Polar Alignment - Getting to grips, or gripping to strangle!

[CKemu]

Last night it was delightfully clear from my neighbours back yard, so decided to attempt a "vague" polar alignment, below is a single sub frame from my second attempt to "capture" M51:

Sub was taken with the following conditions:

ISO3200 | 240s Exposure | Strong LP, so used Orion Skyglow Broadband Filter

I use the Meade LX90 8" Fork Mounted SCT (GPS, UHTC, ACF) on an Eq wedge.

Learning how to polar align currently, and can find Polaris with great ease thanks to learning a few more stars and constellations. I've heard of various methods to align a scope - but recently stumbled across a video that suggests you can get a fairly good align using a scope of this nature:

It' about 2:00 onwards that he explains his alignment method, the point is then made later than Polaris should when rotating the scope stay in the center of the eyepiece, no matter what I did, I couldn't get the star to stay central in the eyepiece, it would drift as I turned.

Now I suspect that my OTA wasn't perfectly parellel to the forks, this was causing an overall error.

Before moving on to drift alignment and other methods, I'd like to perfect/practice this one, does anyone have a fork mounted SCT like mine and can give advice on this method / tell me how to get my OTA parellel to the forks?

[Vega]

I cant comment on aligning a fork mount design as my experience is with EQ mounts. However I would like to say, looking at your image, it looks like initially it was tracking fine but then something caused the rig to either stop tracking at the last few seconds or it got knocked. The reason I say this is the streaks have a star at the bottom and are not simply uniformed streaks of light of the same brightness.

Hope im making sense, hard to explain.

Matt

[CKemu]

You make an interesting point, so I just went back through the other 10 shots I took of this area, I have streaks that are even in brightness on some photos, I've also got streaks that go in the exact opposite direction... confusing.

[mikeknowle]

I have streaks that are even in brightness on some photos, I've also got streaks that go in the exact opposite direction... confusing.

Could that be vibration from somewhere nearby then if its going in both directions, such as a lorry passing near your house, door slamming, wind catching the scope, worried astonomer pacing up and down?

[msinclairinork]

I also have no experiance in aligning fork mounts, but 240s is a long time for any mount to track perfectly without the aid of guiding.

Particularly with high focal legnth scopes such as yours.

So could be a case of it just not being up to the job?

Did you try different exposures? say 60s etc.

Michael

[ollypenrice]

Michael's right, unguided at that kind of focal length will never be possible over anything but very short exposures. If this was at f10 then short exposures won't get much data. Have you the f6.3 reducer? This would be a double benefit, more than doubling the speed and reducing the FL.

I found fork and wedge to be impossible to polar align in less than a matter of hours and converted to GEM.

I agree with Vega's analysis of your trailing though. Some quite good tracking was going on there much of the time.

BTW, Polaris does not lie exactly at the North Celestial Pole so I don't follow the point about its not moving in the EP.

Olly

ollypenrice's Photos

[Merlin66]

I use a 12" Lx on the fork/ wedge with no major drama.

There are many (complicated!!) ways to check if the OTA is sitting in the forks at 90 degrees to the polar axis (RA)

A quick and dirty method is to set up the tripod as level as you can using a Looooong spirit level. Drop the fork assembly on the tripod and point the tube directly upwards. Level the tube using the spirit level north -south and then check the level across the dec arms direction. This obviously should be level. If not, you can get some adjustment from the attachment plates between the OTA and the Dec stub shafts. If you need more then you have to loosen one of the dec arms (the four bolts at the bottom) and wedge it up until level. Over on the Lx200 Y! group Alan Sickling has a couple of files on how to do this with ABSOLUTE precision.