Last nights testing of NCP alignment method

[jasperuk]

OK as there was a slight high level cloud cover last night I decided to forget the scope and just test the alignment theory which I can guarantee is probably wrong but I like to do things slightly different from everyone else

So last night I:

a) Set the mount up EQ3-2 and did the best I could to align the mount and the polar scope so that there was a little deviation when rotating through the RA 180deg but not much (scares me to death playing with the alignment scope trying to get it central having see stories about "don’t over tighten or you can crack the reticule" and "Don’t loosen too much or it will drop out" etc....)

Rough aligned the mount using the compass to north and level the little bubble, the DEC was pre set from another night so Polaris was already roughly in view.

c) Completely missed out the setting circles bit of the process as its very confusing and I just don’t get it at all ( also I was not actually doing any observing so I didn't think it mattered ).

d) Loaded Polarfinder on the netbook set up my location and the time and checked the position of Polaris in the chart http://home.online.no/~arnholm/org/zip/polar202.zip

e) Looked through the polar alignment scope and managed to get Polaris to roughly the right place on the outer circle so the line covered Polaris. (ignored the small circle just covered polaris with the line)

At this point I went back inside and set the Polarfinder software to 2 hours on and waited, just before the clouds rolled in covered Polaris for the evening I checked the scope and yes Polaris was further round the circle as expected but had drifted to just inside the circle, had I left it for a few more hours and the clouds and rain not happened I guess it would have drifted a little further inside.

I suppose my question is does this method of setting things up work for a short burst astrophotography say up to 30 seconds or is it a bit to much like guess work ????

Any opinions would be appreciated

[vulcan]

The 'safe' exposure time depends on the magnification. For a wide field view with a 50mm DSLR lens, 30 seconds can be done on a regular tripod without any tracking. As the magnification increases, either through the telesope or a camera lens, accurate tracking becomes more important. Even you do mange to get the ideal polarscope alignment, this is still not accurate enough for long exposures (2 minutes or more). Drift alignment is more accurate but even that has its limitations and then you are into guiding. 30 seconds is okay for constellations or lunar photography. Planets are best done with a webcam and stacking - where accurate alignment is not quite so essential. As your alignment gets better, you will manage exposures of a bit more than 30 seconds and/or more magnification.

[Rob L]

Hi Jasper,

I can't offer any advice as I haven't done it myself yet. But your approach seems pretty similar to the way I'm going to try.

Like you the setting circles process sounds like complete gobbledygook to me too.

You say there is a slight deviation in the position of Polaris when you rotate the RA through 180, do you think it's enough to cause the drift? Or do you just think our approach isn't accurate enough.

Rob.

[Psychobilly]

I only use the polar scope to "align" my EQ3-Pro and can easily get 4 min exposures at 50mm and 2min exposures at 200mm.. I just took my time making sure the reticule was properly centered...

Billy...

[RogerTheDodger]

Don't worry too much about the reticule falling out, it's easily put back in. When I centered mine I actually removed it to get a speck of dust off that was annoying me.

[jasperuk]

The 'safe' exposure time depends on the magnification. For a wide field view with a 50mm DSLR lens, 30 seconds can be done on a regular tripod without any tracking. As the magnification increases, either through the telesope or a camera lens, accurate tracking becomes more important. Even you do mange to get the ideal polarscope alignment, this is still not accurate enough for long exposures (2 minutes or more). Drift alignment is more accurate but even that has its limitations and then you are into guiding. 30 seconds is okay for constellations or lunar photography. Planets are best done with a webcam and stacking - where accurate alignment is not quite so essential. As your alignment gets better, you will manage exposures of a bit more than 30 seconds and/or more magnification.

Does it make a difference in am using prime focus to take pictures not a camera lense

[jasperuk]

Hi Jasper,

I can't offer any advice as I haven't done it myself yet. But your approach seems pretty similar to the way I'm going to try.

Like you the setting circles process sounds like complete gobbledygook to me too.

You say there is a slight deviation in the position of Polaris when you rotate the RA through 180, do you think it's enough to cause the drift? Or do you just think our approach isn't accurate enough.

Rob.

Is this enought to spoil my asto pictures ???? not sure and this is the small circle in the middle of the alignemnt scope not the whole thing, i was hoping such a small deviation would make not difference

[jgs001]

I use this technique with my HEQ5, using the setting circles to set the time and date, and put Polaris in the middle of the little circle. I've not noticed any issues from the polar alignment in my long exposure imaging. I can get 5 minute exposures at 50mm without guiding it with that method, and 2 to 3 minutes at 600mm. I think you'll find that the mount and drives will limit your exposure times more than your polar alignment (work out how to use the setting circles, it does make a difference, check Astro_baby's heq5 setup pages, if you haven't been already).