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Quick guiding question... (PHD2)

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#1 revans

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Posted 26 November 2022 - 06:30 PM

I'm trying to image IC 342 (Caldwell 5), an open spiral galaxy in Camelopardalis.   It started out a bit breezy but seems to be improving.  Forecast winds are only 6 mph but they seemed higher to me at least initially. They seem to be dying down but I think there are some gusts that must be 10 mph.  I'm doing 2 minute subs with my C6 at 976mm focal length.  I'm using the EQ6-R Pro.

 

PHD2 says my polar alignment is only off 2 arc minutes.  But it gave me notice that my RA was an issue and suggested recalibration.  Stars looked slightly elongated to me and the guiding curve looked more erratic than usual.  So, my response was to engage Guiding Assistant and then I clicked on the correction values it suggested after taking its measurements.  

 

Things look better. PHD2 is not doing any dinging.  The stars look better. I think I'm going to end up with a result I can work with.

  

Was my response OK or was there more I could have tried or could still try?  For example using shorter subs? Taking off my dew shield (which catches what wind there is)?  Other? Too late to build a wind screen... but I think the wind will lessen as the night goes on... anyway exposures are clicking off.

 

Rick


Edited by revans, 26 November 2022 - 06:48 PM.


#2 DirtyRod

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Posted 26 November 2022 - 08:25 PM

If the GA suggestions helped and your results are good then you should be good. Normally removing the dew shield though is enough to require rebalancing the scope. You didn't mention rebalancing so I’m wondering if your balance was off in the beginning contributing to your elongated stars. 
 

Regardless, improving guiding is not generally a one and done. Some days you will need to tweak the PHD2 settings to make it work at longer focal lengths as well as trying east heavy and neutral balancing. 



#3 imtl

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Posted 26 November 2022 - 08:29 PM

You can better control your guiding etc with experience and do not need GA. However, since your steps worked then you did good for your needs/level of skiils. Well done! flowerred.gif


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#4 Sacred Heart

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Posted 26 November 2022 - 08:33 PM

Rick, There are no real answers, in my book anyway, to a given situation. We look for what might be wrong and fix it to the best of our abilities.   PA off by 2 arc minutes is a lot in my book, especially if I can see Polaris.  I use Sharpcap pro for PA and verify with a T Point run.   I have had nights when I'm guiding at .3 - .5 come back 30 minutes later and it is 1.6.  I look outside and it seems to be clear.  All I did was stop imaging, close PHD2 down, restart and recalibrate PHD2 and back in business.

 

Gremlins are out there.

 

In answer to your question,  shorter subs maybe, but not really.  It is signal to noise, you need signal to cancel noise.  Life gets tough when you do really short exposures, you need a train load of them.  By short exposures I'm thinking under a minute.

 

Taking the dew shield off may work, if your mount is strong enough to handle a little off balance, then there is light pollution that the dew shield blocks.

 

Wind screen, a long term fix, quick easy to set up / tear down, effective.   Some T posts, a tarp zip tied to PVC pipe, slide the pipe over the T post.

 

Joe


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#5 revans

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Posted 26 November 2022 - 09:55 PM

Rick, There are no real answers, in my book anyway, to a given situation. We look for what might be wrong and fix it to the best of our abilities.   PA off by 2 arc minutes is a lot in my book, especially if I can see Polaris.  I use Sharpcap pro for PA and verify with a T Point run.   I have had nights when I'm guiding at .3 - .5 come back 30 minutes later and it is 1.6.  I look outside and it seems to be clear.  All I did was stop imaging, close PHD2 down, restart and recalibrate PHD2 and back in business.

 

Gremlins are out there.

 

In answer to your question,  shorter subs maybe, but not really.  It is signal to noise, you need signal to cancel noise.  Life gets tough when you do really short exposures, you need a train load of them.  By short exposures I'm thinking under a minute.

 

Taking the dew shield off may work, if your mount is strong enough to handle a little off balance, then there is light pollution that the dew shield blocks.

 

Wind screen, a long term fix, quick easy to set up / tear down, effective.   Some T posts, a tarp zip tied to PVC pipe, slide the pipe over the T post.

 

Joe

My dew shield on the C6 is improvised and made out of dark blue posterboard cut to fit the scope and taped to the scope with a few pieces of Scotch tape... but it does work and is re-useable.  Also it doesn't have the weight of a leather like dew shield.... it almost weighs nothing.  But it can catch the wind and cause scope vibration or make it worse.  

 

I had good luck tonight as after about 90 minutes the wind became completely calm and has stayed that way.  I started imaging around 5:30 pm but my first x number of exposures are probably going to show some wind effect from the higher gusts at the time.  I'm still imaging and it is nearly 10 pm so I should have a decent number of subs that are OK. At least the stars look reasonably round to me and I appear to be pretty much in focus as far as I can tell.

 

I'm wondering if, when transit occurs just after 11 pm, if I should stop imaging or do a meridian flip.  Doing that isn't a problem for me as of the past few weeks, although it used to be.  If I do the meridian flip I can get several more hours on the galaxy but it will mean getting up at about 3 am or so to shut the rig down and take in the laptop.  There is a possibility of rain tomorrow, but probably not till the afternoon.  Still, I generally try to get the equipment protected as soon after an imaging run is over as possible.  Otherwise, there is an increased risk of dew on the laptop forming in the wee hours.

 

Anyway so far so good.  One thing that I did notice was that after 4 hours of imaging the target which had been perfectly centered gradually moved over tp the left a distance of something like 20 to 30 arc minutes (? almost a moon diameter).  So I stopped imaging and re-centered it and started imaging again.  I can't remember that happening to me in a long time.  Not sure why... but past experience tells me that Siril will not have a problem stacking.

 

Rick


Edited by revans, 26 November 2022 - 10:04 PM.


#6 revans

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Posted 26 November 2022 - 10:03 PM

If the GA suggestions helped and your results are good then you should be good. Normally removing the dew shield though is enough to require rebalancing the scope. You didn't mention rebalancing so I’m wondering if your balance was off in the beginning contributing to your elongated stars. 
 

Regardless, improving guiding is not generally a one and done. Some days you will need to tweak the PHD2 settings to make it work at longer focal lengths as well as trying east heavy and neutral balancing. 

I trying to get used to imaging smaller targets and using longer focal lengths.  It was a lot easier to get good guiding when I was at half the focal length I'm using tonight.

 

Rick



#7 revans

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Posted 26 November 2022 - 10:08 PM

You can better control your guiding etc with experience and do not need GA. However, since your steps worked then you did good for your needs/level of skiils. Well done! flowerred.gif

Yes, but I think if my level of skill was higher I likely would not have chosen to start an imaging run in what seemed like pretty windy conditions.  Based on my experience tonight, it would have been better to wait several hours after sunset to start imaging.  At least in retrospect the winds would have become calm.  But winds are hard to predict at a very local level.

 

Rick



#8 dx_ron

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Posted 26 November 2022 - 10:13 PM

Most everything is easier at shorter focal lengths cool.gif

 

It is usually easy enough to get PA error to under 2 arc-minutes, but PHD2 will guide your scope just fine up to at least 4 or 5 arcminutes error. If I'm anywhere not much over a minute I call it good and stop fiddling with it.

 

As for calibration, that is not part of Guiding Assistant. If something is off with guiding, doing a fresh calibration is probably a good idea. Point near 0° declination, clear the current calibration, and start guiding. When the calibration is finished, go back to your target.



#9 revans

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Posted 26 November 2022 - 10:36 PM

Most everything is easier at shorter focal lengths cool.gif

 

It is usually easy enough to get PA error to under 2 arc-minutes, but PHD2 will guide your scope just fine up to at least 4 or 5 arcminutes error. If I'm anywhere not much over a minute I call it good and stop fiddling with it.

 

As for calibration, that is not part of Guiding Assistant. If something is off with guiding, doing a fresh calibration is probably a good idea. Point near 0° declination, clear the current calibration, and start guiding. When the calibration is finished, go back to your target.

I'm learning quite a bit tonight.  Patience with wind is one thing.  The realization that I need to use  PHD2 in a more intelligent way is another.  Finally, I think I just experienced SCT mirror flop for the first time while imaging.  My focus suddenly went miles off over the course of two images and I was looking suddenly at a screen full of donuts.  I'm not sure that anything but mirror flop or mirror flexure could do such a thing.  The temperature is mild tonight and hasn't changed much over the past hour or so.  I refocused and started up imaging again.  But if I were not monitoring things and had gone to bed then I would have had hours of donuts to look at. I begin to see why mirror lock on the Edge scopes is a nice thing to have.  Unfortunately I have an old C6.

 

Rick


Edited by revans, 26 November 2022 - 10:37 PM.


#10 Sacred Heart

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Posted 26 November 2022 - 10:40 PM

Rick,  If you feel strong and the weather permits,  do the flip and continue imaging.  As for the dew on the laptop, close the lid down, put a kitchen plastic trash bag over the computer but leave one side open for air movement.

 

Minor hurdles, simple fixes.    That card board dew shield is great, it works that is all you can ask for.   I have / made dew shield extensions for my scopes from EVA foam I got from Hobby Lobby.  I got a roll of it 10MM thick.

 

https://www.hobbylob...10mm/p/80982131

 

I wrap this stuff around my scope, from where it connects to the Losmandy dove tail plate to about 4 - 5 inches past the existing dew shield. I have refractors a 92 and a 76MM and a 7" mak.  Being your dew shield slides on, if you go with the foam, just leave it off and use the foam.   The extra length  both covering the tube and going past the dew shield really helps with dew. I use no dew heaters.

 

Good luck tonight,   Joe


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#11 revans

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Posted 26 November 2022 - 11:45 PM

Rick,  If you feel strong and the weather permits,  do the flip and continue imaging.  As for the dew on the laptop, close the lid down, put a kitchen plastic trash bag over the computer but leave one side open for air movement.

 

Minor hurdles, simple fixes.    That card board dew shield is great, it works that is all you can ask for.   I have / made dew shield extensions for my scopes from EVA foam I got from Hobby Lobby.  I got a roll of it 10MM thick.

 

https://www.hobbylob...10mm/p/80982131

 

I wrap this stuff around my scope, from where it connects to the Losmandy dove tail plate to about 4 - 5 inches past the existing dew shield. I have refractors a 92 and a 76MM and a 7" mak.  Being your dew shield slides on, if you go with the foam, just leave it off and use the foam.   The extra length  both covering the tube and going past the dew shield really helps with dew. I use no dew heaters.

 

Good luck tonight,   Joe

Well, I successfully managed the meridian flip and have dialed in 3 more hours.  But I'm going to bed and setting the alarm for 2:45 am and will shut things down then.  Maybe something will go wrong, but at least I'm getting a few extra subs and maybe a lot... I'll know in the morning.  Hope I don't get mirror flop again.

 

Rick


Edited by revans, 26 November 2022 - 11:46 PM.


#12 imtl

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Posted 26 November 2022 - 11:52 PM

Yes, but I think if my level of skill was higher I likely would not have chosen to start an imaging run in what seemed like pretty windy conditions.  Based on my experience tonight, it would have been better to wait several hours after sunset to start imaging.  At least in retrospect the winds would have become calm.  But winds are hard to predict at a very local level.

 

Rick

Nahhh, I would still go image and see if I can get something out of it.



#13 DirtyRod

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Posted 27 November 2022 - 12:17 AM

I trying to get used to imaging smaller targets and using longer focal lengths.  It was a lot easier to get good guiding when I was at half the focal length I'm using tonight.

 

Rick

Yup. As dx_ron indicated, short focal lengths are much more forgiving. The resolution with my 350mm refractor combo is 2.8 arc-sec. I hardly even monitor because guiding has to be terrible in order to negatively impact my images. With my C8, my resolution is .69 so my guiding has to be under 1.0 in order to get decent stars so I have to watch and tune my guiding. Without the reducer it’s even harder.

 

Finally, I think I just experienced SCT mirror flop for the first time while imaging.  My focus suddenly went miles off over the course of two images and I was looking suddenly at a screen full of donuts.  I'm not sure that anything but mirror flop or mirror flexure could do such a thing.  The temperature is mild tonight and hasn't changed much over the past hour or so.  I refocused and started up imaging again.  But if I were not monitoring things and had gone to bed then I would have had hours of donuts to look at. I begin to see why mirror lock on the Edge scopes is a nice thing to have.  Unfortunately I have an old C6.

Maybe it’s time for an EAF if you don’t have one. I autofocus after every flip and every hour. More often if the temp is dropping fast. I normally don’t see donuts after a flip but the focus change is noticeable.



#14 Sacred Heart

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Posted 27 November 2022 - 12:21 AM

Rick, About mirror flop,  you know you have it, so just pay attention as to when it happens, take notes / write it down.  You cannot change the fact you have it, but you can avoid when it happens.  Just guessing here, it happens at a meridian flip and maybe when you are high, at the meridian on the west and let it ride low in the west.  Just refocus at a flip and don't go so low in the west horizon.   If those are your two places.   All that is, is knowing your equipment.  You know it is going to happen, you have a good idea when it is going to happen - work with it.    It may happen when slewing from one side to the other.

 

Joe


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