The effect of PA on PHD's performance

[Tiki]

Does anyone have an idea of how effective  PHD is whilst having to cope with less than perfect PA?

[lensman57]

work provided the balance is spot onDoes anyone have an idea of how effective  PHD is whilst having to cope with less than perfect PA?

Hi,

Depends on how imperfect your PA alignment is. If you are out by 20~30s it will still work provided that the balance is spot on but it will not correct massive misalignment. It was never designed to do so.

A.G

[Daniel-K]

Always best to get it close as possible but it doesn't have to be perfect. Ive done 10min subs on rough PA

[Leveye]

I have no clear view of Polaris from my main imaging site after a basic align and level I use a one star align and then a multi star alignment in my mounts HC controller menu and then back to zero position and then after getting close to my object with CDC let PHD take over I get perfect 3-5 minute subs most of the time. Balance is key you must nail it. one thing you will see is that the RA and DEC lines in the graph will not be on the same line while PHD guides if you do not polar align perfectly this is normal your pictures will still be great.

[ollypenrice]

For shorter exposures (5 to 10 min) an imperfect PA can be an advantage in two ways. It can provide natural dither via mild field rotation and it can allow your autoguider to correct in one direction only. Disable the the other direction (you do have to do this) and you can do away with oscillation through over correction.

All of the above is witchcraft. You hear it said and it does work when it wants to, but given the fact that the same setup with the same settings works one night and doesn't work the next means that I remain a pragmatist. Faff around till it works and don't over emphasize the value of last night.

Olly

[lensman57]

For shorter exposures (5 to 10 min) an imperfect PA can be an advantage in two ways. It can provide natural dither via mild field rotation and it can allow your autoguider to correct in one direction only. Disable the the other direction (you do have to do this) and you can do away with oscillation through over correction.

All of the above is witchcraft. You hear it said and it does work when it wants to, but given the fact that the same setup with the same settings works one night and doesn't work the next means that I remain a pragmatist. Faff around till it works and don't over emphasize the value of last night.

Olly

Olly is spot on, a few nights ago after a whole day of raining I set up to do a quick run of NGC 6946 before the next batch of clouds and rain arrived. The PA errors were 48" in one axis and 36" in the other if my memory serves me correctly, the mount was behaving nicely while I was out once  I returned to the set up to keep an eye on the clouds, the PHD graph had gone through a crazy period. Nothing loose or had moved just random behaviour. I attach two different images of the graph taken at two different times, notice the departure of the DEC plot from the RA in the 2nd capture, no reason but it just went back to normal. BTW, I do these screen captures just incase of some serious error so that I can ask questions later.

Regards,

A.G

[orion69]

PA error under 5 arcmin is OK, PHD should work without problems if everything else is fine.

PA under 2 arcmin is excellent.

Always polar align best you can.

@lensman57

What mount do you have? How did you measure that PA error under 1 arcmin?

[lensman57]

PA error under 5 arcmin is OK, PHD should work without problems if everything else is fine.

PA under 2 arcmin is excellent.

Always polar align best you can.

@lensman57

What mount do you have? How did you measure that PA error under 1 arcmin?

Hi,

It is an HEQ5 PRO, the errors are reported by the handset after each 2 or 3 star alignment and a polar align routine, I used to fuss about that but now I only do two cycles, so long as it is under 1 minute then I leave it alone, the problem is that I set up in the small back garden and depending on which part of the sky I am imaging I set up in a different position every time.

A.G

[orion69]

It's most certainly not under 1 arcmin, unless you are very,very lucky. 

Those values are not actual PA error.

[lensman57]

It's most certainly not under 1 arcmin, unless you are very,very lucky. 

Those values are not actual PA error.

I used to get the same values and graph shape with my now sold EQ5 PRO, whatever it is I will keep on to it I think.

A.G

[Pete Presland]

the best guided scope is the least guided scope! get the polar alignment as good as you can, do some drift alignment if you can and let PHD do the rest.

[Sam]

the best guided scope is the least guided scope! get the polar alignment as good as you can, do some drift alignment if you can and let PHD do the rest.

This advice is the key. The more time you spend getting the polar alignment right the less subs you'll end up throwing away and the sharper images you'll end up with. It's so true that the least amount of work your mount has to do the better the guiding will be.