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AndyUK

PHD and SX Lodestar guiding problems... (PT 2)

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At long last another clear(ish) night tonight, so I was able to have another crack at getting the Lodestar and PHD working through the finderscope with the MN190. I got a bit further than last time - at least it calibrated and said it was guiding :icon_scratch:, but there's obviously a setting somewhere (or something else) I haven't got quite right :)

Here's a few shots of the settings, some graphs and a couple of sample 900s subs - There were no animals out in the garden, no real detectable breeze, and no-one clumping about... and I have the tripod sat on wooden blocks so the legs can't sink into the lawn. In the graphs Dec appears to be fine, but RA appears to be all over the place which is obviously displayed in the images - I'm fairly confident the scope's well balanced, as I've taken an effort to ensure that any and all weight is "on-axis", and the last thing I do before finalising the framing is offset one of the weights by about 1cm to make the mount east-heavy...

Has anyone any thoughts on what I'm doing wrong or what setting I should be playing with to get accurate guiding?

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by looking at your pictures i see there is a movement (look at the two frames back and forth and you will see the motion). this movement can have many causes, i would check the following.

1) good polar alignment. if possible drift align

2) check the colimation of your telescope

3) how is the guider placed at your scope? is it in perfect focus? haw is the camera mounted at the finderscope? check for flexure. if you use the finder bracket of the finderscope get rid of it. it has a spring screw that has a lot of flexure.

4) you said you moved the counterweight 1cm to east in order to make it east heavy. in my opinion you shouldn't imbalance your scope at all since any imbalance at the ra axis will make the mount either too slow to follow the target (and that's what i see from your phd graph) or too fast. the imballance is made in order to lock the gears of the mount and reduce the backlash which phd calculates during the calibration process. so next time try perfect balance at your scope. also check this page Astronomy Shed UK Astronomy Forum • View topic - Complete Setup From A to Z (you need to register to see the rest of the videos or you can go at the author's channel at youtube) at some point he says how to balance the scope for imaging

5) i see that you have the min motion too low incerase it a bit in order to reduce seeing effects at guiding, also reduce the hysterisis to 0 from 17. also 4 secs is a lot of time try lower times form 1.5 secs to 3 secs and also try 1 and 0.5 secs

6) how is the scope mounted on your mount. do you have a vixen dovetail or losmandy. if you have a vixen dovetail i would suggest to change it to an adm dual saddle it makes a huge difference.

Edited by kookoo_gr

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Try using less time between guide signals, 4s is quite long with the EQ6/5's.

ALso you have an orange box around your guide star, it should be solid green. To me this indicates a guide star problem.

These are just problems you have to work through when using such a heavy large scope on the budget mounts :)

Edited by shaunster

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Hi Kookoo - Thanks very much for getting back with the comments and advice... I hadn't checked for drift between images, so thanks for the heads up on that one....

1) good polar alignment. if possible drift align
Admittedly, I didn't drift align, but I never had subs showing such poor guiding with the Synguider(?). Obviously I can try and give it a go to see if it makes any difference...
2) check the colimation of your telescope
I'd like to think the collimation is spot on - I use a Hotech laser collimator as part of the set up process to check not only primary/secondary but also orthagonality of the focuser (an OCD of mine!) and then also perform a star check...
3) how is the guider placed at your scope? is it in perfect focus? haw is the camera mounted at the finderscope? check for flexure. if you use the finder bracket of the finderscope get rid of it. it has a spring screw that has a lot of flexure
The finderscope bracket is mounted on top of the scope in a TS finder scope shoe, which in itself is screwed tight into a 350mm Losmandy plate. I'm pretty sure that the finderscope's reasonably well focused (as per image), although I have read somewhere that it's recommended to slightly defocus to give better guiding(? I haven't done this though!). I AM using the finderscope bracket though (with the spring) so that's definitely something I can look into!
4) you said you moved the counterweight 1cm to east in order to make it east heavy. in my opinion you shouldn't imbalance your scope at all since any imbalance at the ra axis will make the mount either too slow to follow the target (and that's what i see from your phd graph) or too fast. the imballance is made in order to lock the gears of the mount and reduce the backlash which phd calculates during the calibration process. so next time try perfect balance at your scope
You're right - I was unbalancing a tad to ensure that the worm gear had something to push against and wasn't "floating". However, I'll try with perfect balance next time and let PHD work it out!
5) i see that you have the min motion too low incerase it a bit in order to reduce seeing effects at guiding, also reduce the hysterisis to 0 from 17. also 4 secs is a lot of time try lower times form 1.5 secs to 3 secs and also try 1 and 0.5 secs
I'd set the min motion to 0.05 as after some web searching (and advise from others on SGL who use finderguiders) that appeared to be more or less the concensus (due to the small FOV they provide), but again thanks for pointing it out as another possible avenue to look at. I'll try 2s exposures next time though (and remove the Hysteresis - It sounds a nice idea but it makes a lot of sense to keep it simple!)
6) how is the scope mounted on your mount. do you have a vixen dovetail or losmandy. if you have a vixen dovetail i would suggest to change it to an adm dual saddle it makes a huge difference
The OTA is sandwiched between 2 x 350mm Losmandy plates - I don't think there's much else I can do in that department :)

@Shaunster - Thanks for your comments too - As noted above, I'll aim for 2s exposures next time, but well spotted on the amber box - When I selected the star though it did have a solid green box (honest!), but when I took that screen dump I was wondering if the Mass / Vs figures meant anything(?), and I had to time the screen shot to coincide with his info display - When selecting a star, I knew I didn't want to select one that's too bright (saturated), so I was using the Vs figure as a guide, assuming that 57639 was like an ADU value(?) - What exactly makes "an appropriate guidestar" for PHD?

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In the cold light of day, I've managed to find out some more info... and once again, I now feel a bit of an idiot - I wondered why the laptop was ocassionally "pinging"...! :)

Now I know it was because it had lost the star, but presumably PHD was then able to "regrab it" (I thought it was simply PHD telling me it was making a correction :icon_scratch:). So I obviously made a LOT of errors last night, but I think I can now start to put together a checklist for next time - Cheers both (and fingers crossed for next time...)

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Oh well Andy... At least you know now!! :)
Yes, if there's a wrong way to do something, I'll inevitably find it...!

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I wish you well Andy - As I said in my PM, I am reducing my exposure back down after the 3s ones the other day all went in the bin as they were all just a little oddly blobby. Also, you asked about the spring in my finderscope. I initially used a set of guide rings, then changed to the standard SW finderscope bracket with the spring. The change was nothing to do with the guiding, but at the time, I wanted guide rings for my finderscope. I find no movement in there at all. I have also just tied all my cables in with a cable tidy to make sure that nothing is slopping about.

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You say thanks, but I think I've just jinxed myself - Would appear I have an issue 'somewhere' but going by the graph, I don't think it's the guiding.

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Andy, just a quick thought. Your min motion is way too low. At 0.05 pixels you'll be chasing the seeing all the time. If you're using the SW 80 or the ST102 the lowest setting should be 0.23.

I've found the best way to get better guiding is to use the defaults for every thing (apart from min.motion) and adjust the calibration step gradually until the graph and the subs start to look good. My calibration step is 3500 although I set it to 4000 last time I was out and had got 1 perfect 900s sub before the clouds came in.

You should be able to guide really well with an exposure length of about 2.5 seconds.

Also I would continue to use a slight imbalance as the worm does perform better with a little resistance to push against.

Good luck :)

Mark

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Hi Mark - Thanks very much for your input... but your comments have prompted quite a few more questions - I hope you don't mind?

Just for background info, until 5 weeks ago, I used a Synguider with a piggybacked ST102 in guiderings on the MN190, but I was getting differential flexure issues, and everything was pointing to the weight of the OTA and the rings. As I knew from a previous attempt that I couldn't use the Synguider with a finderguider, I ditched the synguider, rings and ST102 and bought the Lodestar to use through a finderscope.

The reason I set the min motion so low is that I'm using a finderguider, not a "conventional" guidescope - All the information I've read seemed to concur that as a 9x50 has such a low FL (c. 210mm), the min motion setting should be much lower than the default 0.25, and the concensus seemed to be that 0.05 was about right(?). One source I found even stated he used a setting of 0, but I thought that sounded a bit extreme..!

However, by the sounds of it, this is a setting I need to play with - Is there perhaps some equation that relates FL to min motion setting?

I'm also intrigued by your comment that you set calibration to 3500 but changed it to 4000 last time - I was rather hoping (perhaps naively) that once I'd found "THE" setting for the finderguider with the MN190 (and presumably it may be slightly different with the Equinox 80), that that would be it, and I could replicate it each time, regardless of image exposure duration or object placement in the sky... but I'm now guessing that I might be wrong here... :)

How do you know whether you need to change the calibration setting...? Is it simply a case of setting it up and watching the graph for 5-10 mins or so... and if the graph isn't reasonably flat do you then stop and recallibrate with a different setting? (And how do you know whether you need to increase or decrease it?)

Just for reference, my calibration last time out (using 2500) took about 8 mins to go through 20 odd steps E/W and N/S - Is this normal, or is this too long? I guess by increasing to 3500 (or 4000), this would speed up the calibration process and would mean less steps(?), but is there an optimum number of calibration steps one should try and aim for?

I'll certainly be trying around the 2-2.5s mark next time out and also trying to choose a better star to guide from - Having read that it wasn't advisable to guide on a saturated star, I chose one that was actually quite dim last time, but of course with a 4s exposure (and with the sensitivity of the Lodestar) that star may have been about mag 10 or so!

Thanks again for helping out here - I have to admit that having such issues getting this to work, and especially given that it's called Push Here Dummy, I feel even more of an idiot than usual!!!

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Hi Andy. I don't mind the questions at all. To be honest I am still looking for a definitive procedure to getting PHD/guiding working every time. It should be called "Push Here Dummy (If you have a PHD you stand a chance of getting it working)".

Firstly the formula for the min.motion:-

I've found a formula to work out the resolution of a camera and scope in arcseconds per pixel. From that you can work out the minimum pixel movement the camera can make out. Here it is:-

as/pixel = (206.3 * ccd_pixel_size) / focal_length.

The Loadstar has 8.2um * 8.4um pixels and you have a focal length of 210mm so:-

(206.3 * 8.2) / 210 = 8.05 as/pixel.

We want pixels per arcsecond so I flip it:-

1/8.05 = 0.124 pixels/arcsecond.

As PHD wants sub-pixel movements I divide that number by half a pixel:-

0.124 / 2 = 0.06.

So as you said 0.05 is just about right.

Calibration Step:-

I adjust the calibration step according to the declination of the target. I watch how many calibration steps the setting takes and try to get them between 10 and 16 for both EW & NS. My last target was IC10 in Cassiopeia and 3500 wasn't getting good guiding. It was working but the stars weren't great. I've found that rather than tweaking the other settings I play with calibration step first until it's pretty good then smooth out any bumps by fine tuning the other settings.

It's a good game this isn't it!

Mark

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Aha....! Thank you so much - There's some light appearing in the fog...!

Next time out, my plan is now to set up as normal (with a slight east weight bias), use 2.5s exposure and select a brighter star than I did previously and try the attached settings to begin with -

PHDSettings-1.jpg

I might up the Hysteresis up to 10 (which I believe is the default), and I'll try tweaking the calibration value to get me in between 10-16 (as you've suggested) and see how it goes from there.

At the moment, I'd be happy with a flat(ish) guide graph - If the star shapes are a bit out, well, that's probably another story... but this is brilliant - I have a plan - Thank you VERY much! :)

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Andy that sounds like a plan and I would start with 10% hysteresis too.

It's looking a little better weather for this weekend so here's hoping you get a chance to get out there. Let us know how you get on mate. Good luck. :rolleyes:

Mark

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Thanks Mark - I'm hoping to be able to get out tomorrow night... I'll certainly let you know how I get on, but I feel a little more confident now... :rolleyes:

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mark, a question about the formula. is it the same if i bin my qhy-5 2x2?

Hi KooKoo. Just saw your question. If you bin x2 you are (effectively) doubling the size of the pixels. So instead of 5.2um your pixel size would be 10.4um for the purpose of the calculation.

Are you thinking of binning the chip for guiding?

Hope that helps.

Mark

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i always bin my guide camera in order to decrease the noise thus making it easier to find a guide star with my oag and also reduce the effects of seeing. i found that with min motion at 1.4 the guiding is fine. i tried with lower and higher pixel values but i had no success, therefore and the question

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In the cold light of day, I've managed to find out some more info... and once again, I now feel a bit of an idiot - I wondered why the laptop was ocassionally "pinging"...! :)

The red band at the bottom of PHD in your capture indicates the loss of the guide star too (my laptop is muted and that's how I notice).

I use a QHY5 and a finder guider. I set up PHD as it comes, and only adjust the calibration steps too. I increase it until it only take a dozen or so. I've used both EQMOD and ST4 guiding, both work fine.

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The red band at the bottom of PHD in your capture indicates the loss of the guide star too
Obviously something else I missed... Thanks for pointing it out - Cheers!

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