Improving PA and GOTO - the easy way?

[matsey]

Sent from my iPhone using Tapatalk

[perfrej]

OK, you set up every night and need to get decent polar alignment. Again, start by defining what is acceptable. The answer lies in your field rotation acceptance, nothing else. You may be surprised to find that you really don't need to be that accurate, in which case the time spent on reaching a certain level of accuracy in arc-minutes may be wasted. Also, when you discover that you only need to be accurate to, say, 20 arc-minutes of the pole, a simpler way to align may be sufficient. How accurate do you want to be?

/per

[Demonperformer]

Ah, I see what you are saying. The basic answer is, at the moment, I don't know.

I definitely need something to get it close (simply lining the tripod post with the 5th fence-post, by line-of-sight will not be sufficient ). The obvious thing to use is the polarscope, but I have found that to be a total pain to use - when I can see anything through it at all. Failing that, Merlin's method looked really simple, but he has now stated it is flawed. I tried AT and that gave me such screwy results that no one has ever managed to explain them. Hence, I am now going to try Alignmaster.

As I currently understand it, Alignmaster requires two slews, centre two stars with EQ controls and centre two stars with Altaz bolts.  Which I reckon should take me somewhere between 2-5 minutes; at which it will not be a major time investment. Then I will see what sort of images I get. (Typically, the weather has been awful since purchasing it.)

The reports on the astronomy shed video are that people trying Alignmaster, by checking it against obsy systems that have been carefully drift-aligned, say that it is spot-on. I suspect that, even if there is some exaggeration in those reports, the alignment will be close enough, such that the limiting factor in any of my subs will be the effects of "streetlight alley" through which I have to shoot, rather than alignment ... unless I take some serious steps towards 30-min-sub-NB imaging, but that's another story.

As I say, this is all pretty much speculation atm, but if it works out like this in practice, I won't be unhappy (although I would always be willing to have a look at something shown to be simpler & faster).

Thanks.

[perfrej]

A guess would be that you should get it to 10 arc-minutes or so. I ran my remote obs on just under a degree of PA error for a few mounts due to misaligned holes in the pier... Round stars but serious field rotation between first and last sub of the night, individual subs OK.

There are field rotation calculators on the net you can try.

/per

[Demonperformer]

Thanks for that.

I used this one: http://celestialwonders.com/tools/rotationMaxErrorCalc.html, and found the worst-case star trails on my system for 10' PA error and a 300 second exposure. Bearing in mind that the error goes up exponentially as you approach Dec=90, I settled for Dec=89.0 and here the trails would be 11.67μ, which (@ my camera's 6.4μ px^-1) works out at just under 2 pixels (each sub). Over a 2-hour imaging run, this compounds to 280.13μ which would be between 43 & 44 pixels. By the time I drop to Dec=85.0, the error is only 1/5 of these figures.

What these figures would look like in practice, I'm afraid I have no idea.

Thanks.

EDIT: just had another play, thinking about it from the other side. With a 10' error, and a 30 min sub (v. unlikely to try anything longer), I would have a maximum trail of 3.19μ (~0.5px) at an declination of 67.5^o (86.35^o for a 5 min sub). So anything with a smaller declination would have a smaller trail. I guess half a pixel is a reasonable estimate for "acceptable"?

So many variables to consider ...

[ChrisLX200]

Maybe this new gizmo from QHY would suit you... http://qhyccd.com/PoleMaster.html#PoleMaster

ChrisH

[Demonperformer]

Thanks, Chris. That certainly looks like an interesting item subject to:

* How is it "installed on front of the RA axis"? - it will presumably only be as accurate as this attachment.

* What sort of price is it? - does not appear to be on the MA website yet.

[ChrisLX200]

Thanks, Chris. That certainly looks like an interesting item subject to:

* How is it "installed on front of the RA axis"? - it will presumably only be as accurate as this attachment.

* What sort of price is it? - does not appear to be on the MA website yet.

Well no it's not dependant on accuracy of installation, the trick is that you rotate the RA axis and you get an offset either side - it (somehow) works out from the star positions where the true pole is from that. No idea on the price, not even sure whether it is widely available yet. I saw it discussed on another forum..

ChrisH

[Demonperformer]

Interesting.

Should therefore be simple to use.

It only has to solve the field around the pole, so the calculations shouldn't be too difficult - Polaris should be the brightest object in the field, so that narrows down the possibilities. That simplicity might have an impact on price (or might just push up the profit margin), but I can't see it being less than 3 figures (based on the prices of things like skyscout).

Still, interesting ...

Thanks.

[Davey-T]

The big drawback to achieving "perfect PA" on most "cheap" mounts is the inability to make very small adjustments.

I have also got different reported PA errors after no alterations so I don't think it's worth getting too OCD about it

Dave

[rwg]

James, DP and all other members,

This process does not work!

It is flawed, and comes from my limited early trials.

I've recently relocated my observatory and instead of using the usual drift method to set-up the mount I went back to this process.

I couldn't get it to work for me.

In hindsight it relied on a "rotation" about the first synced point which doesn't happen in practise.

My apologies to the many members who may have spent frustrating time trying to make it work.

Sorry for the confusion.

I've asked the Mods to close and remove this thread to prevent future frustration......

Try moving the second star half the distance to the correct position using the Alt/Az adjusters - I think that will work much better (ie work). If you adjust the whole way then when you go back to your original star you will be out by the same amount. By adjusting half way you should home in on correct alignment, probably after several repetitions.

Robin.

PS. why zap the thread - as long as it's clear that the process doesn't work then there's a record of an idea and a discussion of why it's no good. Hiding that will mean that the same mistake is more likely to be repeated in future...

[Demonperformer]

Maybe this new gizmo from QHY would suit you... http://qhyccd.com/PoleMaster.html#PoleMaster

ChrisH

Been chasing info on this ...

The results on the attached screenprint seem to be quite impressive (if I am reading it correctly) ... 1.5" error in a 30 min sub without guiding on an EQ6! Which means (I guess) that the alignment is within 36" of the pole? Not bad with the coarse alignment adjustors on the EQ6.

There is talk of a price of $199 (+ connector).

But I liked one of the subsequent comments ...

Uk price? $199 = £129.91

Or will it be a case of drop the $ sign and replace with £...... ???