Stellarium - Can someone check something for me please?

[swag72]

Can anyone who is using Stellarium WITHOUT EQMOD or Stellarium scope (just via the USB / R232 adaptor) and is guiding, please check something for me.

When you goto an object and guide, if you zoom in really close, does the telescope cross hairs move away from the object or stay put on it?

Hope someone can check this out - Another night where it's all going wrong.

[chrisshillito]

I don't regularly use Stellarium and always use EQMOD but non the less it seems quite logical to me that the cross hairs would move away from the object when guiding.

You have to ask yourself why guide - clearly we do this because simply tracking isn't good enough - If a guider is making corrections it is doing so because basic tracking is not holding the telescope on track - any correction to compensate for this will inevitably move the "cross hairs" away form the object in stellarium.

I suspect you simply have a polar alignment error and as a result of this you are experiencing a constant drift in RA and/or DEC. The guider corrects for this drift but this does involve moving the mount away from the objects true RA/DEC coords and that is what you see in stellarium.

All you need stellarium for is getting your mount to point at the object initially (i.e the goto)- once you start guiding the object will be tracked and so long is it keeps in frame it really doesn't matter what stellarium shows.

Chris.

[swag72]

You are saying that the cross hairs will move from the object that I goto? I would expect guiding to keep it in the same place.

I think that my polar alignment is fine. My mount is left up permanently at the moment and my PHD graph shows no sign of drift. I also do use PHD to check my polar alignment and it is bang on.

Here's an example of my problem - Sorry abput the poor photo, but I can't do a screen grab as it's on the astro lappy! So had to take a pic in the dark!! As you can see, the Dec line is causing me no issues, but the RA shoots off for no apparent reason. Would this explain the movement in Stellarium?

[chrisshillito]

You are saying that the cross hairs will move from the object that I goto? I would expect guiding to keep it in the same place.

The whole point of guiding is to move the mount at a non-sidereal rate to correct for drifts and periodic error. If the mount moves at a non sidereal rate stellarum will show it. What stellarium show really doesn't matter, its what your guide scope and imaging camera are seeing that matters.

Please note that Polar alignment errors only affect DEC (for some reason many folks have that impression). The relative amounts of RA and DEC drift vary depending where in the sky your mount is pointing.

Did PHD manage to track your guide star? If it did then your object should be kept in frame irrespective of what stellarium shows. If PHD tracked OH but your imaging cam didn't show the object then that's probably just means your star alignment needs improvement in that area of the sky (i.e. the goto wasn't accurate)

From the PHD graph there is certainly what looks like a constant RA drift present though I can't make out the time frame or magnitudes involved. If you feel the need to diagnose this then try tracking stars in different areas of the sky (remember to recalibrate PHD each time) and see if the rate of drift remains constant. If it does then it looks as though there is some underlying of tracking issue.

Chris.

[lukebl]

It's worth remembering that the variable density of the atmosphere from horizon to zenith means that the sidereal rate isn't actually constant anyway. At the horizon, atmospheric distortion results in the stars being at least half a degree away from their 'true' position. Guiding goes at least some of the way towards counteracting this effect, and results in the cross-hairs on Stellarium to move.

Having said that, I use Carte du Ciel, and suggest that you try it. I think is a much better and more 'serious' program compared to Stellarium. Stellarium tries far too hard to be 'pretty' at the expense of computer memory and technical features.

Not sure about your PHD issues, though.

[martin_h]

I'd agree with the above...goto only gets you where you want to be, it doesn't effect tracking...I'm imaging the iris neb at the moment, I use stellarium to get there and when guiding close stellarium down as I dont need it anymore.

[swag72]

The guiding (when it is working) is fine - I have been doing 9 min subs with no problems. So I will stop worrying about Stellarium!!

Something must be wrong in the goto then and in my star alignment, but I can't work out what. For example, I do a 2 star alignment at the moment using Arcturus and Capella as they are what I can easily see. Alignment is successful and when I goto something easily recognisable using Stellarium for example, such as the moon at the moment, it slews no problem and is pretty accurate.

I just can not work out why the PHD graph goes as it does sometimes.

[chrisshillito]

The guiding (when it is working) is fine - I have been doing 9 min subs with no problems. So I will stop worrying about Stellarium!!

Something must be wrong in the goto then and in my star alignment, but I can't work out what. For example, I do a 2 star alignment at the moment using Arcturus and Capella as they are what I can easily see. Alignment is successful and when I goto something easily recognisable using Stellarium for example, such as the moon at the moment, it slews no problem and is pretty accurate.

I just can not work out why the PHD graph goes as it does sometimes.

To be honest you're probably better of choosing and alignment star based upon its proximity to your target rather than simply going for brightness - all the alignment stars the synscan uses are bright enough to be clearly identifiable in your finder scope.

The PHD graph is just telling you the guide star appears to be moving - this could be because of one or several of the following:

1. polar alignment error

2. poor tracking (lunar rate used rather than sidereal etc.)

3. periodic error

4. Your mount is moving - slipping clutches, (or sinking observatory )

5. The guidescope is moving with respect to the mount (flexure)

6. The guide camera is moving (flexure, rotation)

Of course its best to minimise these effects if you possibly can. However they not uncommon, everyone experiences them to some extent and they are a natural part of the challenge that is astrophotography (it isn't supposed to be easy!). The first four are the reason why folks autoguide in the first place, the last two compromise guiding.

Chris.