2 Min Exposure - Alt/AZ Mount - Horsehead
Posted 24 November 2012 - 01:23 PM
I mentioned that my normal mode of operation is Alt/AZ. That is still true for my primary mount at home, which is an LX-200 with a Meade 80mm ED APO mounted piggy-back. Now, my “travel mount” is a Celestron CG5-GT, which of course is a Polar configured mount.
The image linked below, is the Horsehead Nebula. The image is not what the post is about (although I certainly like it…). The discussion is “Mounts” and “Configurations” and “Imaging”. I captured this image two nights ago, with a 75% moon in the sky. The image was taken using the 80mm, piggy-backed on the LX200, mounted Alt/AZ.
The image is a 9 frame stack of 2 Minute exposures. The camera was my SBig ST-8300-M. The full image was cropped to about 1.3d x 1d image. Very little other processing was involved.
The purpose of this post is to simply show that (what I feel are good) images can indeed be taken with Alt/AZ configured mounts, and that a 2 min exposure can produce good results. This image is taken UNGUIDED. If I do 2 min with the LX200 as the imaging source (which I do), I need to guide, but the results are just as good.
The final image (and thus the entry into my personal observation log) can be found here:
Posted 24 November 2012 - 02:28 PM
Nice results showing that, for shorter FL, unguided imaging in AltAz can be done. Thanks.
Posted 24 November 2012 - 05:04 PM
Granted, the 80mm has a shorter FL (about 500mm). But, as I said, I do this with the 8" LX-200 as well. The focal length there is about 2000mm, or 4 times as long. Thus, magnification is 4 times, and the tolerances are that much tighter.
Below is a link to an image I took on 11-12 of M-33 (NGC-0598), The Triangulum Galaxy, with the LX-200. The image shown is the entire CCD, and catches "most" of the galaxy. This galaxy has a very rich collection of stars in the FOV, consisting of various sizes, from very small, to reasonably good sized stars.
If you "super magnify" the image, you can detect some elongation in the horizontal direction. But to a standard size "display", they look fine to me. So, is it "perfect" ? Of course not. And 2 Minutes is about the maximum I can do in Alt/AZ with the LX-200, and still achieve "acceptable" results. Note... this is a GUIDED image.
But the point is..... The image is acceptable, at least to me. And probably to a lot of other people. It is a matter of personal choice of what each person is willing to accept. I do not seek perfection. But I do like good photo documentation of my observations.
It can be done.
Here is the link:
Posted 24 November 2012 - 06:06 PM
I think your images, guided long FL and unguided short FL, do look good. I'd be happy with results like those. Have you tried guided with the short fl on the CG5? I like my LX80 a lot and don't plan on getting rid of it. But, it looks as if I may need somthing else if I want to get into AP. I had a CG5 that I probably should have kept. Your OTA's sound a lot like what I'm using 2700mm Mak and a 500mm 80mm refractor. At this point, I plan to guide and image with 80mm refractors and leave the Mak out of things.
Posted 17 December 2012 - 05:17 PM
Please post your photos here in this thread. The links you gave no longer seem to work.
You stated that you guided these photos. I am assuming that you were not using a field de-rotator. So, did you guide on a star at the center of the field so that the guiding process would not be affected by field rotation? Did you try some unguided images for comparison?
Posted 18 December 2012 - 12:00 AM
I am working on re-publishing my astronomy web page(s).
I will have the link repaired tomorrow morning.
As to your questions...
No field de-rotator. I was going to play with one to extend the time (beyond 2-min)... but discovered that you really need to use an OAG, and I am not configured for that.
My guiding (both at home and away) uses a Orion StarShoot Camera, and whatever OTA is available. At home, that could be the LX200 or the 80mm ED-APO (which ever one i am not using for imaging). Away, with the Celectron CG5-GT and 6" SCT, i use an Orion 8x50 (was originally a finder).
Center star ?? That would be nice, but not always possible. I just select what appears to be a "good" star.
One of the really BIG issues, especially when imaging with the LX200, is overall mount balance. Properly balanced, and i can get those times. Unbalanced, and troubles start showing up quick.
Posted 18 December 2012 - 12:35 AM
Seriously I missed your picture the first time around, hope you get it reposted soon.
Posted 18 December 2012 - 09:16 AM
Without using a field de-rotator, is it possible that your guider is actually introducing more error unless the guide star is at the center of the field? Assuming the gearing in your mount has very little periodic error (in both axes), it seems to me that guiding on a star that is off-center would be a problem because the guide star would be rotating around the center of the field. In other words, you might be adding a translation error on top of the rotation error. This is just a hunch. I haven't actually done any calculations. But it might be worth trying a few unguided images for comparison. Of course, periodic-error in the drive trains is the wildcard in this equation. It could be that guiding is conquering the greater of the prevailing evils. If your mount has PEC, then perhaps you can set up in equatorial mode and correct for the periodic-error in the polar/azimuth axis. But when operating in alt-azimuth mode, you would still have uncorrected periodic-error in the altitude axis.
Posted 18 December 2012 - 10:51 AM
Your question referenced guiding in this image. As i stated in the original post, if i use the LX200, then i MUST guide for a 2 min exposure. In this case, I was doing a wide field, and thus the 80mm (much shorter focal length). This is a UNGUIDED image (which i stated).
I will post a guided 2 min image where i use the LX200 for comparison, later this morning.
Gillmj24.... i "almost" missed the "sarcasm" note. I was thinking... " Oh no... not more of that *&^% "
I really do enjoy my equipment. The LX200 and the 80mm are perfert for my application at home. The Celestron CG5-GT and 6"SCT are perfect for my travel needs
Half Hitch... have not noticed that, at least that i recognize. I know without the guiding, if i go beyond about 30-40 seconds, the star trails get big in a hurry. With guiding, i don't seem to notice any.
If there were minor "trails" or "bloated stars"... i probably would not object to much. This real question is ... "What do we take the pictures for ?? "
In my case, they create an observation log. I record everything in the record, including the image. I am not trying to create "Publishable" images, nor am i trying to impress the master astro-imagers. I am simply creating an observation log. This is what i "see".
Posted 18 December 2012 - 11:05 AM
That's a pretty good image considering the straightforward manner in which it was made, including use of an alt-azimuth mount. I look forward to seeing the guided image. I was merely asking questions for the purpose of learning from your experience. Thanks for posting these results.
Posted 18 December 2012 - 02:40 PM
These were taken Dec 8th, and 10th 2012, from my back yard (DaHut).
And here is another...
And a last one, which DOES show a little rotation (bloating stars on the right side)... but like I said earlier, it serves my purpose.
This is what my normal "log" entry is like.
The image to the left is a capture from the Palomar Sky Survey, and thus is my "Target". The image to the right is my image.
Posted 18 December 2012 - 06:14 PM
Posted 18 December 2012 - 07:16 PM
Balancing !!! ----- did i say Balancing ???
Lack thereof shows up IMMEDIATELY !!!
Guiding !!! The software i use for guiding is PHD....
I used to use Envisage... got sort of OK results out to about a min to 90 seconds. With PHD, extended out to 2 min.
What a difference quality software makes !!! Also, as part of that change, the guiding became ST4 rather than Meade serial.