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Old 30-04-2013, 09:04 AM
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PRejto (Peter)
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Guiding at longer focal length

Hello,

I have guided quite successfully with my MX mount using an 80mm f6.25 Aquila scope with my ST-i camera. I can normally see errors in guiding below .5 pixel. (TSX camera add on)

Recently I have been trying to guide through the ONAG on my TEC140 (f7)

I am seeing quite large guide errors (1+ pix) and so far I cannot find a combination of exposure and aggressiveness, etc to make this better.

Using my Aquilla guidescope the resolution is 3.05 arsec/pixel and binning 2x2 through the TEC140 = 3.11 arcsec/pixel, thus I would expect to be able to guide with the same accuracy, or is my analysis just wrong?

I would really appreciate any tips to getting this right.

Thanks,

Peter
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Old 30-04-2013, 09:36 AM
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Merlin66 (Ken)
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What do the images look like?
I never really trust the graphs....
With the default PHD settings, a Lodestar guiding with a beamsplitter (similar to your ONAG) with the C11 @f10 and the NEQ6pro mount I can easily hold the target star on a 25 micron slit for > 10mins....
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Old 30-04-2013, 09:44 AM
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Quote:
Originally Posted by Merlin66 View Post
What do the images look like?
I never really trust the graphs....
With the default PHD settings, a Lodestar guiding with a beamsplitter (similar to your ONAG) with the C11 @f10 and the NEQ6pro mount I can easily hold the target star on a 25 micron slit for > 10mins....
Hi Ken,

I've not done too many images yet, but I should add that I'm using a 2x barlow on the TEC140, so close to f15 (around 2,000 mm). Some of the images look ok, others show elongation.

But, can you help me with the original question? If two different scope/camera combinations yield essentially the same guide camera resolution should one expect to be able to achieve the same guiding accuracy? Would there be any other factor making guiding through my TEC140 so much harder?

Thanks,
Peter
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Old 30-04-2013, 09:55 AM
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multiweb (Marc)
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I recently had the same issues guiding with a SX lodestar at 1800mm. I found that some tweaking was needed in PHD.

I was guiding at 1s intervals. The first thing I had to do was increase the minimum distance from 0.15px to let's say 0.25px before a correction was issued. I also had to explicitly set DEC guiding North or South depending to avoid switching direction. Finally had to pay particular attention to balance and drop the maximum pulse time to 30ms in DEC and 180ms in RA. I also had to lower the calibration steps down to 400ms. That gave me approx. 10 to 12 steps on each axis and back.

At the end of the day if your mount is giving you +/- 2" peak to peak and your image scale drops under 1" it will be very hard to stay within the same pixel on the imager. All you can do is smoothe the rate of corrections. One good night out of 4 I went down to 2.1" FWHM and my guiding was within +/- 1". Other nights it would be 3-4" FWHM and I couldn't keep the guiding within +/-2" with the same settings in the exact same region of the sky which was right ahead at the zenith.
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Old 30-04-2013, 10:15 AM
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Peter,
In the CCDCalc sheet you didn't include the barlow...
Notwithstanding, at least in theory your comparable plate scale v's guiding should hold.
As Marc says, it comes down to seeing conditions, telescope balance and luck!
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Old 30-04-2013, 10:35 AM
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Quote:
Originally Posted by Merlin66 View Post
Peter,
In the CCDCalc sheet you didn't include the barlow...
Notwithstanding, at least in theory your comparable plate scale v's guiding should hold.
As Marc says, it comes down to seeing conditions, telescope balance and luck!
Hi Ken,

I did not include the barlow because the barlow is placed after the beam splitter...thus, the guide camera does not see the barlow at all.

Thanks for the plate scale confirmation. The "luck" part I want to avoid by knowing exactly what I'm doing. But, I'll take good luck with seeing!!

Peter
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Old 30-04-2013, 10:41 AM
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Ahhh, OK.
You may find Greg Marshall's guide to guiding interesting reading....
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File Type: pdf GuideToGuiding.pdf (93.0 KB, 45 views)
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Old 30-04-2013, 01:37 PM
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I inspected some of my images with CCDInspector last night, and was a little surprised - actually VERY VERY low pixel shift, considering all the images were taken at f/9.2 (it also revealed the SLIGHT tilt I had suspected, which will be remedied tonight).

I PREFER imaging at f/9.2 - I know that goes against the grain - even on DSO's.

Just got my Lodestar this morning, so will be interesting how better things get too.
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Old 30-04-2013, 01:57 PM
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Quote:
Originally Posted by Merlin66 View Post
Ahhh, OK.
You may find Greg Marshall's guide to guiding interesting reading....
Thanks for this! I'm not using PHD, but there are many good tips!

Peter
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Old 30-04-2013, 02:56 PM
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What package are you using for guiding??
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Old 30-04-2013, 03:33 PM
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Quote:
Originally Posted by Merlin66 View Post
What package are you using for guiding??
I using the camera add on to TheSkyX with my MX mount. Prior to that I used CCDSoft but I never had very good luck with PHD into the MX. I know some are doing it very successfully, but once I got CCDSoft going it worked well and I didn't look back.
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Old 30-04-2013, 06:51 PM
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I assume you have done a PEC curve and are using it? Also using direct guide? I am not sure if that reduces guide errors but it may as its more direct and possibly faster.

Modern autoguiders guide at the subpixel level so your resolution of the camera and guide scope are not that relevant. See the writeup at SBIG site about their STi guide kit about this.

The main problem with long focal length is flexure closely followed by seeing.

Poor seeing plays havoc with current autoguiders. Its a flaw in their design really. I have heard of one person who wrote their own software and it was more advanced. I have several concepts for more advanced autoguiders but I am unlikely to take it any further.

Roland Christen posted once that a guide scope should be mounted to the OTA itself not the mounting rings. I guess that reduces differential flexure.

You may also need to play around with your guide exposure length to see what works best. I use 2 seconds with good results on PMX but I used 5 seconds last weekend and got good results where 2 seconds stopped being effective. I use 5 to 6 seconds on my PME.

PMX can be a tad unreliable as well. The cam system to enable the 3 way switch is a source of unreliability and I personally would prefer my PMX did not have this feature and go back to the simple but bullet proof PME system. For example I had my PMX guiding to perfection on 2 nights. I knocked the counterweight shaft - fairly lightly I might add, and the next night I was getting oscillations in guiding which is indicative of the tension screws on the cam not being tight enough. Perhaps they become loose over time - not sure. Anyway, assuming your PMX is behaving itself (which is not certain with PMX) then what you are experiencing may be simply poor seeing. Everything is amplified when you go long focal length but those errors have to be taken in context that you are imaging at a longer focal length - so don't you have to multiply the magnification factor onto the guide errors to match the numbers?

Otherwise you are not comparing apples with apples. Did you get eggy stars or were they still round?

I get round stars at 3 metres and guide errors of .5 pixel or perhaps a tad more occassionally. But thats 3 metres. I get .00 to .3 errors at 1050mm and get round stars too.

STi with the lens guiding kit may be a good accessory and it looks like it would be less susceptible to differential flexure.

Guide scopes are usually cheap scopes and have poor loose focusers and perhaps not the best mounting rings etc so there is plenty of room for flexure.

With autoguiders everything needs to be tight and firm. No slop or looseness.

I often find that selecting the right guide star can make a huge difference. For example I don't always pick the brightest star. If they are too bright I think it makes the software's job of calculating the centre harder. Don't select a double star or a guide star with another similarly bright star in the frame. A nice sharp medium bright star seems to work best. The first thing I do when I start getting worse guide errors is to pick another star. It often works wonders. Errors can vanish and the guiding settles down. Also watch the guide errors and if you are getting - to + errors after every correction then your guider is correcting itself or you have backlash.

On PMX with a good PEC, good polar alignment (this has to be perfect of course and is assumed in the above), good balance, nothing dragging on the camera, you should be able to get round stars at 1800mm without any trouble.

A thread about autoguiding would be good as its critical for imaging.

All imaging roads lead to off axis guiders in the end. Perhaps SBIGs guide sensor in the filter wheel is just as good. Its a bright idea. QSI's combining an OAG in their camera is genius. FLI needs to lift its game with autoguiding. They have no solutions whatsoever. I'd say their marketing is more to the Xray market more than to the astrophotography market.

Greg.
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Old 30-04-2013, 07:03 PM
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I know I am not much help at all, but my guide scope is mounted SOLIDLY to a V bar, which is bolted to the tube rings on the main scope. There is NIL flexure, and the newer images I believe show that too.

No guide scope rings for me - tube rings holding the guide scope bolted to the main scope tube rings. Solid as a rock - in fact, can use the guide scope as a carrying handle if I wanted to
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Old 30-04-2013, 09:23 PM
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g__day (Matthew)
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One thing I have found with guiding at long focal lengths is that folks often guide too aggressively and end up chasing the seeing.

One I have trained the PE on my scope I often watch the target star in say PHD to see if it moves around randomly much - at 2.3 metre focal length - that is seeing. If I see random movements I up the maximum drift before PHD will issue a correction and up the time between images to 3-5 seconds, else I will be totally chasing the seeing and trying to guide out random atmospheric disturbance!

I wish MaximDL or PHD had routines to measure and account for both seeing and DEC backlash with a lot more smarts!
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Old 30-04-2013, 09:34 PM
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Hello Greg,

Thanks for all the feedback. Much appreciated. First though, I guess it wasn't that clear in my earlier post that I am guiding with an ONAG, so no guide scope at all, and hopefully zero flexture. But, previously I have only guided (successfully!) with an 80 mm f6.25 scope. So, this is my first attempt to guide directly through the TEC140. My camera has a barlow on it but the guider does not see the barlow....thus I'm trying to image around f14, or .59 arcsec unbinned. I think next time out I will just bin 2x2....

To answer your questions. Yes, I have good PEC, and I am aligned within 2 arcsec in Altitude and 7 arcsec in Azi...

My biggest problem with the ONAG so far has been to get good focus on the guide camera. I think I'm close, but it isn't too easy to make fine adjustments. I read contradictory things about focus...some say a bit out isn't a problem, others (like Wodaski) say you need absolute focus (which I agree with if you want to see dim stars). I'm beginning to think my problems really were with seeing the last time I was out. Next time I will try longer exposures to average out the seeing, and I may try disabling guiding in declination.

I appreciate your comments about the 3 way switch! I had an issue just two days ago...I homed, slewed and tried to plate solve and just ran into a brick wall. I wasted more than 1 hour unable to plate solve. Finally I went out to the mount and noticed that one of the knobs on the 3 way switch was not completely seated properly. I re homed, slewed and everything was normal. My fault to not seat the switch properly, but it does illustrate how sensitive it can be.

Thanks again for the help.

Peter
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Old 30-04-2013, 09:37 PM
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Quote:
Originally Posted by g__day View Post
One thing I have found with guiding at long focal lengths is that folks often guide too aggressively and end up chasing the seeing.

One I have trained the PE on my scope I often watch the target star in say PHD to see if it moves around randomly much - at 2.3 metre focal length - that is seeing. If I see random movements I up the maximum drift before PHD will issue a correction and up the time between images to 3-5 seconds, else I will be totally chasing the seeing and trying to guide out random atmospheric disturbance!

I wish MaximDL or PHD had routines to measure and account for both seeing and DEC backlash with a lot more smarts!
Thanks Matthew!

I will follow your advice next opportunity re seeing. I think that may have been the root of my problem last time outside.

Peter
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