Polar alignment - finder scope vs guide scope

[descott12]

Hello,
I am going to try the polar alignment feature and the instructions say to use a "finderscope" but I think we really want a guide scope with no inherent magnification. Is this correct?

Thanks as always.

[admin]

Hi,

Perhaps a poor choice of wording – the typical telescope of choice for this is a 200 mm focal length finder/guider scope with a typical guiding camera (ZWO120/Altair GPCam/QHY5LII) connected to it. This combination gives a field-of-view of just over 1° in the horizontal by a little under 1° in the vertical, which works fine. If your field-of-view is more than about 5° or less about half a degree then you are likely to run into problems.

Cheers, Robin

[descott12]

Thanks for the clarification

[T2krookie]

So when I finally decided to utilize sharpcap after spending significant time comparing the polemaster and Ipolar solutions (Since I already have a Starshooter and guidescope) I just in this thread noticd your comment that sharpcap wants around 1 to 5 degrees FOV? Pray tell what is this when the other solutions are utilizing 13-15 degree fov ? I was planing on simply taking the guidescope and figuring out the Focal length needed and add a small lens to it to minimize space needed or perhaps even repurpose one of my webcams modded to a 15 degree FOV.

What kind of difficulty will this produce? Does sharpcap have difficulty with larger fovs? Is there no way to correct for this if it is so?

[admin]

Hi,

There are a number of reasons to limit field-of-view for the polar alignment tool. Firstly you should obtain better accuracy if you are using a 200 mm focal length lens than if you are using a 20 mm focal length lens – in the latter case with a typical guide camera you will be imaging at about 40 arc seconds per pixel meaning that to get really good polar alignment you need to be pixel perfect in all the measurements, star positions, etc.. Working at a longer focal length reduces the errors caused by pixelation dramatically – at 200 mm each pixel is 3.8 arc seconds.

From an implementation point of view, keeping the working area within about 7° of the pole reduces the size of a star database that needs to be included, speeds things up in terms of the calculations and simplifies things since the effects of spherical geometry can largely ignored.

If you have a significantly larger field-of-view then there is a danger that there would be sufficient bright stars in the field of view, but outside SharpCap's plate solving zone to confuse the plate solving algorithm.

I've personally used a 35mm focal length lens with a standard 3.75 µm pixel guide camera for polar alignment in SharpCap and been successful. I bought the 35mm lens off of eBay relatively inexpensively (although it was advertised as a 50 m lens).

Cheers, Robin