ep. Once declination and hour-angle are accurate this should give good results.

like I mentioned earlier, and this is probably old news to most, the beauty of the hour angle system to me is that once the mount is set up accurately there should be no need for polar alignment. Enter the coordinates of any star then just swivel the mount on the spigot to get it centre ep and everything should be aligned 👍

That all changes though if I move north or south much of course.

It's a very nice mount

 

Edited May 9, 2020 by markse68

[andrew s]

The most likely cause is cone error, remember that from my first reply? If the telescope tube is not parallel to the dec axis if you align on one side it will be off on the other.

The traditional  cure is to shim one end of the telescope to make it parallel, sound simple but is a painful trial and error process. See what Google turns up. I have done it in the past but it was a long long time ago.

Regards Andrew 

[markse68]

Quite possibly Andrew but I’ve no idea how to measure that and I’m putting my trust in Zeiss to have got the mount pretty accurately machined for their telescope but if I could think of a way I’d certainly check it. I guess I could try your test with Polaris?

Anyway I have been a total idiot. All this adjusting was done on the assumption that the spigot was vertical when the tripod bubble showed level. I thought I should check that before packing away the tools 🤔 I used a V- block clamped to the spigot to give a horizontal perpendicular surface for the bubble- moved it 90deg to double check its accuracy which was good . Levelled the tripod to this bubble and...

Way off! There are some dents in the end of the spigot so it may have been good when it left the factory but not now

i tried to shim it with some really thin Mylar tape but the base line is so short the adjustments were too coarse. 

Nothing for it but some gentle fine wet-n-dry  with applied greater pressure in the high side.

Took a while but eventually it looks much better 👍

If that spigot isn’t perfectly vertical then all the other circles will be out- should have been the first thing I checked 🤦‍♂️

Had to re-do all the other checks and both the setting circles needed slight adjustment

I will try to check for cone error tonight if clear 

 

 

 

 

Edited May 9, 2020 by markse68

[andrew s]

Use a star near the meridian not Polaris.  Regards Andrew 

[markse68]

Sorry for being so dumb Andrew but Polaris is about as close to the meridian as you can get isn’t it? Do you mean closer to the zenith or a horizon? I just tried it with Mirfak and it’s not perfect but it’s within my widest ep fov at 33x. Shimming the scope mount by trial and error does sound like a bit of a nightmare having to deal with the target moving all the time! 

Edited May 9, 2020 by markse68

[andrew s]

5 minutes ago, markse68 said:

Sorry for being so dumb Andrew but Polaris is about as close to the meridian as you can get isn’t it? Do you mean closer to the zenith or a horizon? I just tried it with Mirfak and it’s not perfect but it’s within my widest ep fov at 33x. Shimming the scope mount by trial and error does sound like a bit of a nightmare having to deal with the target moving all the time! 

Close to zero dec. My mistake. If you are close to the meridian at zero dec then flip ra axis move only 180 degrees  the star should be in the frame offset by the cone error.

Regards Andrew 

[markse68]

Zaniah sounds about perfect? Good news- it's a small amount off but then I can't get mega accurate with the circles but it's pretty close- I checked a couple of times. Zeiss did good! (thought they would)

I think I might make a better spirit level attached to a V- block type mount that I can stick on the spigot if I still can't get enough accuracy- off now to a better location to isolate under the stars and try it out proper

Thanks for the help Andrew

Mark