Guiding questions

[Guest]

I'm thinking about putting some guiding on my scope but I have a couple of questions.

1. Do you have to guide on the object you are observing or can you guide on anything providing it is in the field of view of the guidescope and camera? I'm thinking you might want to photograph say a nebula which is in the centre of the field but that might not be suitable for guiding on so you might want to pick a star in the same field of view but not in the middle.

2. I've seen reference to off-axis guiding. I can guess what it is but why would you want to do it? 

Cheers

Steve

 

[vlaiv]

1. Yes of course. Guiding works as long as you pick a guide star relatively close to the target (where relatively close is at most few degrees away from the target)

2. Off axis guiding is actually better way to guide than using guide scope for several reasons:

- you'll be guiding at same focal length as you will be imaging. There is no concern about required guide precision, you'll most definitely have enough guide precision. I actually bin my guide camera with OAG to increase sensitivity.

- you avoid differential flexure. Both cameras are attached to the same telescope and there simply is no chance of two scopes moving slightly differently since there are no two scopes. In addition, OAG fixes mirror flop (reflector scopes that suffer that sort of thing) as well as mount tracking issues.

- weight and mounting issues - OAG is lighter device and easier to attach to imaging rig. It is also often less expensive than guide scope.

There are some draw backs as well:

- focusing main telescope changes guider focus as well - this might not be drawback, depending on why you need to refocus. Filter changes sometimes trigger need for refocusing even when filters are parfocal (this happens on refractors), but upside is that you don't need perfectly focused star to guide on - sometimes little defocus in guide star actually helps as seeing influence is spread around

- You need to have plenty of backfocus in your system to allow for OAG - it is yet another component next to filter wheel, reducer, rotators, whatnot ....

- People sometimes complain that it's hard for them to find suitable guide star with OAG - that was not my experience so far, so I can't comment on that.

[Cornelius Varley]

Guiding is normally done with a suitable nearby star. An off-axis guider is a sort of diagonal. The off-axis part is a small prism which can be adjusted so pick up a guide star somewhere on the peripheral is the image.  Using an OAG  means you don't need a separate guidescope.

[Seelive]

1. No, you can theoretically guide on anything. The only factors are the accuracy of your polar alignment, the focal lengths of your imaging/guiding systems and the guiding accuracy of your chosen guide software (I wouldn't rely on accurately guiding on an object near 0 deg declination using Polaris as a guide star 😆).  Polar alignment errors will result in field rotation that will be exacerbated by the ratio of the imaging/guiding system focal lengths and the accuracy of the guide software. In normal circumstances, if the ratio of the imaging/guiding focal lengths is not too excessive (eg less than 5:1) and 'reasonable' exposure times are used (ie not hours) then it's unlikely to be a problem.

2 Looking at replies that have been posted whilst typing this, it has already been answered.

[vlaiv]

1 minute ago, Seelive said:

1. No, you can theoretically guide on anything. The only factors are the accuracy of your polar alignment, the focal lengths of your imaging/guiding systems and the guiding accuracy of your chosen guide software (I wouldn't rely on accurately guiding on an object near 0 deg declination using Polaris as a guide star 😆).  Polar alignment errors will result in field rotation that will be exacerbated by the ratio of the imaging/guiding system focal lengths and the accuracy of the guide software. In normal circumstances, if the ratio of the imaging/guiding focal lengths is not too excessive (eg less than 5:1) and 'reasonable' exposure times are used (ie not hours) then it's unlikely to be a problem.

This is not quite correct for at least two reasons.

First is, as you put it polar alignment. It will not only cause issues with field rotation if you don't choose near by guide star. Imagine that your guide star is 90 degrees off the main target. This is very similar situation to situation when someone is doing drift alignment.

Drift alignment is done at Meridian due south for azimuth component of PA error (if I'm not mistaken) and due East or West for Altitude component of PA error.

This means that if for example your PA error is only in azimuth and you image due south at meridian but your guide star is 90 degrees of - due West  - guide star will drift due to PA but main target will not. Guider will try to correct for guide star and it will move main target without real need.

Second issue that might happen is atmospheric refraction near horizon. In fact - each altitude has some level of atmospheric refraction and apparent star position is not the same as true star position. This difference increases as you near horizon.

As target is tracked - if guide star and target are at different altitudes - this change in apparent position will be different and there will be guide error due to this.

There is also issue of sensitivity. As you yourself pointed out - would there be any point in guiding on Polaris? Having guide star at significantly different declination to target creates this issue. At low declination RA movement is exaggerated. This is the reason we calibrate our guider system at DEC 0  - there RA movement is greatest per unit time - or rather RA movement projection on sensor is greater. If RA movement projection on sensor is small enough, guider might not react in time. This is not an issue if error is such that imaging system also does not see it as large error, but if guide system sees small error (guide star at high DEC) but imaging system sees large error (imaging at low DEC) - then there will be issue.

In principle you want your guide star to be relatively close to target for best performance.

[Seelive]

'In principle you want to your guide star to be relatively close to target for best performance'

To be honest, I think that was what I was alluding to.

[vlaiv]

Just now, Seelive said:

'In principle you want to your guide star to be relatively close to target for best performance'

To be honest, I think that was what I was alluding to.

I misunderstood you then - I thought that you meant that you can pretty much use any star to guide on when you said:

47 minutes ago, Seelive said:

No, you can theoretically guide on anything.

You probably meant - you can guide on "any object" - rather than "any position". Sorry about that.