I think I screwed up !
Posted 09 February 2008 - 10:48 AM
"Image jumping" is certainly not desireable and it's exactly what you are trying to avoid ! It does serve to demonstrate the effect of mis-set antibacklash !
It doesn't matter what figures YOU choose as long as the 'scope responds to them in a smooth manner in both directions and on both axes. If your "street lamp" glides across from left to right and back again and the movement is smooth when it moves up and down in your EP, then your settings are right !
Don't forget that when these are set correctly you might experience a few seconds delay particularly on the low speed slews before the movement "kicks in". Remember though, it's smoothness of movement you seek rather than immediate but jerky response !
Hope this helps,
Posted 09 February 2008 - 12:52 PM
If the action is smooth then it sounds OK !
Don't forget when aligning tonight to use the "Right and Down" buttons if your "GoTo Approach "is set to positive and also, when centralising objects, post alignment, use "Right and Down" for objects to the East of the meridian and "Right and Up" for objects to the West of the meridian. This should give you good tracking.
Good Luck for Tonight !
Posted 09 February 2008 - 01:12 PM
Posted 09 February 2008 - 01:18 PM
Posted 09 February 2008 - 01:21 PM
Just move the object to the central position in this manner.
"Right and Down" when aligning.
Post alignment, always "Right" but vary the "Down" or "Up" according to the object's position East or West of the meridian.
As they say, start at the beginning, go on till the end and then stop !
Posted 09 February 2008 - 02:40 PM
Also for reference, just been out again using London and alignment is out again. Did a two star auto. Didn't play with it much as I was admiring the Orion Nebula with my new clean Corrector Plate. Gorgeous site.
Is it normal for a star (Betelgeuse in my example) to have a funny 2-3inch elliptical halo pointing to the right of it ? This disappears after the object is centered.
Posted 09 February 2008 - 03:22 PM
If the alignment star is in the EP and that means anywhere within its' FOV, move it as a FINAL move to the centre of the FOV by using "Right and Down".
If it is intially, for example, low and right in the FOV to start with, then the sensible way to move it is "Left and Up" and that is exactly what you should do, bringing it to the opposite place in the FOV, namely, high and left.
You then make your approach to centre with the star by means of "Right and Down" as your FINAL action. When centralised then hit the align button. Do the same for the second alignment star.
I'm not sure why you should still be out of alignment because if it worked last night using "London" there's no reason why it shouldn't work tonight.
Stick to an Auto Two Star Align and use, just for the purpose of this exercise, Rigel, Betelgeuse, Procyon, Aldeberan or Sirius as your first star and then Polaris as your second. Don't forget also to Doughnut the stars to make centralisation easier.
As to the "Betelgeuse Elipse", that sounds like a collimation problem. It could be the orientaion of the corrector plate but more likely it's the secondary mirror out of line.
Let me know how the fresh alignment goes.
Hope this helps,
Posted 09 February 2008 - 04:46 PM
The halo problem is still there. Double checked collimation on Sirius, all looks visually fine when defocused. Re-collimated anyway but still the same. The exact description is:
When looking at Star or Planet in FOV, if the object is at the edge of the FOV it throws an ellipse towards the centre of the FOV. Ellipse with empty centre and brightish outline. Tried different EP's with same result. As far as I know, with all the info I have the corrector is the right way up as the serial no behind secondary facing out is lined up and the correct way with dovetail.
Posted 09 February 2008 - 07:02 PM
Sorry for the delay but I've been trying to get some imaging done tonight with my 'scope because the weather is so ideal for it.
I am equally sorry but I feel I really don't know what more I can say or try to do about your situation. Clearly you've got so many BIG problems that I feel only a "hands on" from someone living locally to you and who has experience of these 'scopes, might be able to help solve them. Do you belong for example to a club from which someone there could see what's going on ? I just cannot understand at all why you cannot get a decent alignment after all we've discussed.
As to the halo problem : with it seemingly specific to the edge of the FOV and that objects look OK in the middle of the field, I would not now suspect the secondary mirror being out of line, but that the corrector plate is causing peripheral field distortions and that orientation is, after all important to the 6SE. Purely my guess but this is as far as I think I can go with this, trying to deal with it from this distance.
Hope you get it up and running as soon as possible.
Posted 09 February 2008 - 07:06 PM
Thanks, you have been very helpful. I am not a member but perhaps should look around to join a society around here. I am just really annoyed at myself, I really should have paid a little more and bought brand new scope, instead of second hand. Would have saved me a lot of grief with cleaning etc.
I think I will try Celestron support again too.
Posted 09 February 2008 - 08:25 PM
Posted 09 February 2008 - 11:11 PM
Better yet, buy a refractor, remove the SCT and mount it on your Nexstar goto.
Posted 10 February 2008 - 12:35 PM
I would suspect that what you are seeing is spherical aberration. SCTs, as far as I know, carry a spherical mirror which, by way of its shape, does not allow the light hitting it to be reflected back to a single focal point as is the case with parabolic mirrors (common, for example in Newtonian 'scopes). They are however cheaper to figure and produce than parabolic types.
So, in order to overcome this " optical defect" in spherical mirrors, the corrector plate is introduced into the system.
Thus,and hazarding a guess, I would propose that the corrector is not doing the job, probably as a result of that disorientation, which takes us back perhaps to the origins of this thread.
If you suspect the same, then as I see it, the only way to prove and correct this, is to rotate the "plate" in small increments checking performance at each stage of rotation.
Tedious, I can well imagine, but with the lack of any potentially needed sophisticated equipment, that would be my approach.
Posted 10 February 2008 - 12:36 PM
Posted 10 February 2008 - 12:48 PM
No you won't and you can do it !(LOL)
Check with others first though for their opinions because this might be MY approach to the problem but thereafter I might well find myself working in complete ignorance of an easier solution !
Posted 10 February 2008 - 01:08 PM
There will always be some inherent coma in mirrored SCT's. Some seem to have less than others. My 6SE has it in spades all around the edges of the eyepiece. The more power I use, the less I see. But shove in a 32 or 40mm and they're all around the periphery. Stars have little tails hence the descriptive word, coma.
On the other hand, good quality, longer focal length refractors have none. Using good eyepieces in my CR-4, I see absolutely zero coma. Then I can put the same eyepiece in my SCT and they're all around the edges.
Last night I was using my SCT as well as two other scopes. I have a cheapie Vixen 70mm 900mm focal length refractor and when looking through it using the same 1.25" Baader Hyperion 24mm, I see no coma. Stars are razor pinpoints right to the edge. Shove the same eyepiece into my 6SE and there's coma. No getting around it unless I use a field flattener. Celestron makes one, but I went with one by Lumicon. And they're not cheap either. If I insert the field flattener onto my SCT, they go away, but I'm losing magnification and ending up with wider views thus necessitaing the use of much lower mm eyepieces to see objects up close.
It seems there are always trade-off's.
Posted 10 February 2008 - 01:12 PM
Posted 10 February 2008 - 02:43 PM
Celestron makes this one designed for you scope: http://www.celestron...D=50&ProdID=358
Orion sells this one for an SCT: http://www.telescope...roduct_id=05120
To me it looks identical to the Celestron model, but is somewhat cheaper.
Lumicon has this cheaper model which I have threaded onto my 2" WO diagonal: http://www.lumicon.c...cal Reducer&hn=
It'll only work with a 2" so unless you're going to go with one, buy the Celestron. It's more than double the cost so you might factor that into buying a 2" dielectric diagonal designed to fit an SCT.
And even with a focal reducer in place, I still see a tiny bit of coma. Not nearly as much, but still some is there. But that might be the Lumicon since it's not truly designed for an SCT. Perhaps the Celestron gets rid of all of it.
Anyone have it and can report back as to whether it completely removes spherical aberrations?
Now that I have four scopes, three refractors and one SCT, I've pretty much come to the conclusion that the SCT is only good, at least for me, for deep sky viewing. I do not care for the center obstruction anomaly which I can see when viewing bright objects such as the full moon. But for penetrating into far off nebulas and galaxies, the SCT gets the job done. But I find I prefer refractors. The images are sharper and I don't even have some exotic Astro-Physics, TeleVue or any costly scope. The nearest I have is a Meade 80mm triplet APO and last night when using it to look at the Orion Nebula I could see better contrast than what I saw using my 6SE. Also, a slightly sharper image was available with the refractor.
My dream once we move out of the city is to have a home outside of any metropolitan area where I can enjoy dark skies. Then I'll get myself one of those nice small observatory domes and perhaps a Meade 'coma killer' as well as a large APO refractor. That way I'll have the best of both worlds while viewing other worlds.
Posted 10 February 2008 - 03:11 PM
Posted 10 February 2008 - 03:17 PM