Aligning the finder to the scope axis......important or NOT !

[Ewan]

A bit of a read but as i like problem solving i thought i would share this.

To kick this off i will first say that i do square my finder to the scope or rather the RA & Dec directions, if the scope goes up / down then so does the guiding in phd & the same for left / right.

Now i did query this with another member & also trying to get the imaging target central on the ccd view & in PHD, neither of us could decide wether it improved things or not in regards to guiding.

I sent TingTing a msg regarding his new guide cam QHY5L-II & the fov compared to the DMK 21AU618 i use, as he has only used it twice he kindly sent me a pdf file on how to use his guide cam & in it i read this :-

'Alignment of the QHY5L-II axis with the Mount axis

If using a finderscope, it is a good idea to align the X/Y axis of the QHY5L-II square to the axis of the mount.

This can be done by turning on the coarse reticle and positioning the guidestar so it is at one of the

intersecting horizontal and vertical reticle lines in PHD2. You can the move the mount in RA and DEC and when

the QHY5L-II is correctly aligned, the star should track the line up/down and left/right. This can help reduce

guiding corrections sent to the mount.

Consider the case where the X/Y axis of the QHY5L-II is at 45 degrees to the RA/DEC axis of the mount. Any

star movement will cause both an RA and DEC correction to be sent to the mount. If aligned square, this will

probably result in either an RA or a DEC correction being sent to the mount. The less corrections sent to the

mount is often the best scenario.'

So can someone give a 100% confirmation it does or does not matter ? OR have you tried this & found it did make a differnece ?

I look forward to reading any of your answers & the reasoning behind them.

[ollypenrice]

I think it is best to have the guidescope orientated orthogonally.I always do this. I prefer a corrrection in one axis to be a correction in one axis, not a mixture of two. It is a less massy calculation and the mount is given more information. You can always try it and see if it matters. I never have because it strikes me as silly not to run the guider orthogonally.

Olly

[Ewan]

Agreed Olly, it's one of the things i do setup at the beginning as it only takes a few moments.

It would be nice to have the imaging testing time to trial & error a few things but like others when weather permits outside time it is for imaging not testing.

[tingting44]

interesting, my friend speaks a lot with the person that wrote EQMOD, he said he is going to ask him also, but yes it does make sense and be interesting to know how much of a difference this does make

[russellhq]

I've never thought about this when setting up, so it's unlikely I've ever had the guide scope square. All I can say is I've never had any problems guiding 15min subs.

It's probably one of those things that on paper makes sense though is hard to quantify, and when put in to practice, the effect is insignificant.

[chrisshillito]

One thing you have to consider is how well your autoguider software is able self-calibrate. If it can accurately measure the angle of the guidescope or camera then it can equally well compensate for it. So guidescope/camera angle shouldn't matter.

However, measuring angles isn't particularly easy to do accurately particularly if they are small. Floating point maths routines in the software can also through out a few funnies when it comes to trig. Also some guide cameras have non square pixels which further complicates things (I've never been convinced PHD is able to measure angles correctly when coupled with a non square pixel camera).

If you have a guidecam with non square pixels I think its definitely worth mounting it such that it is orthogonal with the mount. If you have square pixels then there may just be a theoretical advantage in having the guidescaope/camera at a 45 degree angle as a large angles is much easier to measure and process than a potentially very small one that might result if you try for an orthogonal setup.

This all said, from an operation point of view I much prefer to work with everything squared up.

Chris.

[Ewan]

One thing you have to consider is how well your autoguider software is able self-calibrate. If it can accurately measure the angle of the guidescope or camera then it can equally well compensate for it. So guidescope/camera angle shouldn't matter.

However, measuring angles isn't particularly easy to do accurately particularly if they are small. Floating point maths routines in the software can also through out a few funnies when it comes to trig. Also some guide cameras have non square pixels which further complicates things (I've never been convinced PHD is able to measure angles correctly when coupled with a non square pixel camera).

If you have a guidecam with non square pixels I think its definitely worth mounting it such that it is orthogonal with the mount. If you have square pixels then there may just be a theoretical advantage in having the guidescaope/camera at a 45 degree angle as a large angles is much easier to measure and process than a potentially very small one that might result if you try for an orthogonal setup.

This all said, from an operation point of view I much prefer to work with everything squared up.

Chris.

Thanks Chris for the feedback.

I had seen this mentioned before when i first started guiding & so so do it as part of the setup.

I have not done specific tests regarding this but thought it can't hurt, as a lot of things in AP have to be square, aligned & tight it made sense to look at this as well.

Thankyou all for the input so far.