Howdy all, how goes it?
It has come to my attention that I have egg shaped stars turning up in my images all the time, to the point where it has started to become a problem for me as my experience and images get better.
To this end, I have tried a small experiment with equipment. I dont think that the mount and guiding are the whole answer, although I am still using my LXD55 mount, which I would love to replace with an EQ6 as soon as I can.
I dug out an old 102mm Celestron scope that has done a few stints as a guide scope and took a few images through it. Can you please have a good look at them and compare to http://www.iceinspace.com.au/forum/a...se.php?a=44399 http://www.iceinspace.com.au/forum/a...se.php?a=44044 http://www.iceinspace.com.au/forum/a...se.php?a=41568
so I can get your best opinions as I dont want to have to buy another scope as well as a mount, and tell me if you still think its a mount problem or a scope problem, as the 102mm test images seem fairly good with round stars. I have an LX90 with a wedge, however this is my grab & go scope, so I didn't want to set it up and tie it down,and it does suffer from mirror shift when adjusting focus, so mirror flop in imaging will be a problem also I feel. If it is more scope than mount related, I was unsure as whether to get the Skywatcher Pro 100 ED or the Orion 100 ED(Hope thats right) that are advertised on Bintels web site. I would have about $1500 to spend. I dont wanna use the Celestron as it suffers very badly from large blue halos around the bright objects, which is Chromatic Abberation, I think. Is that right? Or is it as simple as buying a filter to counteract this? The scope will need to have a large compatibility with both my DSI Pro 2 and Canon 40D, which Im yet to use as I still cant get it to connect to my pc properly, so the release cable will get a work out!
Thanks heaps.
it looks like a polar alignment or tracking problem, how long do you spend getting your drift alignment right?
does it get worse with longer exposures?
an easy way to see if its a polar alignment problem or mirror shift though would be to put a bright star in the centre of a recticle ep and watch it like a hawk, if it slowly drifts off then its polar alignment if its a more sudden movement then its the mirror
edit: forgot to mention the stars might look round in the 100mm because its a shorter focal length and tracking doesnt need to be quite as accurate
If I had the choice between a Skywatcher ED100 and an Orion 100ED I'd be going for the Orion.....
With your Celestron 102mm refractor, the blue halo's may be chromatic abberation.. Im assuming its a Achromatic refractor... If this is the case, you will more than likely be able to correct this with a 2" Baader Semi APO filter (its on my wish-list for my 6" Achromat)
I haven't checked the drift for a while as I have a pier setup which was dialed in at first use over several nights. I was going to check it again, however, I dont have a reticle eyepiece. What I normally do is use a barlow with a 9.7mm eyepiece and focus the star till it is just inside the eyepiece edges and use that to see where the drift is going. I also have an illuminated polar scope which has markings in it and they are very near to spot on. Also, I can put a star in the camera image plane and watch it on the screen of my pc. Drift is very , very slow, if at all and I sometimes wonder why I guide for shorter exposures. But I will check it. Can someone leave a link for the 127 APO, what is its colour correction like?(Im a lover of natural colour, I prefer reflectors and know very little about refractors). Keep 'em coming, I need to nail this. Thanks.
if you want to get your drift alignment good enough for photography you will need to use a recticle or your ccd camera and a program with a recticle overlay (i think theres a few free ones?) and drift align at the focal length you will be imaging at or more, slow drift is still drift and it will cause egg shaped stars or trails that even the best apo wont fix
plus, if you have a permanent setup theres no excuse!!!
close enough isnt good enough especially in that case
nail your alignment then take a few more images and see if you still need the new scope
It isn't totally permanent. I hafta pull it down and set it up as I dont have a shed around it. The pier is in the back yard, waiting for a shed to go around it to finish it off. Every thing is marked and locked so I can set up and get going.
I thought that according to some of the literature I have read on this site, minimal drift over 15 mins is good enough to guide for a bout 10 min exposures, or else I read it wrong.
Also, drift seems to be that close that if I think its drifting I adjust it, it drifts in the opposite direction, then I have to bring it back. I will check it, dont worry, just gotta wait till I can actually see the sky.
I prefer reflectors and know very little about refractors). Keep 'em coming, I need to nail this. Thanks.
That will change as soon as you get yourself a good refractor. I'm not sure about the 127 'APO' though, I have it in the mail already for testing, might sell it soon after.
As long as aperture isn't everything for you, no reflector will match the image quality of a good refractor.
Im an old C102 owner too (I believe made by vixen). Came on a SP mount.
In relation to your question about achromats you can perhaps get away with things with a good achromat and filters. But they are not perfect. The filters of choice I think is between the contrast booster or the UHCS. If mainly from suburbia maybe go the UHCS. For an ED refractor adding the semi-apo filter is probably sufficient (although probably you might not even bother).
I own both a semi-apo and UHCS. The semi apo is definitely better for visual but I havent tried it photographically yet. It does let some blue fringing through visually (I think the UHCS may therefore be better).
The images you have taken look great with not too much chromatic abberation.
Prickly: Could you stack the UHC-S and the Semi-APO? I have a UHC-S and am going to buy a Semi APO for my Achromat, I wonder if the pair stacked would help photographically??
That will change as soon as you get yourself a good refractor. I'm not sure about the 127 'APO' though, I have it in the mail already for testing, might sell it soon after.
As long as aperture isn't everything for you, no reflector will match the image quality of a good refractor.
Yes, I keep hearing that. But no refractor I look through can match the image size or field of view & clarity of my 10in f4 Schmidt Newtonian for deep space views. Aperture does rule, really!!
My brother agrees, he downgraded to a couple of good ed refractors, which he likes for imaging, but misses the aperture for visual!
I'll keep it in mind!
if you want to get your drift alignment good enough for photography you will need to use a recticle or your ccd camera and a program with a recticle overlay (i think theres a few free ones?) and drift align at the focal length you will be imaging at or more, slow drift is still drift and it will cause egg shaped stars or trails that even the best apo wont fix
plus, if you have a permanent setup theres no excuse!!!
close enough isnt good enough especially in that case
nail your alignment then take a few more images and see if you still need the new scope
So, does everyone agree that its a polar alignment fault? What is the theory behind what causes the egg shaped stars. How does it occur etc?
As you'd know, when you setup in polar alignment you normally never have to correct for movement in DEC as you're following the object's RA to track it...so long as the alignment is good. If your alignment is off, you not only get drifting in RA, you may also even get some drift in DEC and what this does is creates a short trail of the stars in your piccie, hence the egg shaped stars instead of nice round ones.
Interesting - I hadnt thought of that. I will give it a go probably with the UHCS closest to the the main telescope objective to minimise reflections between the filters. I always like new things to experiment so that goes top of the list for the next prime focus imaging session. (Really need to get an autoguider!).
Monoxide is right. If imaging is the only consideration you dont need any more than a UHCS. For visual however the UHCS darkens things considerably and gives a prominent greenish appearance. Not so nice for planets especially. The semi-apo really excels here. Mind you the UHCS lets through lots of light at the key wavelengths of importance for photos.
The other nice thing about a UHCS is that you can stack with a red filter and it acts like an H alpha filter. So its a good multipurpose filter. Havent tried this yet but Baader say it works.
