开云体育

I set up an ASI 120mini camera with a 60mm guide scope. Having a lot of difficulty setting it up and aligning with the main scope. If I use this setup only for auto guiding purposes do the two need to be aligned?
If the answer is yes I can use all the suggestions you can send.

--

I like my two scopes aligned. But they do not have to be exactly aligned to work great.
After all, they work independently of each other.

I will use Polaris for initial aligning. Then when I have as close of a same image between my Main Camera, and my PHD2 image of my Guide Camera, I can pick other stars (Vega, or a bright star) and check or fine tune the guide scope.
But close is close enough, because they are independent in operation.
For me, my guide scope is my "Course View" and my Main Camera is my "Fine View" during Modeling (Alignments).

--
SonnyE


(I suggest viewed in full screen)

Hi Tony,

I don't know what your imaging scope focal length is but as long as they are pointed in the same general place and the imaging scope focal length is not much past 1,600mm or so it should work fine. I have been using a Stellarvue 50mm guide scope rigidly mounted using the matching fixed (as in no adjustments) Stellarvue clamshell fastened directly to a dovetail adapter which is clamped to the top V-dovetail on top of every scope I image with. I've used the same autoguider and camera on a Stellarvue 80mm f/6, bespoke Parallax/AT130 f/6 and LX200 M10" f/6.3 SCT on top of my G11G and the guiding seems to be perfectly fine. I think it works because there is very little flexure because there is no adjustments and everything is solidly mounted.?Obviously you have to be sort of close but they don't need to be centered exactly on the same star.??

?


--

Chip Louie Chief Daydreamer Imagination Hardware?

? ?Astropheric Weather Forecast - South Pasadena, CA?

Can you align them during daylight?? Center main scope on a telephone pole or tree (anything in the distance).? You can use a camera or eyepiece.? Then once centered, repeat using the guidescope but centered on the same object as main scope.? Very ez to do during daylight.

Tony,

Like everyone has said, the imaging and guide scopes do not have to be perfectly aligned, but they should be pretty close. I use the crosshairs in both PHD2 and Sequence Generator to get as close to center as I can on a bright star without over doing it. If the centers are too far off you introduce cone error and over the course of a night on one target, the guide scope will stay on a star, but your image frames will start to travel around it in a small circle. Then when you go to stack your light frames you will see they will not overlay as well, and you will have wasted margins you will need to crop off, reducing the total good image area you might have had.

Just as important is good polar alignment. Even if you exactly center both scopes, the imaged frames will drift off target in one direction (declination), and then the other way after the meridian flip, when PA is bad. Then again you will end up cropping off bad margin areas.?

If you are imaging a target that has a lot of empty space to the edge, these factors are not as important. But if you have something large that just fits onto the sensor, then you could be losing part of your intended target. But with good centering and good PA, the light frames will pretty closely overlaid and you are getting the most from your given sensor size. And we all like bigger sensors :).

Best regards,

John

If you want to line both scopes up, point your main scope at a bright star and center it up in your camera. Take the auto guider camera out of your guide scope and stick an eyepiece in the guide scope. It doesn’t have to be in focus. You are just looking for the same bright star your main scope is pointing at. If you can’t see it, adjust your guide scope until you can see it. Put your guide scope camera back in the guide scope and the bright star should be visible now. Fine tune the guide scope until the star is centered. If you have a green laser it is even easier. Just remove the guide camera and point your green laser in the guide scope. You will be able to see the green laser come out the other end of the guide scope and see where it is pointing and adjust it.

--

As others have suggested, take the guide camera out of the guider (but first use a parafocal ring on it so you don't loose focus)? and insert an eyepiece. Place a bright obvious star in the center of the main scope and then adjust the guider so that the same star is centered in the guide scope. Put the guider back in and re-check focus.

There is another reason (esp. if you don't have a permanent setup) to get this as good as you reasonably can: Field Rotation, which if present causes the images of stars to appear as little arcs center on (and proportional to the distance from) the guide star. Field rotation depends on four things:

1. The precision of the polar alignment.
2. The any difference in the angular alignment of the imaging and guiding scopes ( how well they are co-boresighted, to use the artillery term) - what your question was about.
3. The duration of the exposure. The longer the exposure the worse (if present) field rotation will be.
4. The magnitude and the orientation of the polar alignment error relative to the RA and DEC of the imaging target.

You can not do anything about the 4th unless you just give up and move to another object in a different part of the sky and pray you are lucky. The third you can compensate for by taking more shorter exposures and then using a stacking tool that does a two point stacking alignment.

The first two (polar alignment and co-boresight accuracy) you can do something about. Having the two scopes co-boresighted has two benefits: First, it makes it easier to diagnose (if necessary) polar alignment problems because the arcs of the stars will all be orientated around the center of the image (where the guider was tracking), making it obvious what is going on. Second, a large amount of boresight error can magnify even a tiny amount of polar alignment error.

With regard to 2, the problem (if present) is not less for shorter focal length imaging instruments. This is because the size of the arcs in angular measure (degrees, minutes of arc, whatever)? for a given exposure duration is linear in the angular distance between the star images and the guide star. So while cutting the focal length of the imaging scope in half will reduce the plate scale (Image size in inches per degree, for example) the width of the field, in degrees will proportionally increase (for the same chip, obviously)? so the two effects cancel each other out. Likewise cropping won't do anything in general, nor will going to a shorter focal length (wider field) instrument.

For a general discussion see:



For a detailed discussion, see Michael Covington's book "Astrophotography for the Amateur", pages 276 to 282 where he goes through all the mathematics of polar alignment error, DEC drift, and field rotation. On page 281 he gives some nice illustrations and a summary (B.3.3 How Much Field Rotation is Tolerable). The reading is not required, just a general understanding.

The Calgary (Canada) section of the Royal Astronomical Society has a detailed qualitative discussion of Field Rotation, including the extreme case of ALT/AX mountings:

https://calgary.rasc.ca/field_rotation.htm

The bottom line is that the better the polar alignment the less all this matters. And for off-axis guiders improving polar alignment or shortening exposures (if feasible) is the only cure.

And of course what really matters is how satisfied you are with your final images.

Clear skies and Happy Holidays.

Mark Christensen

PS: Can you do the alignment during daytime? Of course, just make sure the alignment target is far enough away. How far? Far enough so the parallax between the two scopes (guider and imaging) doesn't matter much. If your 60mm guider is the type I am familiar with its focal length is between 240 and 300mm.? Let's call it 300mm. Most guide cameras have small chips in them so let us suppose you want to get within 1mm of the center. That means the parallax error should be reduced to less than 1/300 or so (about 0.2 degrees). If the spacing between the center lines of the guider and the imaging scope is something 6 inches then the alignment target should be at least 300 * 6 inches or 150 feet and 300 feet would be better. Further is you wish to achieve more accurate co-boresighting. The same discussion applies to finder scopes.

You know I wish I had an image stack I processed when the guide cam was off from the main scope to illustrate how the frames skewed over the course of the night. But it? is lost somewhere in processing sessions of days gone by :). If someone has a night to kill sometime, they could try to exaggerate this distance, say to 2 or 3 degrees from center to center, then watch how the edges migrate hour by hour. The final stack will be misaligned to where only a central region might have enough good overlapping data. Perhaps when there is a full moon with no good targets to image I could run this as an example. But there's probably someone out there who knows the math and can illustrate what will happen exactly.?

--

Yes. I guess I am.?