Quote:

---

Just a question, since I see you're using an EQ6.
Are you using EQMOD by any chance?
That way you could add VS-PEC to substantially get
rid off most of the PE-error.
The remains will have to be guided out but over all
the performance is pretty good!

---

Post Extras:

Quote:

---

You will need a second camera (& guide scope). But yes. this is autoguiding & it will keep your image from shifting as long as you are polar aligned.

---

Post Extras:

Quote:

---

I think most have shown now that guiding through the HC vs the guide port has little or no effect on guiding accuracy. If any small delay is added (milliseconds) it can easily be discounted when compared to the long image capture time (typically 1 to 2 seconds), at least this is what I've read on the subject. Also, I've had no issues guiding through the HC when imaging at modest focal lengths (200-1100 mm).

Steve

---

Good to know. My (apparently outdated) information was that the delays could be long enough to affect guiding accuracy--in excess of 500ms. Craig Stark even referred to this issue in a presentation he gave on autoguiding at AIC earlier this month. Perhaps he was referring to issues with earlier systems not current models.

--------------------
- Jared Willson

• A few refractors
• A couple of GEM's
• One camera
• Two eyeballs
• Some heavily light polluted skies

Post Extras:

Quote:

---

Quote:

---

You will need a second camera (& guide scope). But yes. this is autoguiding & it will keep your image from shifting as long as you are polar aligned.

---

That answers my other question. When I check the "Guide" section of the camera-settings menu, then select "settings", or some such, a menu-item said "Camera Settings". When I selected it, it showed the same offset and gain as my "main" camera, an Orion StarShoot DSI version 1. I was hoping it would let me use the Orion, instead of a separate camera. Another post said that the guider exposures are a lot faster than the "main" exposures so it sounds like I'll need a separate camera.

I have a Meade DSI, a Meade LPI, a Meade Electronic Eyepiece with video-capture board. For the guide-scope I was thinking of using my 2-inch refractor.

K-R.

---

As far as I know, the only system that will allow you to use the same chip for both imaging and guiding is the Starlight XPress series. Some of the Starlight cameras will image with even columns while guiding with odd columns. I think you then alternate subexposures so you aren't losing resolution. The advantage to this system is one camera and no guide scope. The disadvantage is double the imaging time to achieve a given exposure.

Most of the SBIG cameras also do not require a separate autoguider because they are really two cameras in one--an imaging chip and a guide chip built into the same camera body. That works quite well as long as you aren't doing narrow band imaging.

Other than that, I believe you always need a second camera (though it can be a small, inexpensive camera--perhaps even your LPI would work!)

A 2 inch refractor should work fine as a guide scope. As long as you are over about 200mm in focal length you should be okay. Guiding software can calculate the star centroid with astonishing precision, so the old rule of thumb that you needed more focal length in your guide scope than in your main scope doesn't apply in the world of autoguiders.

--------------------
- Jared Willson

• A few refractors
• A couple of GEM's
• One camera
• Two eyeballs
• Some heavily light polluted skies

Post Extras:

Quote:

---

Quote:

---

I think most have shown now that guiding through the HC vs the guide port has little or no effect on guiding accuracy. If any small delay is added (milliseconds) it can easily be discounted when compared to the long image capture time (typically 1 to 2 seconds), at least this is what I've read on the subject. Also, I've had no issues guiding through the HC when imaging at modest focal lengths (200-1100 mm).

Steve

---

Good to know. My (apparently outdated) information was that the delays could be long enough to affect guiding accuracy--in excess of 500ms. Craig Stark even referred to this issue in a presentation he gave on autoguiding at AIC earlier this month. Perhaps he was referring to issues with earlier systems not current models.

---

When I was starting off autoguiding I remember reading articles from people on both sides, and I think you're right that it might be related to older systems. I think some of the new PDF articles I found (EQMOD people I guessing?) tested both systems and found no real difference. It would be interesting to hear what Craig Stark has to say as I'm sure he's got the latest knowledge and testing information.

Initially I was going to build an ST4 interface but given the new info and that I have a newer mount I tried the HC interface first. So far I haven't had any issues other than ones I caused myself.

Steve

--------------------

Post Extras:

Quote:

---

Quote:

---

Quote:

---

I think most have shown now that guiding through the HC vs the guide port has little or no effect on guiding accuracy. If any small delay is added (milliseconds) it can easily be discounted when compared to the long image capture time (typically 1 to 2 seconds), at least this is what I've read on the subject. Also, I've had no issues guiding through the HC when imaging at modest focal lengths (200-1100 mm).

Steve

---

Good to know. My (apparently outdated) information was that the delays could be long enough to affect guiding accuracy--in excess of 500ms. Craig Stark even referred to this issue in a presentation he gave on autoguiding at AIC earlier this month. Perhaps he was referring to issues with earlier systems not current models.

---

When I was starting off autoguiding I remember reading articles from people on both sides, and I think you're right that it might be related to older systems. I think some of the new PDF articles I found (EQMOD people I guessing?) tested both systems and found no real difference. It would be interesting to hear what Craig Stark has to say as I'm sure he's got the latest knowledge and testing information.

Initially I was going to build an ST4 interface but given the new info and that I have a newer mount I tried the HC interface first. So far I haven't had any issues other than ones I caused myself.

Steve

---

Craig's talk was fairly interesting--aimed more at beginning and intermediate imagers than advanced imagers, though, which was a bit surprising given the particular audience at AIC. Still, he clearly knows his stuff and is a good speaker as well. I did learn a bit about what makes for a good guiding setup. The major points he emphasized were:

• Keep your guide scope and guide camera as light as possible in order to minimize the chance of differential flexure
• You don't need a high quality guide scope to get good results. Any inexpensive refractor with a focal length of 200mm+ will do fine.
• The lower noise in a high quality guide camera can offer some small improvements in guiding accuracy, but don't stress about it
• Don't work at getting focus too good in your guide camera. In fact, slight mis-focus can be beneficial (especially if you are undersampling) since it spreads the light from the guide star across multiple pixels

He also provided some of the history of autoguiding (which is where I picked up my information on the timing issues referenced above). I'm sure there were other salient points as well, but that's what I picked up from the lecture.

--------------------
- Jared Willson

• A few refractors
• A couple of GEM's
• One camera
• Two eyeballs
• Some heavily light polluted skies

Post Extras:

Quote:

---

A few days ago I noticed the stars starting to move off of the top of the screen during 150-second exposures. There were no trails or oblong stars, but they would change position by a few pixels between subsequent exposures. I had done a drift-alignment, so I figured this "should not happen."

---

What you're describing sounds less like periodic error and a lot more like field rotation due to polar misalignment. Even though you said you did a drift alignment, I suspect it was not accurate enough. You didn't say what telescope and focal length you are imaging with but I suspect you're using a shorter focal length imaging scope and a camera with decent sized pixels. What's most likely happening is that during your 150 second exposure, you're close enough in your alignment and tracking that stars are not crossing from one pixel to the next to give you noticeable star trails, but the polar misalignment is introducing a slight amount of field rotation which you're seeing as star field movement from frame to frame.

To correct for that you will need to do a more accurate polar alignment which means that you'll need to use a higher power eyepiece and drift align for a little longer period of time. But in reality, if you're not seeing star trails, how important is the field rotation? Stacking the images in your pre-processing software using two star alignment (translate and rotate) will correct for field rotation.

Patrick

--------------------

Celestron CPC 1100 XLT
10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount


My Astronomy Pages
The heavens declare the glory of God; the skies proclaim the work of his hands.

Post Extras:

Quote:

---

Quote:

---

A few days ago I noticed the stars starting to move off of the top of the screen during 150-second exposures. There were no trails or oblong stars, but they would change position by a few pixels between subsequent exposures. I had done a drift-alignment, so I figured this "should not happen."

---

What you're describing sounds less like periodic error and a lot more like field rotation due to polar misalignment. Even though you said you did a drift alignment, I suspect it was not accurate enough. You didn't say what telescope and focal length you are imaging with but I suspect you're using a shorter focal length imaging scope and a camera with decent sized pixels.

---

I forgot to mention that the telescope is a Stellarvue 80mm Nighthawk aplanat and the camera is an Orion Star-Shoot Deep-Sky v1.

Quote:

---

To correct for that you will need to do a more accurate polar alignment which means that you'll need to use a higher power eyepiece and drift align for a little longer period of time.

---

I don't recall seeing the image rotate, just crawled "upward" in the frame as I looked at the files in order of time-stamps. I'll have to take another look at them.

Quote:

---

But in reality, if you're not seeing star trails, how important is the field rotation? Stacking the images in your pre-processing software using two star alignment (translate and rotate) will correct for field rotation.

Patrick

---

It bothered me because, during a Really Long Exposure run the object (in this case the Horsehead Nebula) would "crawl" off of the frame. Since I run my telescope remotely I probably would not notice until it was too late. As it was, I only noticed because a nearby "obnoxiously bright" star was no longer in my frame after a while, when I checked the telescope's computer.

As always, any advice or "observations" (pun intended) are appreciated.
K-R.

--------------------
Stellarvue Nighthawk Tenth Anniv. (NH7?) - 3-inch Refractor
Stellarvue 2-inch Refractor
1973 Sears Discoverer 3-inch Refractor - "Nemo"
Orion EQ-G Atlas with EQ-MOD - "Charles"
Orion Star-Shoot DSI v1
Meade LPI & DSI

Post Extras:

Quote:

---

I don't recall seeing the image rotate, just crawled "upward" in the frame as I looked at the files in order of time-stamps. I'll have to take another look at them.

---

Quote:

---

I forgot to mention that the telescope is a Stellarvue 80mm Nighthawk aplanat and the camera is an Orion Star-Shoot Deep-Sky v1.

---

Short focal length, fairly large pixels... As mentioned, the tracking is good enough not to get star trails because of the focal length and pixel size, but there is a very small element of field rotation that shows up as star field movement.

That's my story, and I'm sticking to it!

Patrick

--------------------

Celestron CPC 1100 XLT
10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount


My Astronomy Pages
The heavens declare the glory of God; the skies proclaim the work of his hands.

Post Extras: