More questions - 20Da and SkyMemo

Earlier I had some problems with my 20Da and 300 F4 IS so I tried my 70-200 IS. I am sure I had my mount close to perfectly aligned - with all three stars.

I get significant star trails at 6 minutes with this combo. So my question - is there a way to narrow down my problem? Am I pushing the weight limit? bad balance? motor not accurate? Also - I know it is probably a stupid question, but - I do not have to level the unit to polar align, because it only moves in RA, right?

I would just think I need to use a shorter FL, but I see the results Jeff H is getting and know I could eventually do better!

Any help diagnosing the issue would be appreciated!

Six minutes is a lot to go for any mount unguided. (Well, any mount that I could afford.) (Although with a 200-300 mm lens, you should have some leeway.)

What are your streaks like? Are all the stars streaked the same? Or are some streaked more than others? Are they streaked evenly, or do they look jagged? (Post a pic, please!)

Technically, one does not have to level the whole mount to get a proper polar alignment--as long as the axis is parallel to the earth's axis--you got it. However, It is generally easier to get the whole thing right if you level the head.

Try severely mis (polar) aligning your scope/camera.

THen take a long exposure.

You will get star trails, of course. But the interesting thing is to look at the shape of the trails. If the path is smooth, then the motors/gears are working well enough. If they are jerky, then you have some issues in the drive train, periodic error, etc.

Alex

Gary, An unguided 6 min on a 300mm lens on a polar-aligned mount is not a severe condition. I used to do 150mm on my medium format camera on a Vixen GP-DX with 30 min exposures and everything is pin point- and that was with a polar alignment scope.

Do you have a small scope you can mount on it? Instead of your camera, mount your scope, balance it and then polar align and use a high but usable magnification to center a star. You can quickly tell if the tracking is decent. Around 40-50x, you should be able to step away for 5 min and come back and find it in the middle.

Being a city guy, I am always stunned when I make it out to the country. Once or twice, in an excessive burst of enthusiasm, I have mistakenly aligned on what I thought was Polaris (delta Ursa Minor). When you do that, you get star trails galore, and my STV autoguider politely tells me to kneel down and eat a mouthful of grass (just kidding). You are probably much more intelligent than I am so I presume this is not the source of your star trailing!

Alex is right- your mount does not have to be level.

Ivan

I double checked every thing and the trails are much less in this 14 minute exoposure - 200mm on 20Da...

Looks acceptable, no??

(100% crop B&W)

Attached Image:

Gary,

Balance is very critical on this mount. Make sure it's a tiny bit heavier to the east. Also, let it run for about 10-15 minutes to settle in, and it will be fine--the gears needs to take up a little slack. A few other things to check:

Make sure there is no slip in the RA; tighten it very hard and also see if you need to adjust the clutch. I tweaked mine.

Check to make sure the polar alignment scope is properly centered/aligned. You can do that during daylight--there are lots of instructions online for how to do it. I also tweaked my polar alignment scope a bit.

You should be fine at 10 minutes plus with 200mm. I almost always shoot 10 minutes at 200mm and have gone as long as 15 minutes with no trailing.

Jeff