Seem to have a weird issue with EQ5 Pro Goto accuracy

[acharris77]

Hi all I need some advice. I am having a weird issue with my EQ5 Pro Goto accuracy. I polar align and then begin a 1 or 2 star alignment. I use the first star as Vega and select Enif as the second as I tried a 2-star alignment tonight and I use a 25 mm eyepiece BTW.

The scope slewed from the home position to Vega and it got close-ish and I brought it into the eyepiece using the handset and centered it. Great, I then went to Enif and the star was in the eyepiece, but off center so I centered it and alignment successful. Great.

So I send the scope back to Vega and switch from the eyepiece to my DSLR. The star is middle. Send it to Alkaid as a test, star still center, send it to M57 appears in the middle of the live view. Send it to M31 or M45 and M31 is bottom right in the live view and M45 appears only showing 2 stars. Slew back to M57 bang middle.

IS this normal with the goto and SLR and are there any tips to help make the goto more accurate. Also no matter how good I get my polar alignment and tripod level, I still get slight trailing/odd shaped stars on 60 sec exposures. Could the mount be slipping and that is why the goto off and my stars are odd shaped/trailing?

Any help is appreciated. Thanks.

[jambouk]

Anthony,

This all sounds reasonably normal given what you have described.

GOTO:

Your two alignment stars are pretty similar in terms of azimuth and altitude, which is fine if you then want to only GOTO in that area of sky, but M45 is miles away, so so I wouldn't be surprised if the GOTO wasn't that accurate for that target. To get your GOTO as accurate as possible for the widest area of sky, you need alignment stars which are much further apart, and ideally pick three star alignment to get the maximum coverage of sky. I always imagine drawing a line between the three alignment stars, a triangle; say for arguments sake you picked the three stars of the summer triangle as the three alignment stars. The area inside your triangle will have pretty good GOTO accuracy, but a long way outside of that triangle your GOTO accuracy will be poorer. So when picking three alignment stars pick them over a large area of sky, and ideally have the advanced filter turned on in the handset so the handset picks the best three star combination. So this is easy to fix!

Unguided subs, limit of 60 seconds:

60 seconds sounds reasonable for unguided subs at your focal length with an EQ5. There are many reasons why you are getting trailing including:

- poor PA (which can be improved)

- periodic errors in the mechanism of the mount (which are difficult to improve)

- wind and movement (fewer beans and don't dance around the mount when imaging)

- poor power supply (not sure how common this actually is, but relatively easy to resolve)

- and others I'm sure

I think the easiest way to improve your chances of getting the longest possible unguided subs, is to improve you initial polar alignment. Are you just using the polar scope to align at present? If so, what version of the firmware on your Synscan handset do you have? If something like 3.35 or higher, there is a polar alignment routine on the handset which will allow you to make much finder adjustments to get a tighter polar alignment. There are various threads about the handset polar alignment routine on SGL, so just a case of hunting for them. If your handset firmware is 3.27 or so, then you'll need to update the handset firmware as these older versions don't have it (it is an option in the alignment option on the handset); there are various threads on SGL about how to update the firmware on your handset too.

But, you have to be realistic. A 60 second unguided sub is pretty good in my opinion, but then I have a very long focal length and struggle to get 30 seconds unguided. I think if you can consistently get 60 second unguided subs you will be doing very well, but you should be aware that any periodic errors in the mount will result in an occasional dodgy sub or two, there is little you can do about this I suspect, other than accept out of every eight 60 second subs you will have two that will not be usable. You could switch to shorter subs I guess, else look at guiding... Again, lots of threads on SGL about periodic error if you are interested in reading about it, but in my opinion I would just accept it is going to be there and concentrate on the things you can make a big impact on, which is better star alignment and better polar alignment.

James

[jambouk]

The other thing I meant to say is:

For polar alignment of an equatorial mount, it makes no difference if you level the mount of not, as your adjustments in azimuth and elevation when polar aligning bring the axis of the RA into "perfect" [in theory] alignment with the axis of the north and south celestial poles, so in theory it wouldn't matter if your mount was at 45 degrees to the ground, you could still achieve perfect polar alignment.

And, there is little relationship between polar alignment and GOTO alignment. You can have perfect polar alignment and still get awful GOTO alignment, and vice versa. The key is if you want to be imaging, you need your polar alignment to be as good as possible as this is going to affect how well the mount and telescope tracks the object you are looking at; your GOTO alignment just makes finding things much easier but has no impact on how well the system tracks the target once you've found it.

It sounds like you are doing very well. Looking forward to seeing your images.

James

[Owmuchonomy]

Great advice from James.  If you want imaging accuracy then you need to concentrate on your polar alignment following a 2 star alignment on the SAME side of the meridian as your target.  If you are more concerned about GOTO accuracy, as said you need the 3 star alignment which attempts to reduce any cone error in your setup.  60 sec unguided subs is pretty good, believe me.

[acharris77]

Thanks all. Yeah I am happy with 60 secs subs unguided, but still get some strange shape in my stars so need to improve my PA. It is the goto that I wanted more accurate, as trying to position M31 by moving the scope and firing off 5 sec subs to see where I am is a little tricky. I will try a 3 star alignment and using the advice given above of using the wide area of the sky instead of close starts and try it. Also I got the V4 handset and software and a different polar scope with my EQ5 pro.

I have uploaded a few images already in the Deep Sky imaging section.

Thanks again.

[jambouk]

When framing with the DSLR on the scope, whack the ISO right up to the max, then just take a short sub, and you'll hopefully see if the object is in the FoV or not and how well framed it is; will save you a bit of time than waiting for a 5 second sub to complete at ISO 400.

Do give the handset polar alignment routine a go, you'll be impressed at how tight it gets it.

James

[acharris77]

That is a very useful tip thanks James.

This is my tracking on a 80 secs on a single, unprocessed sub taken last night as a test. It was a bit windy so probably not helped but am quite happy even though the stars are slightly out of shape I am thinking which I don't know is cause errors in the tracking or something else.

Thanks.

[dph1nm]

If you have trailing in the RA direction the problem is almost certainly periodic error. If it is in the dec direction then it is almost certainly PA.

NigelM

[Stu]

That is a very useful tip thanks James.

This is my tracking on a 80 secs on a single, unprocessed sub taken last night as a test. It was a bit windy so probably not helped but am quite happy even though the stars are slightly out of shape I am thinking which I don't know is cause errors in the tracking or something else.

Thanks.

That looks like coma to me, rather than a guiding error. The stars are smeared away from the centre like little comets.

[jambouk]

Dis you get it all sorted?

James

[jambouk]

Dis you get it all sorted?

James