Field rotation/cone error and guide scopes

[Entropy]

Hey all
yet another newbie question :)
I recently purchased a 90mm guide scope, and just slapped it on top of my current scope using the mounting rings that came with the scope.

The other night while i was doing my three-star alignment on my goto mount i suddenly remembered why i do a 3-star alignment, to eliminate the cone error and this led me to the thinking that perhaps slapping the guide scope haphazardly onto of the main telescope was a bad idea and perhaps the cause of me not being able to get round stars in long exposures?

Am i on the right track here? Do i need to buy guide scope rings and align my guide scope and my finder scope to the main scope? Or am i just really bad at polar alignment? Or even can i polar align/3-star align using my guide scope to eliminate this problem in my main scope?

I'm quite confused with what i'm actually doing when aligning... i just follow the steps :)

Thanks in advance
Scott

[renormalised]

If you add a guidescope to your main scope, you have to align it. Otherwise what is the point of having the guidescope in the first place. Same with your finder. Take your time to make sure that what you see in both reflects what you can view through your main scope, making sure that what's centred there is in the crosshairs of your finder and centred in your guidescope. Otherwise polar aligning your main scope to take piccies or whatever will be a waste of time.

Do an approximate alignment of your guidescope and finder with your main scope on some terrestrial object at some distance to you....say a light pole a few hundred yards away. Then later on, at night, adjust your alignment using a bright star.

[Entropy]

I just assumed that a guidescope was to make sure the star wasnt moving.... and it wasnt until i did my 3-star alignment, and saw guidescope rings for sale online, that it hit me that there was probably some cone error in relation to my guidescope and mount.
I just wasnt sure if this would actually effect guiding or not, obviously it does. I knew the finder had to be aligned to the main scope, but i had never thought about alignig the guidescope.

[skwinty]

Hi Scott
It is my understanding that the guide scope does not have to be aligned to the imaging scope. It could for example be 30 degrees off the main scope axis. (to find a suitable guide star). If your mount is accurately polar aligned , then the guide corrections would only be in RA and not DEC. If however, you are not accurately polar aligned the your guide corrections would be RA and DEC which is not so good as there are corrections in two different axes.

This is my understanding and if I am wrong please correct me.:thumbsup:

[Entropy]

This was my understanding as well, as i thought i had heard that the benefit of using a guidescope over a off-axis guider was that if there was no suitable star in your field, you could just find one with your guidescope....

Then i started thinking about the cone-error correction, and what guidescope rings are actually for and if my guide scopes cone error would cause field rotation as it over corrected in the guide scope.....

Im very confused...
(which, as you may a;ready know, isnt all that hard)

[Entropy]

Im not sure if this will help or not, but i suppose there is no such thing as to much info.

This is a picture of my setup
http://i241.photobucket.com/albums/ff170/entropy22/Photo0085.jpg

This is a close up picture of how my guide scope is mounted on my main scope
http://i241.photobucket.com/albums/ff170/entropy22/mounting-1.jpg

And this is the source of my endless frustration...
http://i241.photobucket.com/albums/ff170/entropy22/omegacent10minprocessed.jpg
http://i241.photobucket.com/albums/ff170/entropy22/lagoon3000s15.jpg

Also, with that last image, i thought i was looking at the lagoon nebula... but now i am not so sure.. can i get a confirmation on that as well please?

[skwinty]

Come on guys,
Put Entropy out of his misery here and give some advice!;)
I dont have enough experience to help with this problem and can only comment on what I think I understand.:help:

[Zuts]

I am a bit of a beginner myself but,

1) If your guide scope is not pointing where you are imaging then in general you will be over or under correcting. Skwinty says 30% is OK but i dont believe this is true or would only be true if you were exactly polar aligned which most people are not. They use the guide scope to correct minor polar alignment errors.

So your guide scope does not have to be pointed exactly where you are imaging but close. If you are close then cone error i dont beleive would effect the corrections.

2) The benefit of using a guidescope over an off-axis guider is that you have the whole field to use to find a guide star and more light. The drawback is that if your setup is not good then you will get different rates of flexure as the mount tracks and the guiding will attempt to correct for this. An off axis guider because it is seeing what the imageing ccd is seeing does not suffer from differential flexure. However it is not so much of a roblem with widefield short focal length, more of an issue with SCT's say which even at F6.3 can have a focal length over 2 meters.

3) To me your setup looks OK. As long as it is all tied down nicely. If you are imaging through the Newt then you may have problems if the focal length of the guide scope is not close to that of the newt; as they will see different rates of error.

4) The last image is the lagoon nebula. It is just a bit dark as you dont have enough data. I have overstretched it and you can see the central part of the nebula.

[Entropy]

Thanks for that reply zuts.
Do you think the elongated stars in my, now identified, lagoon nebula shot are from guiding errors due to my dodgy mounting, or more from field rotation due to poor polar alignment, as that shot was my longest exposure attempt to date(3000s) and was mostly done at that length just for the sake of the fact that i found the option to do 1000+ exposures in my software and i wanted to give the camera a real whirl.

[Zuts]

Hi,

I think you mean 300 seconds, not 3000 seconds. Anyway I think your polar alignment needs to be better. What are you using for guiding, whatever it is, it normally gives you an idea on screen what sort of corrections it is making.

Ideally they should be under 1 pixel. If they are and you are still getting elongated stars then your guidescope is not seeing what the imaging cd is seeing and you should look at tightening everything up.

Paul

Edit:

If it is really 3000 seconds or 50 minutes then probably the alignment is pretty damn good and you should go back to 5 minute subs, however I dont see enough data for a 50 minute sub so am assuming you meant 300 seconds.

[Entropy]

It was a 3000s exposure, i had turned the gain and offset down to 1 and 5 respectively, so that probably explains why it was so dark (ive got no idea how to use those gain and offsets yet, and this was also an experiement in that).
I have been averaging about 1000s exposures for most of my shots, but tonight i want to get a good polar alignment... actually do program my PEC on my mount (i havent been confident enough in my polar alignment to do this yet) and have a go at some 300-600s subs... as the subs people have are amazing.....

If anyone reading this is willing, would someone be able to post a single sub of an image (the lagoon nebula if possible) so i can see what i should be aiming for.

Also, does anyone have any hints and tips about shooting a galaxy ?
This is my life ambition... and the way ive been going, it just may take that long :D

[Zuts]

Hi Entropy,

If that was a 50 minute sub then I think your alignment and guiding may be fine. I usually go 5 to 10 minutes max. Try this, say 12 by 5 minutes and stack. I am not sure what your camera gain settings do so maybe someone else can help here and give you the best settings.

I have posted a Lagoon stack of 3 by 5 minute subs. On this night my scope was dripping with moisture and the guiding consequently was very bad. After 3 subs i gave up and went home coz i forgot my dew heater. So this image suffers from bad guiding, not enough data and a wet dewey lens.

Anyway good luck

Paul

[Entropy]

You wouldnt happen to have any of the individual images you used to stack would you ?
Just for a guide line (even though it probably wont be a great comparison due to the fact that its basically comparing a SBIG to a SAC10)

[Zuts]

Hi,

You may be surprised with the comparison, everything looks better from dark sky with lots of subs. Anyway i was out last night and took some images. Here is one 5 minute sub, flats and darks, simple gama stretch in maxim dl. Moon at 80%, no filters.

For comparison, the other 11 five minute subs, stacked and processed. Shrunk to 1200 pixels and compressed to 30 percent.

Cheers
Paul

[Geoff45]

In principle, you don't have to have the two scopes aligned to do imaging. However, if you are not properly polar aligned, then the drift in the two scopes will be different because they could be pointing at parts of the sky with different dec and drift depends on dec. This may or may not be significant for imaging, depending on the size of the errors. In practice, if you are reasonably polar aligned and you shift the guide scope slightly to get a suitable guidestar, it will not affect imaging.
Geoff

[avandonk]

As usual it is all a compromise. Ideally you want perfect polar alignment and the guidescope guiding on the center of your image. If your mount has both scopes, imaging and guiding at right angles to the polar axis that is your declination axis then there will be no drift or image rotation.

In the real world this is not true. You will get drift and rotation which gets worse as the alignment errors get larger.

Misalignment of guidescope with imaging scope will tend to give image rotation and this gets really important if you are doing widefields. This is especially true even at short focal lengths. The center star (guidestar) will look fine and the outer ones will be tangentially smeared due to rotation.

Drift is caused by slight polar misalignment or so called cone error or both.

A small amount of drift and or rotation as long as it is not noticeable on a single exposure can be beneficial. This occurs when you stack a series of exposures as the noise will be in slightly different places on each exposure. The noise will then not add but tend to cancel out especially if you median stack. It works even better with INCR as the difference is bigger between exposures. Or you can stack the ODD and EVEN frames seperately and simply then average the two stacks.

Hope this helps.

Bert