Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Setting the wedge angle on 80's era C90 Mak Wedge

  • Please log in to reply
5 replies to this topic

#1 shawnmcarter

shawnmcarter

    Lift Off

  • -----
  • topic starter
  • Posts: 15
  • Joined: 10 Apr 2020

Posted 03 August 2020 - 08:17 AM

I have computer controlled mounts (Nexstar 8SE and a 127mm F6.8 achro on an AVX) but I don't want to depend on them.  Both of those mounts are heavy and take a good 20-30 minutes get out of the workshop, set-up, aligned, and finally pointing somewhere interesting.  I sometimes want to just take a small grab-n-go, plunk it down in the yard and look at two or three things and then put everything away.  So, to do that I have an early 80's era Celestron C90 Mak Astro with a wedge. 

 

So, I understand setting circles and how to use them...in theory.  I haven't been able to find anything with them so far which suggests that my polar alignment, particularly the wedge angle is incorrect.  So, hopefully someone can show me where I have boneheaded the wedge set-up.  My procedure thus far is:

  1. With the mount and scope removed from the wedge, I level the OTA with bubble level and ensure the azimuth gauge is set to 0.
  2. Place the mount on the wedge, raise the elevation, now declination, so it reads my latitude 33 degrees. 
  3. Adjust the wedge so that a bubble level resting on the lens cap becomes level...that should put the wedge at 57 degrees, right?

Something is wrong, because setting the RA dial to current sidereal time and slewing to targets doesn't work, nor does centering a known bright star in a high magnification eyepiece and setting the RA to that works...the Dec is usually fairly wrong as well.  That tells me the wedge is wrong or I am missing a step.  Any advice would be appreciated.  The mount is aligned to Polaris or as close as I can get it.

Also, maybe I'm reading getting the celestial coordinate pairs incorrect.  I am using Stellarium and selecting a star, and reading RA/DE (of date) coordinates.  For example, Vega is 18h37m38s/+38 48' 15"...but there is also the Hour angle/DE which is 21h54m35s (changing)/38 48' 15".  I've tried both to no avail.  Is there a time adjustment I need to account for?



#2 KBHornblower

KBHornblower

    Mariner 2

  • -----
  • Posts: 288
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 03 August 2020 - 08:48 AM

I have computer controlled mounts (Nexstar 8SE and a 127mm F6.8 achro on an AVX) but I don't want to depend on them.  Both of those mounts are heavy and take a good 20-30 minutes get out of the workshop, set-up, aligned, and finally pointing somewhere interesting.  I sometimes want to just take a small grab-n-go, plunk it down in the yard and look at two or three things and then put everything away.  So, to do that I have an early 80's era Celestron C90 Mak Astro with a wedge. 

 

So, I understand setting circles and how to use them...in theory.  I haven't been able to find anything with them so far which suggests that my polar alignment, particularly the wedge angle is incorrect.  So, hopefully someone can show me where I have boneheaded the wedge set-up.  My procedure thus far is:

  1. With the mount and scope removed from the wedge, I level the OTA with bubble level and ensure the azimuth gauge is set to 0.
  2. Place the mount on the wedge, raise the elevation, now declination, so it reads my latitude 33 degrees. 
  3. Adjust the wedge so that a bubble level resting on the lens cap becomes level...that should put the wedge at 57 degrees, right?

Something is wrong, because setting the RA dial to current sidereal time and slewing to targets doesn't work, nor does centering a known bright star in a high magnification eyepiece and setting the RA to that works...the Dec is usually fairly wrong as well.  That tells me the wedge is wrong or I am missing a step.  Any advice would be appreciated.  The mount is aligned to Polaris or as close as I can get it.

Also, maybe I'm reading getting the celestial coordinate pairs incorrect.  I am using Stellarium and selecting a star, and reading RA/DE (of date) coordinates.  For example, Vega is 18h37m38s/+38 48' 15"...but there is also the Hour angle/DE which is 21h54m35s (changing)/38 48' 15".  I've tried both to no avail.  Is there a time adjustment I need to account for?

So far you have done nothing to get the wedge facing true north.  I would rotate the RA movement until the declination movement moves the scope up and down, set the scope so the declination circle reads 90, and then rotate the wedge until the scope is aimed in the direction of Polaris.  Then I would fine tune the elevation of the wedge, just in case the tripod is not perfectly level.



#3 kathyastro

kathyastro

    Vanguard

  • *****
  • Posts: 2,175
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 03 August 2020 - 08:48 AM

For setting the wedge, start with the base of the wedge perfectly level.  Use a "torpedo" level to ensure that it is level.  (Polar alignment, as such, doesn't care about level, but setting the wedge angle does.)  Set the wedge angle to your latitude.  At 33 degrees latitude, the wedge plate should be 57 degrees from horizontal.  My compass has an inclinometer built in, so I check the wedge angle with the inclinometer.

 

I aim the base of the wedge towards true north, using a compass corrected for magnetic variation.  After dark, I'll double-check visually that it is aimed at Polaris.

 

Once the wedge is set up, I will install the mount on it.  Make sure that doing so doesn't move the wedge base.  The standard wedge is made for a bigger scope than the C-90, resulting in the weight being cantilevered out to one side.  I mounted my wedge to a plywood base plate that offsets it so the centre of gravity is directly over the tripod.  The wedge is unlikely to tilt with the weight of the scope, so it is probably not a factor.

 

It sounds like your setup is fine, pretty much the same as what I do. Except that I set the wedge angle without the scope mounted.   I think I understand what you are doing with setting the angle on the scope, but there is room for misinterpretation and confusion that way.  Setting the wedge angle without the scope leaves less room for ambiguity.

 

The problem is likely in using the setting circles.  Be sure to use the RA/Dec coordinates, not the HA/Dec coordinates.  You won't find HA useful with this mount.

 

On my mount, the sidereal time index is on the south side of the motor base.  With the scope aimed south, the RA index is on the north side of the fork base, same side as the RA slow-motion knob.  There is also an RA index on the south side, but it is not the one to use unless you have "tumbled" the scope and have the fork arm on the right side.  With the fork arm on the left side, use the RA index on the north side.

 

Be sure that your declination setting circle is properly calibrated.  You can calibrate it roughly by eyeball, by aiming the scope at Polaris.  The scope will then be parallel to the polar axis.  Set the setting circle to read 90 in this position. 

 

A more accurate calibration of the dec circle can be done in daytime.  Aim the scope at an object on the south horizon.  Note the dec reading.  Now "tumble" the scope by rotating the RA axis 180 degrees and tilting the dec so that the scope is again pointing at the same object on the south horizon.  (The scope will be upside-down, with the finder on the bottom.)  Note the dec reading in this position.  If the two readings are different, adjust the circle to half the difference.

 

Let me know if any of this information helps.



#4 Andrekp

Andrekp

    Viking 1

  • -----
  • Posts: 617
  • Joined: 14 Apr 2020

Posted 03 August 2020 - 09:46 AM

You need to be careful that you don’t have accuracy errors adding up when you start combining measurements that can each have accuracy errors.

 

the first step is making sure that your 90 degrees marker on your setting circle is actually the mechanical axis of your scope.  If it is not, then that error will be compounded into all the others.  The easy way is to forget about polar aligning for a moment.  Set the scope at 90 degrees, then manipulate the REST of the mount to point at Polaris.  Don’t change the 90 degree setting.  Now rotate the scope around its RA axis.  If your set 90 degrees IS your actual 90 degree mechanical axis, then Polaris will stay centered as you rotate and that Declination setting circle is set correctly.  If Polaris moves out of the center, then that 90 degree circle is off by some amount.  There are different ways of resetting it, but just experiment with very slightly changing the alignment of the tube until you can get Polaris to stay centered, then THAT is your 90 degree mechanical axis.  Until you know that is correct, the rest doesn’t matter.  (And remember, you have not polar aligned, just lined up that Dec axis at this point.  Use Polaris because it basically stays in one place and is convenient for this use.)

 

now, what I would do, YMMV, is get ahold of an accurate angle gauge.  Digital ones can be found cheap.  Look up your latitude, and set the wedge angle to your latitude as exact as possible.  It doesn’t matter where the wedge is when you do this, just that the wedge flat is at your latitude’s angle to the horizontal part that attaches to the tripod or whatever.  You can figure out how to make this work, depending on your gear.  It’s the angle that matters, not whether things are level at the moment.

 

now put everything together, set the scope to 90 degrees and point it as close to Polaris as possible.  Don’t worry about centering it, you are just aiming for close enough.  Now use a level to get the mount itself absolutely level.  Don’t adjust the tube.  Don’t adjust the wedge.  The mount (probably tripod),  not the scope and wedge.  Presumably you would do this by using the horizontal flat part of the top of the tripod, or whatever is available.  You’ll need to figure out what works here.  
 

at this point, you can just slightly tweak the positioning of the mount (tripod) to center on the north celestial pole (Slightly off from Polaris) and you are set. Sound more complicated than it is, I know.

 

the point here, is not to compound errors.  If you use the level tube to set your latitude on the wedge, it presumes that the Declination circle is correct.  So you’ve combined the error of the added circle and the error of your level.  It’s more accurate to get your scope mechanically set, then just vary the position and tilt of the tripod slightly to get the final alignment.  All you should need to do is haul out the scope, properly aligned with itself, and tweak the tripod a bit to get it aligned with the sky.  Most of these steps, if done correctly, will only need to be done once.  Then you just need to point the self-aligned scope at the correct point in space.  A scope not aligned with itself can’t be aligned with the sky.

 

it is much easier, I think, to just tweak the tripod each time than messing around with the wedge each time.  Some small wood shims, or whatever make it quick and easy.



#5 shawnmcarter

shawnmcarter

    Lift Off

  • -----
  • topic starter
  • Posts: 15
  • Joined: 10 Apr 2020

Posted 04 August 2020 - 10:12 AM

@kathyastro I think you're right, the best tool would be a magnetic inclinometer which, I do not have nor do any my neighbors.  But the problem was in calibrating my settings circle as you and @Andrekp illustrated so well.  So I did spend some time yesterday afternoon really zeroing in on what my mechanical axis is with the scope and mount.  Then, I saw another thread where someone recommended using Stellarium in GPS mode and laying your phone against the wedge and reading the coordinate - it should be what your polar angle is * -1.  That showed me my original method was 2 or 3 degrees off.  Still, there is inaccuracy in a phone's GPS and gyro set-up (or whatever phones use for directions).  So, as soon as it started toward dark last night I polar aligned the mount and centered Vega in the eyepiece and read the Dec value on my mount and adjusted the mount till that was correct.  I'm fairly certain the polar alignment is ok from using my other scopes.  After I got the wedge adjusted so that Vega had the correct declination, I set the RA for Vega's value.   Then slew the telescope to the coordinates for Arcturus.  Arcturus missed being in the eyepiece by one half turn of my RA knob...I'll take that.  So thanks for everyone's help, sometimes it's just good to hear from other folks and get out of your own headspace.  

I can't wait to try it on targets I can't see through the god awful 5x24 finderscope....but I'll probably wait for the moon to get on towards first quarter first.


Edited by shawnmcarter, 04 August 2020 - 10:13 AM.


#6 Andrekp

Andrekp

    Viking 1

  • -----
  • Posts: 617
  • Joined: 14 Apr 2020

Posted 04 August 2020 - 10:38 AM

You don’t need fancy tools to mechanically align the tube to the RA axis.  Spinning it around a target at 90 degrees should get you exactly there.

 

as for setting your wedge angle, yiu can do it precisely as I suggested, but you can also just do it generally as well.

 

mechanically align the tube to the axis.  Then point the tube toward the north celestial pole with the tube at 90 degrees and adjusting the wedge.  Then maybe move the wedge so you are pointing just a tiny bit below the pole to give some wiggle room.  Lock it down in that position.  Then adjust the tripod legs, maybe use some shims, and line up properly on the pole, which is largely a visual thing.  Using no tools at all but the scope, you should be able to get as aligned as you will need for most any use.

 

if you need even more accuracy for AP long tracking, you can do this as well by drift aligning.

 

really, you don’t NEED a tool, just patience and an understanding of what you are trying to do.


Edited by Andrekp, 04 August 2020 - 10:38 AM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics