Probably stupid question about goto

[burningbarricade]

Hello.

I'm completly new to astronomy and I've got a new telescope with goto but so far have only had one clear night where I could have a go at aligning it for the first time. I had to give up after a while coz it got too cold and I was having problems getting a clear sight at some of the stars it was attempting to align with - I was using the 2 brightest stars method.

I've since done my homework whilst waiting for a clear night and I would rather use two stars which I have picked myself to align the scope. My question is: when I was using 2 brightest stars method of alignment, where the scope suggests the two brightest stars in the sky, it would invariably pick 2 stars which were in completely different parts of the sky. Is there a reason for this or can I pick 2 stars which are in a similar direction as the 2 stars, for example Deneb & Vega?

cheers

[Psychobilly]

The larger the separation ideally in both Dec and RA (or ALt and Az) the more accurate the alignment should be...

I would be inclined to go with the ones the scope controller is suggesting unless as you say one of the stars is obscured in which case you can normally select an alternate.

If your manually picking them then good separation in both "axes" should be OK.

Billy...

[ottUp]

You should pick 2 (or 3) stars that are fairly far apart if possible. Two reasons:

1. Your centering of the star in the eyepiece will not be perfect and, the closer the two stars are together, the bigger your error is as a percentage of the total distance sampled. So stars farther apart will give you a more accurate model for calibration; and

2. Most telescopes have something in them that shifts position when the scope crosses the meridian (the line from the North Star, passing through "straight up", to due South). i.e. if the scope is "leaning to the left", then later is "leaning to the right", it's common for components like the mirror assembly to shift position slightly. This shift will change the position of observed objects. For this reason, it's a good practice to ensure you have at least one alignment star on each side of the meridian, and the goto computer can then calculate what shifted and allow for it. I would bet that the "automatic selection" option picked stars on opposite sides of the meridian.

I, too, prefer to use the option where I pick the alignment stars. This lets me pick stars I'm sure I can identify, and in parts of the sky I can see. The automatic feature often picked stars behind my house, or whose identity I wasn't sure of.

Regards

Richard

[Zaphod_B]

Hi burningbarricade

Which GoTo are you using? It might help to know as some are "quirky"!

A few general points on alignment:

[list type=decimal]

[li]If using a GoTo on battery power, be aware that the GoTo's are quite sensitive to voltage drops. The scope slews to opposite sides of the sky (or seems like) and the voltage given by the batteries can drop a little making alignment problematic. Best to use a 12V mains adapter or 12V Power Tank rather than battrries. Also best to avoid rechargeables (such as NiMH and Ni-Cd) as they only have 1.2v per cell instead of the 1.5V for alkaline cells. With 8x alkaline cells you are getting 12V. With 8x NiMH or Ni-Cd's you are only getting 9.6V to start with![/li]

[li]Some scopes are "fussy" for want of a better term as to how the final approach to the alignment stars is made. On Celestron GoTo's the manual suggests approaching the target stars from below and the left (in other words the telescope OTA moves up and right for "fine-tuning" of the alignment stars).[/li]

[li]Make sure that the alignment stars are NOT low-down and near to the horizon but are about mid-way to zenith. Similarly, don't go for alignment stars too close to zenith either. Best to have alignment stars that are 90 degrees or more apart.[/li]

Hope these points are of some help.

Zaph