SharpCap Forums

Sharpcap provides polar alignment. This involves pointing the scope to Polaris, visualizing 15+ stars in the camera, plate solving, then rotating in right ascension by 90 degrees and plate solving again. My camera is not portraying 15+ stars, only Polaris and 1 other star. My F/10 Celestron EdgHD has diameter 280 mm (11") and a focal length of 2800 mm. With this telescope my Meade DSI IV gives a FOV=22' x 16' and my ZWOASI2600 gives a FOV = 29' x 19'. I've done autostretch, and I've set the gain up to the maximum. But still nothing.

Is the problem that I don't have a sufficient FOV? I don't think so. The example shows 17 stars within 32' of Polaris with 1/4 of the image truncated. So there should be at least 17 stars around Polaris within 32'. (I could install a focal reducer, which would bring me to 55 minute FOV).

Is the problem light pollution. I'm at Bortle scale between 6 and 7. An astrophotographer showed me his many stars and he images in Bortal scale 9 ... essentially New York City. He was imaging with a 5 inch diameter scope. Mine is 11 inches.

Any ideas?

Hi George,

It is definitely a FOV problem. My FOV is 1.6 deg x 0.9 deg and I have no problem with Polar Alignment. I can't use my main imaging scope for PA because the FOV is too narrow. So instead I have a small 200mm focal length guide scope. My guide camera is an Altair 290M with 2.9 micron pixels and a resolution of 1920x1080. If you can do it, I recommend that you use a guide scope instead of a reducer on your image scope, unless of course you want to use the reducer all the time. It's just a pain swapping in and out the reducer between PA and imaging. That's what my experience tells me. Keep it simple.

Here is a handy calculator for FOV:
https://astronomy.tools/calculators/field_of_view/

Brian

Hi,

the FOV problem is the most likely - simply put, the star database for polar alignment only goes down to Mag +13, so once you get down to small FOV, there may not be enough stars of less than mag 13 in the database in the area you happen to be looking at to do the alignment.

If you have the focal reducer already, give it a try, it should help a lot.

cheers,

Robin

Thanks Brian & Robin... I will try the focal reducer. Lousy weather the next 7 days. Best... George