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Learning to use setting circles

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#1 Michael Lomb

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Posted 10 April 2012 - 06:22 AM

Manual setting circles are built in to most telescopes, but often they are too small and inaccurate to be functional. Digital setting circles are almost standard now. Their apparent advantage is that accurate alignment is not a requirement for visual observation as any inaccuracy can be factored out by calculations. The setting circles of the Questar predate any of this but it was designed to actually work. The only requirement is that the platform that the telescope sits on, table or tripod is level. The base of the telescope should be 90 degrees vertical minus the latitude. If you lived on the north or South Pole, the telescope would lie horizontally flat, and on the equator 90 degrees from horizontal.

My Questar sits on a surveyor’s tripod. The three legs can be adjusted to any height, and the platform can be levelled with a bubble level. These are very accurate, even a tenth of a degree will move the bubble off centre.

From there my wooden equatorial wedge is bolted down, and set to 35 degrees latitude. It is not adjustable. The Questar is then attached with the central screw, and if everything was cut and levelled correctly, the declination should read 35 degrees with the telescope optical tube in the vertical position. You can check this with the bubble level by having it rest on top of the optical tube assembly and read declination. Moving the optical tube horizontally and placing the bubble level on the side of the tube should in my case read 55 degrees (55 + 35 = 90). It does to within to less than one degree error (this will be close enough). Declination is now set, and does not need to be adjusted again.

Imagine now that the Quester telescope is a compass and can draw circles in the sky (the celestial sphere). If the optical tube is 90 degrees to the fork arms, it draws the largest circle when the right ascension drive turns 360 degrees. This is the celestial equator. If the optical tube is in line with the fork arms it should only rotate around the celestial pole. Finding this spot in the Southern Hemisphere is difficult, but you do not need to use this.

I remember from physics 101 that if you have one equation and two unknowns, you cannot solve it. You know declination, its set and not an unknown. You just need to know how east or west to rotate the equatorial platform to match up the celestial centre to avoid an ascension error. The easiest way is with a magnetic compass. True south is not the same as magnetic south. You can look up in a map to find out the discrepancy. In my case celestial south is located 19 degrees east of magnetic south. Use a map compass to adjust the direction of the mount. You are now aligned to within a degree in ascension (it will be close enough).

Now wait till a bright star shows up, and use the finder to locate and centre this. You do not need a pole star, any bright star will do you just need to know what it is. Look up in an atlas what its declination and ascension is. The declination should already be correct, now rotate the ascension plate till the coordinates match. If you have the clock drive running, there are no more adjustments to make. The eye piece plus the long focal length of the Questar gives you a narrow field of view, and the degree or less of inaccuracy in ascension combined will get you off centre, but it will always be in the view finder which covers 10 degrees in a 16 mm eyepiece. So that is the theory, does it work?

Last night I used the setting circles for the first time using this method. The moon was coming up, and only the bright stars were visible, and a picked some random targets around the sky, Alpha Centauri, Acrux (both double stars), Omega Centauri, Mars, Canopus, Sirius and the Jewel Box. I wrote the coordinates on a card, and rotated the fork arm in ascension and optical tube in declination. All targets showed up in the view finder. Flipping back and forth between the finder and telescope view, I was able to centre all the targets in a few seconds. You only have to adjust ascension and declination less than one degree. Once centred in the eyepiece I checked the setting circle readings. They were accurate to within ½ degree declination and 2 to 4 minutes ascension. It was a lot faster than trying to line up the optical tube by sight alone.

There are more accurate ways to align the telescope perfectly for astrophotography, but I found this was not needed for visual observation; you can make minor tweaks manually. You can really get bogged down in the fine details to the extent that you start to think of getting digital encoders, (and then get bogged down in the manual).

If you don’t use setting circles, you are missing out on a useful half of the Questar’s capabilities. It is all very retro, but that appeals to me also. You can actually make use of a star atlas to plan an evening rather than randomly hunting for things to see, which is what I have been doing so far. The best way to do this, may be to make small cards (size of playing cards) on an object of interest, the RA and Dec reading, some notes on this, and put them in your pocket, and go through them one at a time with a red led flash light. (You do not need a data base of 40,000 objects with a 3 ½ inch telescope). I have not done this yet, but I can see how star hoping via setting circles could work, by calculating the difference in right ascension and declination of a small target in relation to a local big target.

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#2 justfred

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Posted 10 April 2012 - 03:04 PM

Michael,

Setting Circles are one of the secrets of the universe. Bravo to you for learning them. They make moving around much simpler. I still have an older ETX 90RA which taught me how to use setting circles. Those little scopes have wonderful optics but are hindered until you use the setting circles to locate things. The Questar is as accurate as you want it to be. Here in the northern lattitudes I have used the technique you describe in the daytime to find planets and stars and that always seems to amaze folks - and me.

Does your RA have both Northern and Southern numbers? My '66 has only the Northern and I'd have to subtract the numbers from 24 to get the correct RA from a Southern perspective.I've attached a phot. I couldn't use my tracking either! I'd still love to head that one way one day and see your sights.

Fred

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#3 Michael Lomb

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Posted 10 April 2012 - 03:36 PM

It is always good to hear from you in your replies to posts. When I sent my Questar for servicing at Company Seven, I had them install a 220 volt 50 Hz motor, and a dedicated southern hemisphere ascension plate. When I first got the telescope last year, sorting out the direction of southern and northern hemisphere ascension dials caused no end of confusion.

For those who have not read these dials before, each small line is 4 minutes, the longer lines 20 minutes. Coordinates of Sirius is 6 hr 45 min R.A. and -16 degrees 43 minutes declination (60 minutes to a degree).

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#4 Michael Lomb

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Posted 16 April 2012 - 05:04 AM

Here are some fine tuning tips to the method I described above. To find the exact magnetic declination of your location, you can go to http://magnetic-declination.com/ and enter your country and city. Magnetic compasses are usually accurate +/- one degree (some more expensive ones state +/- 0.5 degrees)

It is also worth checking that your Questar telescope declination dial is aligned correctly with the axis of the optical tube assembly. On a perfectly level table point the optical tube upward in a vertical position and put a bubble level on top. You can double check this with bubble levels that also work in vertically. The declination should read 90 degrees. In my case it does not. I was surprised that it was about one degree out (checked with two different levels). There is no way to adjust this that I know of. It does not really matter, as long as you pick it up. When you dial in the declination coordinates, just offset the reading by the degree of error.

#5 munirocks

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Posted 22 October 2013 - 07:31 AM

Your tilting board based on latitude is an excellent start, but there is a complication. Because the earth is an oblate spheroid instead of a perfect sphere, earthly latitude lines (which are based on gravimetric equipotential surfaces and not on perfectly measured angles at the center of the earth) do not exactly map to celestial declination lines (which are based on perfectly measured astronomical angles). The pictures here are worth a thousand words:

http://en.wikipedia....Geodetic_System

In other words, a plumb-bob hanging in the middle latitudes does not point exactly towards the center of the earth. Similarly, the angle between the "straight up" direction and the celestial pole is actually smaller than it would theoretically be if the earth was a perfect sphere. How much smaller? I don't know, but with your accurate setup you could probably measure the difference as well as anybody.

Even if I knew what the difference was for my latitude, constructing such an accurate mount is probably beyond my interests and capabilities (although it looks to be within yours).

So what's an astonomer to do? Start with an excellent tilt board like the one you've made (or an equatorial wedge set for your latitude, like I've got) but make sure it has an extra bolt or other device that allows you to make fine adjustments in the declination of the mount. A fine adjustment in the azimuth is also handy but not so important, as you can nudge the mount left or right in small increments.

You can then set the declination on your Questar to 90 degrees, fine tune the mount until the celestial pole is in the middle of the view, and your declination is perfectly aligned within a few minutes of arc. You then just need to turn on the drive and calibrate the Right Ascension of the scope on an any known object.

When I first got my equatorial mount (non-Questar) it had a wedge, and a facility for coarse declination adjustment (based on my latitude), and a socket for a fine-adjustment bolt. But the bolt was missing so the wedge was resting on the coarse-adjustment stop instead of the fine-adjustment bolt. I made a bolt that can be hand-operated in the dark (without tools) by finding a long bolt that fit the threads, and putting a large wing-nut on it all the way up to the head.

Having the fine declination adjustment made a HUGE difference that is hard to overestimate. With it, using setting circles is a breeze; without it, using setting circles is impossible. And if I couldn't use setting circles, I wouldn't own a Questar - it's that important.

It's hard to match the feeling you get when you dial in the coordinates of an object and there it is, smack in the middle of of the field of view.

I've always used a tripod. I've never tried using the portable legs for alignment, but I most certainly plan to when my 50th anniversary model shows up. The fine adjustment capability of the central leg looks like they've thought it through, so it's looking promising.

The accuracy of this method, of course, depends on having a view of the celestial pole (north or south) and being able to recognise its location in the eyepiece. Perhaps in my next post...

#6 ColoHank

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Posted 22 October 2013 - 11:28 AM

Here's a photo of the tilt adjustment on my home-built wedge. The pivoting nut was salvaged from a discarded satellite dish, and the end of the threaded rod bears on a recess drilled into a small block of HDPE on the rear of the tilt-plate.

I don't worry about minor discrepancies between real and perceived latitude caused by our not-perfectly-spherical Earth or the even larger 3/4-degree discrepancy between the location of Polaris and the north celestial pole. I just level my tripod, lock the declination of the scope at +90 degrees, and use the tilt-plate adjustment on the wedge and rotation of the wedge on the tripod to center Polaris in the viewfinder. That usually gets me close enough to position most targets somewhere in the field of view using the setting circles. If not, I can usually flip over to finder view to locate what I'm looking for (night skies are blessedly dark and transparent where I live).

On occasion, when Polaris is obscured by clouds, I've had to use a compass to roughly align the scope with geographic north and a protractor to set the tilt-plate. Even then the scope tracks reasonably well and I'm able to locate objects using the setting circles.

Accuracy is a wonderful thing, but not a preoccupation with me.

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#7 munirocks

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Posted 22 October 2013 - 01:49 PM

Call me a perfectionist, but when I dial in the RA Dec I like my target to fall bang in the middle of the field of view. It's part of the enjoyment, and it means less time hunting and more time viewing.

If your field of view is 1 degree across and you're looking at Polaris to align on it, the NCP (which is 50 minutes of arc from Polaris) isn't even going to be in the field of view, which means your targets won't be either.

It's a very simple matter to squeeze out the last bit of accuracy. There are two stars between Polaris and the NCP which make an equilateral triangle with the NCP. Learn where those stars are. The human eye is extremely good at imagining equilateral triangles and centering things in a circular field of view.

Once you've got a coarse fix on Polaris, use those two stars to center the NCP in the view, using the fine declination adjustment on your mount and little nudging in azimuth. The goto folks will be asking how you manage to find things so fast without a *computer* (as they often ask me). The computer is in my smartphone, which shows me the RA Dec coordinates of the object I want to look at.

I'll ty to attach a pdf file of the stars that you need to memorise (first attempt).
It's two pages - one showing a normal view, one showing a reversed view. Use the right one for your scope.

I don't know if there are any equally friendly stars around the south celestial pole. Anyone else?

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#8 munirocks

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Posted 22 October 2013 - 02:22 PM

Wow, that's a fine and beefy looking wedge you've got there, ColoHank. Love the large fine altitude/declination adjustment rod.

#9 munirocks

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Posted 22 October 2013 - 03:26 PM

It is also worth checking that your Questar telescope declination dial is aligned correctly with the axis of the optical tube assembly. On a perfectly level table point the optical tube upward in a vertical position and put a bubble level on top. You can double check this with bubble levels that also work in vertically. The declination should read 90 degrees. In my case it does not. I was surprised that it was about one degree out (checked with two different levels). There is no way to adjust this that I know of. It does not really matter, as long as you pick it up. When you dial in the declination coordinates, just offset the reading by the degree of error.


Because the optics are hand-figured, there's no guarantee that the optic axis will be parallel with the optical tube. What is important is that the optic axis is parallel to the mechanical RA axis, even if the aluminium tube is slightly off. That is, when the Dec is on 90, the optic axis should be parallel to the mechanical RA axis, so if you rotate the tube in RA while looking through the scope, the stars (or anything else in the view) should rotate around the center of the field. If it doesn't you need to recalibrate (rotate) your Dec circle relative to the tube, but as you say, I don't know if you can do this on a Questar - it might be factory set.

I learned all this by configuring the setting circles on a B&L Criterion 4000 some years ago. You could loosen a bolt, rotate the Dec circle, and retighten it. Unfortunately because it was adjustable it would occassionally come out of adjustment! :foreheadslap:
After I calibrated the Dec on that scope I found that the alignment was still out in the third axis, whose name we do not speak. To fix that I had to put a shim under one of the fork arms, between the fork and the base. (Not suggesting that you attempt such a thing on a Questar.) In the end it was worth it, as I could usually go from suggestion to eyepiece in about 60 seconds.

#10 munirocks

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Posted 23 October 2013 - 05:59 AM

Accuracy is a wonderful thing, but not a preoccupation with me.


I guess I'm pre-occupied with it because the (non-Questar) scope on which I learned to use setting circles had a finder that was very difficult to use, so I had to hunt about with the main scope if the targetting wasn't spot on. The extra 30 seconds that it took to move from Polaris to the true North Celestial Pole paid for itself in the first few objects that I tried to find, so the high-accuracy method just became a habit.

Maybe I'll relax my pickiness once my new Questar arrives, which of course has a finder that is extraordinarily quick and easy to use. I must admit it will be nice to have a working finder again.

On the other hand, if your setting circles technique becomes so accurate that you hardly use the finder any more, then perhaps "must focus in the Questar finder" will no longer be a criterium when you go eyepiece shopping, and you can buy any eyepiece you like.

#11 munirocks

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Posted 25 October 2013 - 08:48 AM

In case anyone's interested, here is the one-page handout from a lecture that I gave many years ago on setting circles, at the Maidenhead Astronomical Society.

It's a mindmap. Start with Basic concepts in the upper right-hand corner and read clockwise. Questar users can probably skip the section on Mechanical setup in the lower right-hand corner.

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#12 munirocks

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Posted 25 October 2013 - 03:33 PM

In the close-up pictures of the RA setting circle in previous comments, I noticed that for both the northern and southern circles, the outer scale increases in the opposite direction of the inner Hour scale. Does anybody know what the outer scale is supposed to be for? :question:

It doesn't seem to be the same as the outer degree scale on my planisphere. On the planisphere the degree scale increases in the same direction as the hour scale, so the two are interchangeable and measuring the same thing but with different units.

#13 ColoHank

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Posted 25 October 2013 - 05:40 PM

On my RA circle, the inner scale shows two sets of numbers. The larger hour numbers are for use in the northern hemisphere, and the smaller hour numbers, whose progression is opposite to that of the larger numbers, are for use in the southern hemisphere. The markings in the outer scale divide each hour into 4-minute increments.

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#14 munirocks

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Posted 26 October 2013 - 04:14 AM

All three of your scales make sense to me. I think you've got the latest multi-hemisphere version that they now supply, at least on systems with the portable Powerguide II drive system. In the southern hemisphere you just have to swap the 20,40 labels in your head and use the outer scale in the other direction, in combination with the southern hours. No problem with that.

On the other Questars I can see that you also use the outer scale for the same thing - four minutes, or one degree, per mark - and you still use them in combination with the hours, albeit ignoring the 0-360 labels. It's these extra 0-360 labels that run in the opposite direction to the hours, and opposite to the 0-360 scale on my planisphere, that have me stumped. I'm just wondering where they come from, and what coord system they are based on.

In other words, on the other Questars the outer scale isn't labelled in the way that I would use it. They are obviously doubling up on the labelling to fit several systems onto the same scales, but I can't recall ever seeing a system that ran in the opposite direction to hour angles.

#15 munirocks

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Posted 02 December 2013 - 02:03 PM

When I suggested earlier that Questar users can probably skip the "Mechanical Setup" in my handout I was in error. I made that statement based on two assumptions: 1) That all Questar telescopes probably have an accurately calibrated Dec circle and 2) if the Dec circle is not accurately calibrated then there's nothing you can do about it anyway because it is fixed to the optical tube at the factory.

Both of these assumptions are in error. Some Dec circles appear to be out by a degree or two (which is a *lot* when you are trying to use them), and although the circle position is fixed relative to the tube you can still "adjust" it by putting a sticker on the top of the fork arm, over the existing Dec gauge point.

Find the real 90 degree point by adjusting your Dec until an object stays in the middle of the field of view when you rotate the scope in RA. (I'm not convinced that using a level is accurate enough, and it's measuring the aluminum tube instead of the mirror optical tube, which admittedly are probably pretty close to each other.) Once you've found the real 90 degree point, put the new sticker in place on the fork arm, exactly under the 90 degree mark on the Dec circle.

And possibly remove the sticker and point out the issue the next time you send it in for service.

#16 Michael Lomb

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Posted 04 December 2013 - 06:31 AM

Thanks for all the input here on this post. I was not aware of some of details of this. Experimenting with the declination marking with a removable marker is worth experimenting with. An idea that comes to mind is to use electrician’s tape with two colours. The line could be where a red and blue edge meets. I may make it easier to read also. I did have my Questar serviced, and checking the accuracy of the declination dial is not routinely done (the problem is there).

I do remember seeing advertised somewhere spare declination dials for the Q3 that someone had. The dials had two holes in them which is how they screw into the fork arm. The screws are adjusted, and set and then covered with the red metal disk which is recessed into the fork arm. If it was possible to remove the disk without damaging it (I don’t see how) there would be access to the screws. Once adjusted and set, they could be kept from coming loose again with “locktite.” This is a red colored adhesive that is used to keep nuts and bolts from coming loose, but still removable with extra force. I suppose the ultimate authority on this issue is with Questar themselves.

#17 munirocks

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Posted 04 December 2013 - 12:14 PM

Interesting. It looks like the Dec circle is adjustable after all, assuming that the holes in the Dec circle have a bit of slack or are oblong. I wouldn't try it on my new 50th model with the special logo on the red disk, but if I had a used Q with an inaccurate Dec circle I'd have a go at that red disk with a slow, persistent pull with a wet suction cup, to see if I could get at those Dec screws without removing the OTA from the forks. If the adhesive for the red disk was spoiled in the process you could replace it with double-sided carpet tape.

Am I right in saying that the Dec brake works by pinching the Dec circle? - which means that any Dec stress is going through those screws that hold the Dec circle in position. I can see how the Dec circle might come out of alignment if the screws are not as tight as they could be and/or by accident you tried moving the OTA in Dec with the brake on. It seems to me that it would be better if the braking disk was blank and the Dec disk was on the other, non-braking fork arm, but this might interfere with the Dec control knob on that side. As a matter of fact, I'm sure I remember seeing a Q on ebay that had two Dec disks - one on each fork arm. Does anyone know what the "norm" is?

It would surprise me if the process of checking the Dec circle alignment is missing from the factory checklist, as the setting circles seem to be such an important part of the Questar experience. Everthing else on the scope seems to be shaved to within 1/1000 inch of its life, so why not the Dec circle calibration? Such an omission feels out-of-character.

#18 munirocks

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Posted 05 December 2013 - 02:57 AM

I just saw a Q on ebay with the red fork disk missing, and underneath it looks like solid aluminium, other than the big central bolt. I don't see any way to accesss the Dec screws from there, unless your scope is different. Abort! Abort! I'd stick with the, err... sticker method.

I'm not so sure now that I've seen a dual-Dec Questar. Maybe I was looking at one of those scopes that have been flipped over through the forks, which you can do if you temporarily remove the eyepiece holder.

#19 Michael Lomb

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Posted 05 December 2013 - 05:34 AM

Well perhaps my mental image of what I saw was incorrect. In an attempt to resolve this issue, I have just sent a detailed e-mail to Questar summarising this post and asking them for some input.

Here is what I sent...

Hello from New Zealand.

I have a Questar 3.5 standard telescope. This is probably the only telescope in current production that has manual setting circles that are functional. Knowing how to use manual setting circles has been the subject of a recent post on the cloudy nights forum. This is a post I started and others who are more knowledgeable than me have contributed.

I live at 35 degrees south in New Zealand. When using the Questar I mount this on a wooden equatorial wedge that is fixed for this latitude. The platform plate is exactly 55 degrees from horizontal. This was measured with a 12 inch diameter protractor. I checked it again just before composing this e-mail. I mount the wedge on a surveyors tripod and use a spirit level to make sure the platform is level. When the Questar is mounted, and the optical tube assembly is pointed to zenith, the tube should be 90 degrees from horizontal. Checking this involves resting the spirit level on top horizontally on top of the tube assembly. The declination reading in my case should be exactly 35 degrees. 35 + 55 = 90 I have attached a photo.(that is the photo at the start of this thread).

The reading is out by less than a degree, probably about half a degree. The spirit level is sensitive to even a fraction of a degree, and in fact an error of one degree is clearly detectable without a spirit level. This level of error is consistently reproducible. I have a solid level workbench in the garage, and resting the wooden wedge on the table top gives the same reading. Other members of the forum have noted discrepancies with the declination reading also. As long as you know this, you can alter the declination setting by this amount.

It was suggested by some members of the forum that the declination dial setting is determined by the optical path rather than the tube assembly itself. Others have wondered if the declination dial can be reset by removing the red disk on the fork arm and adjusting some screws. An alternative solution to the discrepancies involves taping a new mark on the fork arm.

Are you able to provide some advice and comment on the apparent discrepancies that I and others have noted?

#20 Billydee

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Posted 05 December 2013 - 04:22 PM

Micheal,

Please go to Google earth and check the exact latitude of your location. I doubt it is exactly 35 degrees South. I see references on North Island between 34 and 36 degrees.

Bill

#21 Michael Lomb

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Posted 05 December 2013 - 07:24 PM

In my location it is 35.066 degrees latitude.

#22 Michael Lomb

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Posted 06 December 2013 - 07:15 PM

A rather obvious way of checking the accuracy of the declination dial that does not depend on the accuracy of a wedge is simply to put the base of the telescope on a flat level table. This is the setting if you lived on the poles. With the optical tube assembly vertical the declination reading should be 90 degrees exactly. Resting the level on top of the tube should be level. In my case it is not. It is exactly one degree out. You do not even need a level to pick this up.

#23 Billydee

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Posted 06 December 2013 - 08:42 PM

Michael,

Well 35.066 is about as close to 35 as you can get. I wonder if the level of the OTA tube is the same as the optics level? Darren Drake posted a number of pictures of the famous Q cut-a-way under "Inside Questar". You can look at the mechanical insides on page 1 and page 3 of this listing.

http://www.cloudynig...5549953/page...

It may give you an idea or two about adjustments.

Luck, Bill

#24 Les

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Posted 07 December 2013 - 08:22 PM

Michael,

Just a thought - just because your spirit level can resolve fractions of a degree doesn't mean it is accurate to that resolution.

#25 ColoHank

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Posted 07 December 2013 - 09:02 PM

just because your spirit level can resolve fractions of a degree doesn't mean it is accurate to that resolution



It's easy to check the accuracy of a level. Use the level to set a reference surface so that it appears (according to the level) to be truly horizontal. Then, simply reverse the level end for end and set it on the same reference surface. If the level indicates that the reference surface is still level, it's accurate. If it doesn't, then the level is off.






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