Wedge testing

[Simon E]

Setup the other night the SCT on it's new wedge.

Aligned the wedge/scope pointing directly to Polaris. And then did a EQ Two star align on Mizar (The plough) and Rigel (Orion).

Once setup tried to find some deepsky objects, found the Horsehead first time and found Mars first time as well.

So all in all the wedge was worth it and very easy to setup and align.

I would totally recommend one.

Now all i've got to do is attach the Skywatcher to the Celestron and start tracking to see if the wedge makes much of a difference when taking photos.

Simon

[Mac]

what method did you use to align the wedge?

Mac.

[Simon E]

Connected the wedge to the field tripod and then put the scope onto the wedge. I then tilted the scope so it was in line with the arm.


So the tube was pointing the same way and parellel to the Nexstar arm, unlike the picture above. I then adjusted the angle of the wedge so polaris ended up in the middle of the field of view of the scope. I had to move the tripod around a bit but eventually got it dead centre in the middle of the scope. You must remember not to touch the scope though.

I then selected the EQ North 2 star alignment procedure on the head control and proceded to align the scope this way.

It all aligned correctly and off i went. Easy

si

[MarkS]

It all aligned correctly and off i went. Easy

You may find this isn't accurate enough for long exposure imaging (e.g. 5 minutes) so you might end up refining your wedge alignment procedure a little.

Mark

[Simon E]

What's the best way mark? I was just following the manual.

si

[Mac]

simple.

1) select one star and use the goto on the controller to goto it.  (make sure it is at least 2h 30mins away from polaris)
Use the telescope controls to get the star in the middle of the eye piece,
once the star is in the middle use the handset to sync on that star.

2) using goto, select polaris and goto it.
Trust me when i say it wont be in the middle of the eye piece, unless you have some of the luck of Mr Mark.
using the adjustments on your wedge NOT THE HAND CONTROLLER!!!!!!!!!!!!!!  move polaris towards the middle of the eyepiece, reducing the error by about 1/2 to 2/3.
Dont move it to the middle in one go, just reduce the amount that it is off by... the error in alignment will then decrease on each itteration, untill polaris is bang in the middle.

3) repeat from 1), the error in alignment will get smaller and smaller, until polaris is bang in the centre. this will probably take 4-5 iterations.

Using this method will allow you to take long exposures with the minimal of guiding.

see also.
http://www.astrocruise.com/articles/polalign/polarold.htm
http://www.allaboutastro.com/Articlepages/iterativemethod.html
http://shadycrypt.com/pages/Polar/Polar.htm

[Simon E]

Thanks Mac, I will have a go when the stars come out. What sort of exposure lengths do you reckon i can get with this mount, wedge aligned and good tracking via my guide scope.

si

[Mac]

What sort of exposure lengths do you reckon i can get with this mount

Depends on the length of the string.

Too many variables to say exactly.
but assuming the following.
Wedege is absolutely polar aligned.
Gears are 100% accurate,
internal speed of clock /  gears 100% then theoretically you should be able to guide 24/7/365 absolutely perfectly .

assume the following.
wedge is 99.5 % aligned. (using an illuminated 12mm eye piece and 4 * barlo ), this is probably the best you are going to get, unless you want to drift align for hours.
gears are pretty good / accurately made,
telescope / camera is stable, ect ect.

you should be able to get at least 5 mins, probably 10 mins at a push, before any trailing is noticed

Stick a star in the middle of the eye piece and leave it for 10 mins, see what it does, hopefully it wont move.
mine drifts 1/4 an eyepiece in an hour at X212.  10" meade 12mm eyepiece.

but i've got other problems

Mac.

[The Thing]

Hi Simon,

I have a Meade LX90 and a standard Meade wedge. The key point is the instructions tell you to point the scope at Polaris. This is not the celestial pole. I have made a plywood disc that sits in the hole in the centre of the wedge and has a central hole to fit which holds the Meade finder scope at 90deg to the wedge face. I use this to align the wedge after very careful levelling of the tripod using a bubble level (not the built in Meade one). I use Polar Finder (small app) to show me the offset and factor this in to what the finder scope is showing me.

When you then do an a alignment the Meade controller works out how badly you are polar aligned and then factors in compensation when you GoTo. This does not seem to apply as the scope tracks, so long exposure imaging is rubbish unless you do more polar alignment as mentioned below. Guiding can compensate for some drift but it's hard work for the fork style mounts, my Meade grumbles to itself as it tries to do the corrections!

[Carole]

Simon,

I am not as technical as the guys, but I have to say having experienced an Alt/Az Mount on my ETX (even using it with a wedge), and now having an Equatorial Mount with a polar scope and reticule to get the exact celestrial pole, I have found a huge difference in accuracy and thus alignment and tracking.  It was not even possible to find the true celestial pole on my alt/Az set up.  The guys kept telling me my mount was crap for long exposure imaging, but it wasn't until I got myself an equatorial Mount that I really understood what they were saying.

I know this is not particularly helpful to you, but just to let you know that you will find it a great challenge to get accurate alignment with an alt/az mount.  Having said that there are people who quite successfully image using a wedge, and hopefully with their advice you will get there.

Carole

[The Thing]

This year I'm saving up for a GEM.

[mickw]

If it helps, Telescope Planet as knocked £2000 off the price of a Paramount ME 

[Carole]

 
You are funny Mick

Carole

[Rocket Pooch]

Thanks Mac, I will have a go when the stars come out. What sort of exposure lengths do you reckon i can get with this mount, wedge aligned and good tracking via my guide scope.

si

Hi,

I used to have an LX90 and a wedge, upto 600mm scopes ED80 for example I could guide with no issues, I did many 10 and 20 minute subs.  They key is to polar align properly and guide through the LX90, I used to use a modded web cam and either a .5 focal reducer or .33 reducer.  I could swap the setup the other way around with the small scope as the guide scope and image at F3.3 with the Meade, I has lots of fun with this.

Having said that the day it all went on the EQ6 everything became very verr simple.

Chris