Modifications to my 200P Dobsonian

[Chefgage]

Part 1.

Having purchased a skywatcher 200p dobsonian telescope at the end of last year i feel it was time  to start modifying it.   This is a thead about all the modifications i have done so far and any that i plan to do. It might help others who have purchased the same scope ( i certainly needed help  ) The first thing i noticed was i was struggling sometimes to locate objects in the 9x50 finderscope due to the stars sometimes being dim due to light pollution even using the both eyes method. So the first modifications was to install a rigel quickfinder.  I mounted this ontop of the existing 9x50 finderscope. When all three are lined up (scope, finderscope and rigel) it makes putting objects in the scopes field of view alot easier!! (I especially like the illuminated circles  )

Here is the first modification. I know its to the scope and not the mount but the rest of the thread will be about the mount.

 

[Chefgage]

Part 2.

The next thing on my list was to sort out the stiff azimuth movement. Even with the centre bolt nut slackened off it made keeping objects in the fov difficult due to the jerky movements. So a bit of reasearch on this forum and i read about installing the lazy susan bearing. So a 12" bearing was ordered.  This was installed easliy by removing the bottom base board and fixing the bearing centrally on this board.  When put back together and by adjusting the centre bolt nut i could get the required tension so that the mount now moves smoothly in the azimuth movement.

I found i did not need the slide guides as others have used.

 

[Chefgage]

Part 3.

A quick post for the next modification i did .  When moving the scope to the parked position i.e. Upright, if i am not carefull the scope tends to 'bang' on the mount. A sticky foam pad sorted that.

 

[Chefgage]

Part 4.

Having been out using the scope a number of times i felt i had got used to all its features and had observed the moon, venus, saturn, jupiter and a few double stars.  I decided i wanted to observe the many many dso's that are out there.

This is where I started to struggle. During early summer I noticed that due to the bad light pollution and reduced/lack of astronomical darkness in my area i was struggling to see most stars.  Take the constillation lyra for example. I can always see vega and sometimes one of the other stars in the constillation but the rest of the stars of lyra are usually not visable to the naked eye.  This makes getting them in tje fov and star hopping very difficult. The same goes for alot of the other stars in various constillation.

So after a bit of googling and reading on here i read about push to systems. I could of course gone out and bought a goto scope but i really like the 200p dob so i wanted to see if something could be done about it.

The first thing I bought was a wixey angle gauge to give me the altitude reading. This gauge comes with a magnetic base. So installation is very easy

 

To use the wixey gauge the procedure is as follows.  Use the finderscopes to get a known star such as vega in the fov of the scope. Then use an app such as sky safari to get the current altitude reading of that star. The zero button on the wixey is then pressed to zero the gauge. Then move the scope downwards untill the reading on the gauge matches the altitude reading of the star in sky safari. The gauge is again zero'd. This then means the gauge is zero'd to the horizon, i.e. Zero degrees. If you then look at skysafari to see the altitude reading for the same star and lift the scope so the gauge reads that value, the star will be in the fov  This is providing you have not altered the azimuth movement. 

(More on that later).

[Chefgage]

Part 5.

The next part of the push to system was to install either a setting circle or a digital rotary encoder. I liked the idea of having two digital gauges for both the alt reading and az reading. So a digital rotary encoder it was

A bit of googling showed a digital rotary encoder with a remote display. This was ordered and then the method of connecting the shaft of the encoder to the mount was investigated. I have seem some people have drilled the centre bolt and then using a grub screw conneted the shaft this way. Having used encoders at work i knew you can get small flexable couplings ideal for this situation.

I first replaced the centre bolt of base with a 14mm setscrew. This setscrew is longer that the existing 12mm bolt to allow the connection of the coupling. The 14mm setscrew also stops any lateral play caused by the hole through the base board being bigger that the existing 12mm bolt. This setscrew was locked in place by a 14mm half nut (half nut so the lazy susan bearing still works and does not get stuck due to the height of the nut).

 

 

I filed down the last bit of the setscrew to allow the connection of the coupling. The coupling is a 12mm to 6mm coupling (6mm to fit the rotary encoder).

The top board was then fitted and a nylock nut and washer was used to secure the bases.

 

[Chefgage]

Part 6.

Now it was time to fit the coupling and encoder and then check there was enough clearance under the scope.

 

Only just enough clearance!!

A bracket was then made so that as the mount is turned the shaft of the encoder turns as well.  This bracket was made from a strip of thin steel (it started off as a joist hanger  ).

 

 

The scope was then replaced and the remote display was connected via the supplied cable.

 

The method of using the rotary encoder for the azimuth reading is basically the same as the altitude reading. Again first get a known star in the scopes field of view and note the azimuth reading of this star in the app (sky safari). Zero the rotary encoder. Then rotate the scope mount untill the reading on the rotary encoder matches the azimuth reading in sky safari.  Zero the gauge again, then rotate the scope mount back around to match the reading in sky safari and the known star should be in the field of view ( providing the altitude reading has not moved). This should now mean the push too system has been 'calibrated'.  For this type of encoder, after it has been calibrated you must always turn the base clockwise when moving to a new target. From a zero reading a turn clockwise counts up in degrees, a turn anticlockwise counts down but not from 360 degrees. So if you turn anticlockwise you will end up at the wrong reading. This is why i never seemed to get to the correct position when looking for tatgrts after the calibration.

[Chefgage]

Part 7.

For this push too system to work the scopes base mount must be level. A bubble level gauge will show if the base is level.

 

As you can see this mount is not level

As a starting point i was just going to try and find a level part of my driveway (where i do my observing).

[Chefgage]

Part 8.

First light with the new push too system

I noticed on clear skies that last night was going to be fairly cloud free. So i thought i would give the push too system a spin!

I took the scope out side at around 23:30 and tried to find a level surface where i normally observe, conclusion - not much level surface on my driveway

Anyway it was levelish. I have been wanting to see M57 - the ring nebula so that was my target for that night.  Using Vega as my known star (it was the only star in the constillation of lyra that i could see!!!!) I calibrated the push too system as decribed in part 4 and 6. On sky safari i searched for M57 and it gave me the current altitude and azimuth degree readings. I moved the scope to these readings. I must add that a wide field of view eye piece is used at this point (25mm).  I could not make out the M57 in the field of view but by moving the scope slightly in the azimuth direction i saw a fuzy star. I centered this and then swopped to a higher magnication eye piece 10mm and by using averted vision there it was  !!!!

I then put the 25mm eye piece back in and randomly moved the scope about, the idea being to see if i could find M57 again. By using the new current alt and az readings from sky safari and by slighltly moveing in the az direction i found M57 again. I did this a couple of more times just to be sure.

 

 

 

 

[Chefgage]

Part 9.

Conclusions of the push too system.

1.  The mount needs to be level. Where i had it, by turning the mount through 360 degress there was a change in altitude reading of 1.5 degress - not good  I need to come up with a method of leveling the base.

2.  The wixey gauge display when lit up is very very bright green, a red filter needs to be fitted so as not to destroy your night vision

3.  The rotary encoders digital display does not have a lit up display so a red torch was needed every time i wanted to see the display reading. This needs sorting by somehow lighting up the display. The display also needs moving up to where the wixey gauge is so you can see both gauges at the same time. Not being able to see both gauges makes it difficult.

4.  More testing is required. The clouds started to roll in so i had to stop. I did notice however that when trying to move to a different target the azimuth reading on sky safari did not seem to match up with where the object was physically in the sky, it was out by about 30 degress. I seemed to have to take the app azimuth reading from 360 degrees to give me my gauge azimuth reading. It was only after going in i realised that i use two different apps (just for accuracy). The other app i use uses the azimuth degrees as 0 to 180 degrees and then -180 degrees back to 0.  So i may have been using this app when the confusion set in. More testing required using just sky safari which uses 0 360 degrees for the azimuth.

 

[Chefgage]

Part 10.

I am going to start addressing the points made in the conclusions in Part 9. 

1.  I have seen some brackets that will accept a threaded rod. With some sort of feet on the end of the rod and a small handle this will allow me to level the base/mount. I think three of these feet (the number like what comes as standard) should do the trick.

Edit - I have found an easier way. I have used the plastic shims that are used for packing out door frames. They come in various thickness from 1mm to 5mm. A couple of these under two of the feet of the mount and the base was level. Took less than a minute to do

4.  After a bit of daylight testing I have know understood the problem of the azimuth reading being out by quite a few degrees. This stems from the fact that the azimuth encoder from the zero position reads 0 to 360 when turned clockwise and 0 to -360 when turned anticlockwise. So if you turn the base from the zero position anticlockwise it will read as follows : 0'  -1'   -2'   -3'   Etc...

This cause a problem as it should go 0'  -359' -358'   Etc...

The way to get round this is to always turn the base in a clockwise direction. As obviously azimuth degrees increase from 0' to 360' clockwise. If you need to push to an azimuth reading that is anticlockwise past the zero point then this would mess up the alignment/claibration that was done on a known star. SO ALWAYS turn the base clockwise

[Chefgage]

I am very happy with the modifications so far. With the terrible light pollution in my area (on the light poliution maps i am at the top of the scale!!)  I am able to find many DSO's that I never thought would be possible. There is an interesting point i found out when using the azimuth encoder (see point 4 above).  If you remember this point about alaways turning clockwise it works spot on!!

[wookie1965]

Well done some great modifications, anything that increases accuracy has to be good. 

[Chefgage]

10 minutes ago, wookie1965 said:

Well done some great modifications, anything that increases accuracy has to be good. 

Thanks.  Tomorrow night is looking good cloud wise. Hopefully get back out there with it

[JOC]

On ‎13‎/‎07‎/‎2017 at 09:57, Chefgage said:

To use the wixey gauge the procedure is as follows.  Use the finderscopes to get a known star such as vega in the fov of the scope. Then use an app such as sky safari to get the current altitude reading of that star. The zero button on the wixey is then pressed to zero the gauge. Then move the scope downwards untill the reading on the gauge matches the altitude reading of the star in sky safari. The gauge is again zero'd. This then means the gauge is zero'd to the horizon, i.e. Zero degrees.

I read it 5 times and it finally makes sense - penny has dropped as to why this works, I felt a right thicko when it first dawned on me LOL!

[Chefgage]

Glad it makes sense I have used this method a few times now and it works well.

[JOC]

The more I read it, the more it seems important to have the telescope base level.  As I think about it I believe this needs to be the case even for the Goto system like mine - it's the one bit of the whole setup that I don't think is given enough attention.  I get that the scope might do a 2 star calibration, but my guess is that even this approach doesn't take account of the orientation of the telescope base itself - do you think that a level base is even important for the Goto system?  If so then I'm certainly going to get a bubble level for mine.

[brantuk]

Nicely modified scope - welcome to the "dob mods" club.

[Chefgage]

29 minutes ago, JOC said:

The more I read it, the more it seems important to have the telescope base level.  As I think about it I believe this needs to be the case even for the Goto system like mine - it's the one bit of the whole setup that I don't think is given enough attention.  I get that the scope might do a 2 star calibration, but my guess is that even this approach doesn't take account of the orientation of the telescope base itself - do you think that a level base is even important for the Goto system?  If so then I'm certainly going to get a bubble level for mine.

I would say so. I found that when my base was not level i.e. just placed on the driveway the wixey gauge read 1.5 degrees difference through it turning 180 degrees.