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Old 13-12-2013, 07:14 AM
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Trying to use an OAG

Hello.

I am trying to use an OAG with little success. I have been mucking around with the settings etc and it tends to still do weird things in RA and looks like its bouncing in Dec. I am using MaxIm with a Lodestar.

If I go to my guide scope I can get very nice guiding(for an EQ6) with round stars in my images with near on bog stock standard settings. I have not tried PHD as yet but I want to so I can move back to SGP.

I also am struggling with the Venetian blind look in MaxIm with the Lodestar although I have that option checked in the driver.

Any thoughts, hints or tips?

Thanks in advance.

Bart.
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Old 13-12-2013, 09:03 AM
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Hi Bart,

have you entered your guide scope's focal length in Maxim, site settings?
if using an OAG, it'll be the same as the main scope so you'll need to update that when on the OAG and change it when using the guide scope.
Have you also calibrated your main scope? that's primarily to determine the image scale, does it show the correct scale?
apart from that, if you calibrate and it has moved sufficiently in x and y, then you shouldn't have issues.
if it is drifting in dec, have you checked your polar alignment?
guiding will correct to a certain extent, but if the PA is poor, then it will drift off pretty quick.
as for the venetian blinds, its one of those settings. I had to play with a few of them till i got it right. i'll take a screen shot later on.
also turn off "display progress" during download. that creates a horizontal line.
what scope are you using and are you using pulse guiding or st4?
I use an OAG with a lodestar and maxim and find that I have to sometimes switch to ST4. other times I use pulse guiding. both work very well though.

Cheers
Alistair
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Old 13-12-2013, 10:01 AM
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Autogiding needs to be tuned for a given setup. A guide scope would be one setup, an OAG another.

Assuming everything is tight and no looseness or flex and the camera is well secured etc etc (the basics are being done properly) then the next step is getting the settings right.

Can you post your exact procedures for setting up the OAG guide camera in software?

The usual problem with an OAG and I take it from your post you are past that is to get the stars in focus. I find if the stars are not close to being in focus they disappear off the screen and it looks like the OAG is not working. So assuming you have a focused guide star then it just leaves the autogudiing setup.

00. Polar alignment must be close to perfect. Autoguiding is not a substute for excellent polar alignment. Most autoguiding problems in my experience are actually not good enough polar alignment. So drift align or use software to achieve an accurate polar alignment before bothering with anything else. Same with balance and cable drag. Get your scope well balanced and sort cables so no drag.

0. Try to have your guide camera square to the scope ie not an angle so north on the image is straight up. That way the software does not need to correct for the camera being rotated.

1. You will need auto dark subtract with a Lodestar as it can have hot pixels that the software mistakes for the guide star. I think these software detect the brightest point in the framed image and it is assumed that is the guide star. Without a dark subtract the relatively noisy Lodestar needs a dark subtract, especially in summer now its getting hot. So setup a master dark for your Lodestar and set up Maxim to do a dark subtract on every image. If you don't you will get erratic guiding where the software will do massive moves to try to correct for a hot pixel instead of the guide star. I have seen it do this many times whenusing CCDSoft which will not do a library dark for autoguiders.

2. Callibrate the autoguider. The amount of a correction needed for the
mount to shift to correct for the guide star moving needs to be calculated as a scale. Also non squareness of the guide camera needs to be accounted for if its not done per the first step above.

3. Guide exposures vary with mounts. Well polar aligned and low PE mounts go better with longer exposures like 4 to 6 seconds but a mount with high PE needs fast corrections like 1 second guide exposures. Same with aggressiveness. I would start at about 7 and I reduce that to 5 or so when the mount is well polar aligned and PE is low.

4. Are you using PEC? This will help and you will see larger guide corrections because of it but the stars will be more round.

5. I set min/max move as I don't want a moment of bad seeing sending the mount into an overcorrect. I usually set mine to about min .1 and max 2.

6. Select a suitable guide star. Not too bright and fat and not a double star. Not a star with another star of similar brightness just outside the framing box. That could easily drift into the field of view and get confused with the guide star. Also it will mess up callibration where the mount moves, takes an image, moves again so stars just outside the framing box will move into the frame and mess things up.

7. I often recallibrate when on the other side of the meridian or have slewed a long way away from the last point of callibration. Not an important point but I think it reduces guide errors a tad.

8. The first thing I do if I get too large guide errors is to pick another guide star. Perhaps one a bit tighter. Often the errors drop enormously just doing that.

Greg.
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Old 13-12-2013, 10:14 AM
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Hi Bart,
You could have too much backlash in the EQ6 mount.
That might explain why it works with a small guide scope & not an OAG.
I had the backlash adjusted out by an expert to solve a similar problem.

PM me if you want the name of that person.

cheers
Allan
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Old 13-12-2013, 03:58 PM
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Merlin66 (Ken)
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I use the equivalent of OAG with the reflective slit guiding on the spectroscope. A C11 at f10, on a NEQ6pro mount.
I use AA5 for image acquisition and processing, but PHD for guiding with the Lodestar.
99.9% of the time - no issues, no drama.
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Old 13-12-2013, 06:07 PM
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Quote:
Originally Posted by alistairsam View Post
Hi Bart,

have you entered your guide scope's focal length in Maxim, site settings?
if using an OAG, it'll be the same as the main scope so you'll need to update that when on the OAG and change it when using the guide scope.

Yep, that is done.


Have you also calibrated your main scope? that's primarily to determine the image scale, does it show the correct scale?
apart from that, if you calibrate and it has moved sufficiently in x and y, then you shouldn't have issues.

I guess you mean calibrate it under the guide tab? If so, that is done. Or do you mean under the telescope tab in the observatory applet?


if it is drifting in dec, have you checked your polar alignment?
guiding will correct to a certain extent, but if the PA is poor, then it will drift off pretty quick.

I realise that my polar alignment could be better although I am pretty happy with it over ten minutes using StarTarg.


as for the venetian blinds, its one of those settings. I had to play with a few of them till i got it right. i'll take a screen shot later on.
also turn off "display progress" during download. that creates a horizontal line.

A screen shot would be great and I have the venetian blind option checked I think and all other settings off.


what scope are you using and are you using pulse guiding or st4?
I use an OAG with a lodestar and maxim and find that I have to sometimes switch to ST4. other times I use pulse guiding. both work very well though.

I am using a TSA120 with a QHY9M through a SXUSB filter wheel and a SXOAG with a Lodestar on an EQ6 Pro with belt mod. I have never had good luck with pulse guiding and have always used a GPUSB and ST4 with good results. Should I try the on camera guide port?

Cheers
Alistair
Answers above, thanks Alistair.
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  #7  
Old 13-12-2013, 06:26 PM
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Autogiding needs to be tuned for a given setup. A guide scope would be one setup, an OAG another.

Assuming everything is tight and no looseness or flex and the camera is well secured etc etc (the basics are being done properly) then the next step is getting the settings right.

Can you post your exact procedures for setting up the OAG guide camera in software?

The usual problem with an OAG and I take it from your post you are past that is to get the stars in focus. I find if the stars are not close to being in focus they disappear off the screen and it looks like the OAG is not working. So assuming you have a focused guide star then it just leaves the autogudiing setup.

No problem setting up, making sure everything is tight and I am able to get good focus using a Bahtinov mask. I also used my light box to adjust the prism until it cast a shadow on my main image and then just backed it off, so I have enough penetration into the optical path as well.

00. Polar alignment must be close to perfect. Autoguiding is not a substute for excellent polar alignment. Most autoguiding problems in my experience are actually not good enough polar alignment. So drift align or use software to achieve an accurate polar alignment before bothering with anything else. Same with balance and cable drag. Get your scope well balanced and sort cables so no drag.

Yep, all this has been looked into and all is well. As I said to Alistair, I am happy with my PA and will refine it further when I replace the bolts for the azimuth adjusters. Balance is good and there are no snagging cables, I'm very fussy about all of that.

0. Try to have your guide camera square to the scope ie not an angle so north on the image is straight up. That way the software does not need to correct for the camera being rotated.

Yep, makes sense and is done and checked.

1. You will need auto dark subtract with a Lodestar as it can have hot pixels that the software mistakes for the guide star. I think these software detect the brightest point in the framed image and it is assumed that is the guide star. Without a dark subtract the relatively noisy Lodestar needs a dark subtract, especially in summer now its getting hot. So setup a master dark for your Lodestar and set up Maxim to do a dark subtract on every image. If you don't you will get erratic guiding where the software will do massive moves to try to correct for a hot pixel instead of the guide star. I have seen it do this many times whenusing CCDSoft which will not do a library dark for autoguiders.

Yep, darks are a must and that is also done as well as choosing a good star not near the others and away from the edge etc.

2. Callibrate the autoguider. The amount of a correction needed for the
mount to shift to correct for the guide star moving needs to be calculated as a scale. Also non squareness of the guide camera needs to be accounted for if its not done per the first step above.

Always. Been at this for some time now.

3. Guide exposures vary with mounts. Well polar aligned and low PE mounts go better with longer exposures like 4 to 6 seconds but a mount with high PE needs fast corrections like 1 second guide exposures. Same with aggressiveness. I would start at about 7 and I reduce that to 5 or so when the mount is well polar aligned and PE is low.

Traditionally with a guide scope I can do 1 to 2 seconds with good results, and that is also because the EQ6 is not know for its great PE.

4. Are you using PEC? This will help and you will see larger guide corrections because of it but the stars will be more round.

I have had a foray or two into PEC and find that it can be obtrusive using ST4 guiding. Maybe I am doing something wrong here and should give pulse guiding another shot.

5. I set min/max move as I don't want a moment of bad seeing sending the mount into an overcorrect. I usually set mine to about min .1 and max 2.

Tried this last night but will have another go again tonight.


6. Select a suitable guide star. Not too bright and fat and not a double star. Not a star with another star of similar brightness just outside the framing box. That could easily drift into the field of view and get confused with the guide star. Also it will mess up callibration where the mount moves, takes an image, moves again so stars just outside the framing box will move into the frame and mess things up.

Answered above.

7. I often recallibrate when on the other side of the meridian or have slewed a long way away from the last point of callibration. Not an important point but I think it reduces guide errors a tad.

I too recalibrate generally when I move to a new target even though it says you shouldn't need to.

8. The first thing I do if I get too large guide errors is to pick another guide star. Perhaps one a bit tighter. Often the errors drop enormously just doing that.

I will look into this also, although I must admit that I experience a huge difference in available light to the camera between guide scope and OAG. I would like to go to a bigger focuser and open up the imaging train however it seems the only flattener for the TSA120 is a 40mm imaging circle style (TOA35), which makes me wonder if I am wasting my time using 42 mm gear instead of 54mm or 72mm and should stick with a guide scope.

Greg.

Thanks Greg, made me run everything through in my mind again and gave me a few thinkers as well.
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  #8  
Old 13-12-2013, 06:29 PM
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Quote:
Originally Posted by alpal View Post
Hi Bart,
You could have too much backlash in the EQ6 mount.
That might explain why it works with a small guide scope & not an OAG.
I had the backlash adjusted out by an expert to solve a similar problem.

PM me if you want the name of that person.

cheers
Allan
Hi Allan,

Thanks for the thought, I will keep that in mind and will PM you if I think I need to. The mount is a belt drive modded EQ6 and has recently had the belt tensions checked and the worm and ring gear adjusted so there is no slop or binding although I will check again.

Thanks, mate.
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Old 13-12-2013, 06:32 PM
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Quote:
Originally Posted by Merlin66 View Post
I use the equivalent of OAG with the reflective slit guiding on the spectroscope. A C11 at f10, on a NEQ6pro mount.
I use AA5 for image acquisition and processing, but PHD for guiding with the Lodestar.
99.9% of the time - no issues, no drama.
Hi Merlin,

I will give PHD a go too, although I like to use MaxIm so I can dither. If I use PHD I will try to use SGP as well as that will allow me to dither also.

Thanks again,

Bart.
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Old 13-12-2013, 09:23 PM
ericwbenson (Eric)
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Hi Bart,

After you get the basics setup, such as:
1) OAG orientation: square to main imager and above main chip is a good start
2) Camera orientation: up-down = North South. Verify by turning on image crosshair and slewing star with hand paddle, it should stay near the line. Image might be flipped left-right or upside down, don't worry about that.
3) Set the Lodestar binning to 2, always, it cleans up the venetian blind effect and makes for more sensitive pixels.
4) Calibration: nice L shape (just make sure the calibration times are long enough). DEC backlash will show up here, not much you can do about it.

The following is optional but will improve picking up faint guide stars. If you are going to choose your guide star manually you can skip:
5) Capture a set of bias and 10 sec darks (10-20 frames each should be fine) using the Autosave in Camera Control, you won't be using longer than 10 sec guide exposures I expect!
6) Go to Process...Set Calibration and import these frames, use sigma clipping (default settings) and Autoscale darks. Before exiting hit the "Replace with Masters" button. Exit dialog
7) Now you can use Options...Full calibration on the CCD guider window to calibrate the guider frames

Even more optional:
8) Process...Create Master frames, pick the Dark master frame for the guider and go into Process...Replace Bad Pixels. Hit Auto generate, use Dead pixel threshold=95 and hot pixel threshold= 300, ok. Change the map name to something like SXL2x2_c95_h300, save map, exit, don't save changes to master file.
9) Go back into Process...Set Calibration and pick the master dark and assign a Bad Pixel Map.


10) Tune the AG loop. Find a star that hits 20-40K counts in ~1 sec exposure. Start with aggressiveness zero on both axes, watch the guider graph. The ugly RA sinusoid and erratic DEC drift line are what you're AG is fighting, it won't always win, but you hope it can get a draw...
Increase aggr to 2 on one axis, watch a few minutes hope thing improve, increase by two more, watch some more, etc. Never go beyond 8, try to use 6 or 7 if your mount is smooth enough. Repeat for the other axis.

Watch the RMS number it is more important than peak-to-peak. NB it might be in pixels, multiply by guider image scale to get arcseconds:
(guider scale = 206 * binning * pixel_size_um / focal_length_mm)
so in your case gs = 412 * 8.3 / EFL.
RMS < 0.5" is ok, < 0.3" is good, < 0.1" is fantastic, beware this will scale with guiding quality AND local seeing conditions, so a good moonlit night is best (you don't feel guilty about wasting imaging time!)

That's it. If your still not happy, better look at a bigger mount :>

HTH,
EB
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Old 14-12-2013, 05:46 AM
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Quote:
Originally Posted by Bart View Post
Hi Allan,

Thanks for the thought, I will keep that in mind and will PM you if I think I need to. The mount is a belt drive modded EQ6 and has recently had the belt tensions checked and the worm and ring gear adjusted so there is no slop or binding although I will check again.

Thanks, mate.

Hi Bart,
sorry - I thought you had a worm drive like my older NEQ6 pro.
The belt drive should be ok.

Another suggestion - try PHD guiding.

cheers
Allan
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Old 14-12-2013, 06:07 PM
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Quote:
Originally Posted by ericwbenson View Post
Hi Bart,

After you get the basics setup, such as:
1) OAG orientation: square to main imager and above main chip is a good start
2) Camera orientation: up-down = North South. Verify by turning on image crosshair and slewing star with hand paddle, it should stay near the line. Image might be flipped left-right or upside down, don't worry about that.
3) Set the Lodestar binning to 2, always, it cleans up the venetian blind effect and makes for more sensitive pixels.
4) Calibration: nice L shape (just make sure the calibration times are long enough). DEC backlash will show up here, not much you can do about it.

The following is optional but will improve picking up faint guide stars. If you are going to choose your guide star manually you can skip:
5) Capture a set of bias and 10 sec darks (10-20 frames each should be fine) using the Autosave in Camera Control, you won't be using longer than 10 sec guide exposures I expect!
6) Go to Process...Set Calibration and import these frames, use sigma clipping (default settings) and Autoscale darks. Before exiting hit the "Replace with Masters" button. Exit dialog
7) Now you can use Options...Full calibration on the CCD guider window to calibrate the guider frames

Even more optional:
8) Process...Create Master frames, pick the Dark master frame for the guider and go into Process...Replace Bad Pixels. Hit Auto generate, use Dead pixel threshold=95 and hot pixel threshold= 300, ok. Change the map name to something like SXL2x2_c95_h300, save map, exit, don't save changes to master file.
9) Go back into Process...Set Calibration and pick the master dark and assign a Bad Pixel Map.


10) Tune the AG loop. Find a star that hits 20-40K counts in ~1 sec exposure. Start with aggressiveness zero on both axes, watch the guider graph. The ugly RA sinusoid and erratic DEC drift line are what you're AG is fighting, it won't always win, but you hope it can get a draw...
Increase aggr to 2 on one axis, watch a few minutes hope thing improve, increase by two more, watch some more, etc. Never go beyond 8, try to use 6 or 7 if your mount is smooth enough. Repeat for the other axis.

Watch the RMS number it is more important than peak-to-peak. NB it might be in pixels, multiply by guider image scale to get arcseconds:
(guider scale = 206 * binning * pixel_size_um / focal_length_mm)
so in your case gs = 412 * 8.3 / EFL.
RMS < 0.5" is ok, < 0.3" is good, < 0.1" is fantastic, beware this will scale with guiding quality AND local seeing conditions, so a good moonlit night is best (you don't feel guilty about wasting imaging time!)

That's it. If your still not happy, better look at a bigger mount :>

HTH,
EB
Hi,

Thanks for the informative post. I will give this a go.

Cheers.
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Old 14-12-2013, 06:08 PM
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Quote:
Originally Posted by alpal View Post
Hi Bart,
sorry - I thought you had a worm drive like my older NEQ6 pro.
The belt drive should be ok.

Another suggestion - try PHD guiding.

cheers
Allan
No dramas mate, thanks for being part of the solution!
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Old 18-12-2013, 01:39 PM
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Well, thanks for the advice everyone, however; I just cannot get any further with this.

I am unable to detect any stars in the field after it is set up. I can get Canopus on the chip and using the Bahtinov mask can get both cameras focused well and all works in harmony.

Using 2x2 binning etc, I point it to an object I wish to image and I just cannot find any stars. In frustration I popped the Lodestar into my guide scope and voila, how many stars do you want? I slipped a par focal ring onto the camera when focused in the guide scope so on clear moonlit nights I can keep chipping away.

Thanks again, everyone, your input is much appreciated.
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Old 18-12-2013, 01:53 PM
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Bart,
As you've probably already guessed, there's something wrong here...

You say:
I am unable to detect any stars in the field after it is set up. I can get Canopus on the chip and using the Bahtinov mask can get both cameras focused well and all works in harmony.

So you can get Canopus in both cameras? You should not be able to do this without re-setting the position of the mount.
The pick-off prism only sees the extreme outer edge of the field (hopefully just outside the imaging FOV...)
Have you tried during the day focused on a distant object?

The attached images are not mine, but show both the OAG set-up and the relative size and position of the guider image relative to the main image.
Hope this helps.
Attached Thumbnails
Click for full-size image (OAG setup.jpg)
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Click for full-size image (OAG FoV.jpg)
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Old 18-12-2013, 02:24 PM
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LOL!

Sorry, no, not at the same time! I know around about in my finder where my OAG field is and first focused Canopus in the main camera with the focuser and then moved the OAG to pick up Canopus and slide the Lodestar in and out to focus.

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Old 18-12-2013, 02:57 PM
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The Lodestar should be able to pick up stars in the guide field...
I use one with my reflective slit plate and never have had any issues...
what's different?
If the imaging camera and the guide camera are par-focal?????
Very strange.
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Old 18-12-2013, 08:09 PM
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Quote:
Originally Posted by Bart View Post
Well, thanks for the advice everyone, however; I just cannot get any further with this.

I am unable to detect any stars in the field after it is set up. I can get Canopus on the chip and using the Bahtinov mask can get both cameras focused well and all works in harmony.

Using 2x2 binning etc, I point it to an object I wish to image and I just cannot find any stars. In frustration I popped the Lodestar into my guide scope and voila, how many stars do you want? I slipped a par focal ring onto the camera when focused in the guide scope so on clear moonlit nights I can keep chipping away.

Thanks again, everyone, your input is much appreciated.
that is really wierd
have you detailed which oag and whether its in front or behind the filter wheel?
are you able to post a screen shot of your maxim session with an exposure from the lodestar
and the lodestar settings window

have you tried without dark subtraction and say a 10 sec exposure?
unfortunate that you're giving up on this.
an oag makes guiding so much tighter.
can you post a pic of your gear with the oag and the lodestar
I use mine from horribly light polluted skies and always have atleast 5 stars to guide on with a 1m focal length

cheers
Alistair
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Old 19-12-2013, 08:48 AM
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It is a SX OAG that bolts to the front of my filter wheel, so it is in front of my filters. I have not tried a long exposure like 10 seconds or tried it without dark frames. When the weather clears up again I will set it up again and give it another bash and try to capture some screen shots to post.

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Old 19-12-2013, 08:23 PM
ericwbenson (Eric)
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Quote:
Originally Posted by Bart View Post
I am unable to detect any stars in the field after it is set up....Using 2x2 binning etc, I point it to an object I wish to image and I just cannot find any stars.
It is pretty unlikely that you find a bright enough star in the oag "by luck" (i.e. if you just point the main camera at the object you want to image). Remember the FOV of your OAG is tiny compare to the small guide scope. The reason rotators are so popular is to fix this problem, the accessible area around the main chip with the rotator is 10-20x bigger.

You can manually rotate the image train in most setups. So plan ahead with a star chart (e.g. TheSky) and find a bright enough star near the object and put the OAG in that orientation (hence why put the OAG above the main imager and remember which way is north on the camera). Otherwise you will need pretty long guider exposures, which may negate the whole point of guiding if the mount needs fast corrections (i.e. gear noise).

To determine how bright a star you need is easy use 1 sec exposure on a 6th mag star to start, look at the Signal to Noise Ratio (SNR value in Maxim information window). Every magnitude fainter needs 2.5 times more exposure for the same SNR, so an 11th mag star needs a 100 sec exposure for the same SNR as the 6th mag star, ouch. Usually SNR > 10 is desired for guiding, SNR > 100 is fantastic, you can go as low as SNR~3, but things get dodgy below ~10 in my opinion.

The optional steps I listed in the previous post are there to reduce the noise term hence making the SNR bigger for the same exposure and allowing you to guide on fainter stars. Which means sometimes no rotating the camera!!!

Best,
EB
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Astronomy and Electronics Centre
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