Illuminated reticle measurements

[Davey-T]

I have an illuminated reticle which can be used for measuring stuff but don't know exactly what the graduations represent , it has a centre line marked from 0 to 50 with 25 at the centre, are these degrees ? and do they represent different sizes in different scopes ?

Sorry about the blurry picture

Dave

[Oddsocks]

It looks like a Meade MA 12mm Astrometric Eyepiece and it's main use is to measure the angular separation of double stars as well as angular relationships.

I use one to aid with mount polar drift alignment when I am working away from my home observatory.

The central linear scale is used to determine angular separation between objects, in arc seconds, arc minutes or degrees, depending on the focal length of the telescope being used.

In order to use the scale you have to calibrate the scale by converting the measured number of divisions according to the true focal length of the telescope which is not necessarily exactly what the scope's maker publishes!

Even if your eyepiece is not the Meade 12mm version the instructions for use and the calibration method is generic and can be applied to any type, you just need to adapt the linear scale accordingly.

The calibration is a simple affair though, you just point the scope at a bright star on the zenith and switch off the telescope drive.

At the zenith stars drift across the sky at 15 arc seconds per second so with a watch you time how long it takes the star to drift from one end of the linear scale to the other with the mount motors switched off, do this several times to get an accurate average.

If, for example, the star drifts the full length of the linear scale in ten seconds then the star has travelled 10 seconds at 15 arc sec per second = 150 arc sec, to determine the calibration of the linear scale divide the 150 arc seconds travelled by the full length linear scale which is 50, so each small division on the linear scale = 3 arc seconds.

You have to calibrate the linear scale when you move the eyepiece between scopes, or use it in combination with a barlow etc.

I have scanned and attached a pdf copy of the Meade instruction for you, I still had the original instruction sheet for it.

William.

Mead MA 12mm Astrometric Eyepiece.pdf

[Qualia]

If it helps, Dave, I wrote a little something about using the micro-guide a few years back. I haven't used it for a while, but it's good fun to practice with from time to time   

[Oddsocks]

Dave, Qualia

Please ignore the linear scale calibration method I wrote in post #2 as it is incorrect.

It is strange how you read something from a "respected" manufacturer and believe it must be correct, it was only when I read Qualia's micro-guide, digested that and thought about it overnight that I realised the published linear scale calibration method in the Meade document is garbage!

In the Meade document they state " Stars at or near the Zenith move across the across the sky at 15 arc sec per second (sidereal rate)" , well that may be correct if you are on the geographic equator and your Zenith coincides with the celestial equator but if you are anywhere else on the planet the star will drift through a different distance in a given time as seen through the eyepiece, if you were at the North Pole for instance and you measured the drift of a star at the Zenith you would have to wait hours before anything moved at all....

So the Meade drift calibration method does not work, as it is laid down, but I guess it would work if you pointed the telescope not at a star on the zenith but at a star on the celestial equator i.e. declination "0"?

The Meade document is useful in other details but the linear scale calibration method can be ignored unless you are living on the equator

Thank's to Qualia for this, otherwise I would have been believing the Meade stuff for who knows how long.

William.

[Davey-T]

Thanks Rob, that's really useful, been using it for drift aligning but wanted some meaningful measurement 

Yes William it's a Meade one but basically identical to the Baader Microguide, it didn't come with any instructions so thanks for those.

Dave