Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Anyway to do "rough" and low cost autoguiding with an SE mount without using a wedge?

  • Please log in to reply
20 replies to this topic

#1 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 01:12 PM

A few months back I was searching for info for doing autoguiding on my 6SE and frankly I couldn't find any info that really matched my needs (or thoughts).  Most of the info was from an AP POV for long exposure tracking, that the SEs simply can't do in alt/az, or a few successful but mostly unsuccessful attempts with a wedge and RA autoguiding,.  But I really couldn't find much of anything if just wanting to keep the object being imaged "near" the center of the imagers FOV from occasional nudges in alt/az mode and taking many short exposure (< 30 sec) frames (lucky imaging).  Similar to what I guess Firecapture can do on planets?

 

Maybe this is in part an autoguiding software issue too as I think about it, but my initial thoughts are to nudge the object back to center every few secs up to once a minute or so to keep it better centered for live stacking or the acquisition of FITS, SER, or AVI files.  Sure you'd lose a few frames due to scope travel during the "nudge" but big deal if enough good frames are captured.  Best case would be if the autoguiding software and acquisition software could be synced such that for the "longer" short exposures (say the 1 to 30 secs ones) the object is "nudged" back near center with say a 1 sec mount settle time before each exposure.  Field rotation is still an issue but just having the object or region of interest kept close to the center of the FOV during the whole acquisition would be a big plus.  Enough times I've had the object slowly drift out of the FOV of my 6SE while livestacking and then need to recenter, wait for the mount to start tracking again as best it can, and then restart the image acquistions.  Some days (er nights??) the tracking is pretty darn good but others not so much so having the ability to have the object automatically recentered would be fantastic!

 

I figure (with little real autoguiding knowledge here) even with unsynced nudges and exposures that doing a recentering every ~60 secs with 5 sec or less exposures should give a lot of useable frames for the non-live stacking type acquisitions for post processing and when live stacking the frames with mount movement would just immediately get thrown out.

 

Somewhat more specifically to the SE mounts, gear backlash is an issue, but I would think setting the backlash settings so there is a small jump in the image upon direction of travel reversal would pretty much guarantee every time the autoguiding software says to move in a certain direction all the backlash would be taken up and the image at worst would at least move some small distance back towards the desired center location albeit with a small jump. (EDIT:  Maybe backlash wouldn't be that much of an issue here after all.  Even if the backlash wasn't taken up every time the software sends move commands it would eventually be taken care off once the objects drifts too far of the center and then a larger magnitude move command would be issued getting the object at least close to center again.)

 

I don't understand why a cheap, could be ~$150??, Svbony SV106 (60 mm, F/4, 240mm FL) guide scope with centering rings and dovetail (~$83) and there SV105 2Mp 1/3" CMOS imager (~$60) couldn't be the starting point for an autoguiding setup.  The Svbony states the AV105 imagers works best with 400mm to 800 mm FL scopes so maybe throw in a cheap 2x Barlow to get the scope to 480mm FL and F/8.  Right now I don't know what the FOV at the native F/4 is, and of course at F/8, so there may not be enough stars to guide on with this particular setup.   And that 400mm to 800 mm optimum for the SV105 is for obtaining the best image quality and that shouldn't really be necessary for guiding unless the stars are so poorly focused that the now diffused signal doesn't allow for guiding.  Another more expensive option at about $350 would simply to use the Orion Magnificent Mini Autoguider for a one stop shop solution.

 

Now continuing to show my ignorance and thoughts on autoguiding on an SE.  Could something like SharpCap, which I mostly use for my image acquisitions, autoguide by using a bright star near the center of an object being imaged and nudge the scope based on that?  For example, many galaxies have a very bright central core.  Why couldn't SharpCap, or other acquisition software, sync on that bright core and occasional push that core back to the center while livestacking?

 

I really appreciate any inputs or thoughts as to how to keep the object being viewed with the 6SE, 8SE or similar alt/az scopes near the center while doing lucky imaging or while livestacking.


Edited by rnyboy, 16 September 2020 - 01:35 PM.


#2 sg6

sg6

    Cosmos

  • *****
  • Posts: 9,103
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 16 September 2020 - 01:40 PM

Guiding would extend the period the object is in the center of the field of view, there is a guide port on the 6SE. But what guiding cannot do on the mount is prevent field rotation of the object.

 

It is not the tracking on an Alt/Az mount that is the problem, it is the Alt and Az movement that is the problem.

 

If you did good alignment of the 6SE the target should remain in the center or at least central area for a few minutes. adaquate time to get an exposure, but it that time the object would have rotates slightly and then you would end up stacking a collection of rotated exposures. That is the problem.

 

Every 4 minutes the object will have rotated 1 degree different to the previous and to the next, and the rotation is occuring while you get the image, so it is not sharp.

 

People come up with assorted ideas to get around this and the easiest and best is buy an equitorial mount.

That is what they were built for and that is what they have been doing for a few hundred years.

 

Buying or using a mount that is wrong for the purpose, then trying what can best be described as "hacks", and likely expensive hacks, to get around it will not work as well as simply using the right equipment.


  • PolyWogg likes this

#3 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 02:17 PM

Hi sq6,  yes I know field rotation is an issue.  I actually put that in an edit after I had posted but you may have read my post before the edit.  I'm always stacking field rotated images and if too bad I crop or toss it.  Everything I do is pretty much lucky imaging and the longest exposure I've ever used is 15 secs.

 

But, I think for me the known issue of field rotation is often enough a secondary issue compared to the object moving too far from center over a 1/2 hour or so of wanting to image on the same object.  I typically crop out the area at the edges lost to field rotation after live stacking in SharpCap or processing FITS and SER acquisitions.

 

I once did a 45 minute image of Bode's galaxy that had about 10 degrees of rotation during that time but the mount was really behaving that night and the final cropped image was really nice considering it was done with a 6SE.  On the other hand twice I lost out on getting a decent image of the Trifid nebula because while livestacking its centroid area would drift too far from the center of the imager and whatever diffuse edge went off the edge of the sensor was then lost.  The direction of drifting can be rather random and sometimes it's up, down, or in small circles or spirals that drift up/down, left/right and back and forth in the FOV.  Nudging the image back to center occasionally would have kept everything in the FOV even with the field rotation and I would have had a nice image.

 

I certainly know the SE mounts are far from great but if a $150 investment would keeps things near the center it would be worth it to me.

 

Edit:  Another issue possibly related to drifting for me is that my property runs north/south and is rather narrow with many trees on the east and west so much of my imaging passes through the meridian and my mount goes from tracking right and up to right and down when imaging to the south and most often left and up to left and down when imaging to the north.  How much of an issue this is compared to imaging to the east/west I don't know since I can't look east or west from my backyard.


Edited by rnyboy, 16 September 2020 - 09:57 PM.


#4 Terrybythe sea

Terrybythe sea

    Mariner 2

  • -----
  • Posts: 268
  • Joined: 13 Feb 2020
  • Loc: Gaspe Peninsula

Posted 16 September 2020 - 03:00 PM

I've no idea myself, but typing "6se guide port" came up with a few results that may or may not be helpful. (At a glance, the first one didn't sound encouraging).

 

https://www.cloudyni...iding-in-altaz/

 

https://www.cloudyni...n00b-questions/

 

https://www.cloudyni...exstar-5se-6se/

 

https://www.cloudyni...h-nexstar-8-se/



#5 Don W

Don W

    Founding Member

  • *****
  • Posts: 24,546
  • Joined: 19 May 2003
  • Loc: Wisconsin, USA

Posted 16 September 2020 - 05:40 PM

The real answer to your question is, yes, you can guide an alt/az mount. However, as has been pointed out, you will get field rotation on exposures over about 30 seconds. The people in the EAA section do this on a regular basis. The caveat is, that you need to take multiple short exposures and stack them for a final image. You won't get those Astronomy Picture of the Day images, but you can achieve some good results. I suggest you visit the EAA (Electronically Assisted Astronomy) section here on Cloudy Nights.

 

https://www.cloudyni...-astronomy-eaa/



#6 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 05:41 PM

Thanks Terry.

 

Your third post I don't think I've seen before, or at least I don't remember seeing it, the others I've looked at when I was first searching on autoguiding last spring.

 

The majority of the AP people do seem to have long exposures stuck in their heads and most of their replies around the SE mounts not being suitable are certainly correct in the part about not capable of long exposures.  But they just seem to miss that the idea is just wanting to keep the image "near" the center of the imagers FOV  with some type of rough autoguider setup.  Or it could be I'm still simply missing something that makes it impossible to roughly auto-recenter an image while doing many short exposures either as individual frames or doing live stacking if enough stars are present to allow for that.

 

From your forth link It does look like my initial worry about needing higher backlash settings to guaranty the mount actually moves is important.  On a second thought, which I edited into the this topic's starter, I was thinking maybe not that important because it would be corrected in after a couple of mount recenterings but apparently it isn't.  Maybe that's due to not having the encoders for the alt and az showing a movement somehow causes the autoguiding software to get confused.  But at least a couple of individuals have got the SE to guide.  I'm not sure if any of those were in alt/az mode or all used a wedge.

 

Although you've not experienced the joys of plate solving yet, when I do a "plate solve and sync" when running Sharpcap the object of interest is usually NOT centered in the FOV, occasionally not even in the FOV, and I know that's the fault of gear slop in the mount.   I really think that's a demonstration of this backlash issue they talk about in your fourth link.  Early this spring I put on my ever lengthening list of TBD things to try experimenting with the backlash settings to see if I can improve the plate solve and syncs (Really just the sync part, the plate solves are great if enough stars are in the FOV!) I get with SharpCap.  My bet is that if there is a backlash setting that gives a reasonably good level of syncing the object near the FOV center then it's possible to do the rough autoguiding I'm looking for. 

 

I still haven't seen anything that says it really can't be done.  I'm reasonably willing to try it simply because I could eventually move stuff over to that new scope and  mount I'm thinking about so little if any money is wasted in trying.


Edited by rnyboy, 16 September 2020 - 05:46 PM.


#7 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 05:46 PM

Thanks Don, that certainly gives me hope.  I don't mean to bug you but do you have any EAA links on autoguiding the 6SE?  I really did search for information last spring and the last couple of days and simply didn't find much of anything useful around how to do it on the 6SE without a wedge and what equipment was used.  Probably just missing some key word but I thought things like centering, autoguiding, 6SE, SE mounts would have found what I was looking for.



#8 PolyWogg

PolyWogg

    Ranger 4

  • -----
  • Posts: 345
  • Joined: 26 Jul 2017
  • Loc: Ottawa, Canada

Posted 16 September 2020 - 07:42 PM

I think the reason you're having trouble finding a solution is three-fold.

 

First and foremost, the SE mount already tracks. It should keep your object almost dead centre if your alignment was good. Maybe not "forever", but I have had planets sit in the centre of my FOV of my 8SE at 17.3mm for over an hour while at a star party with only minor drift. The nudges you are looking for are already part of the mount's design. It does continuous stutter steps to keep it there. So most people looking for a simple solution would just rely on that without trying to attach a guiding scope. I'm not sure why you are seeing so much drift if your 6SE is aligned, unless you're talking about drift because it isn't aligned and you're just using it in manual mode?

 

Secondly, I think if you go beyond the basic tool it already has, the next step up is probably some sort of wedge to handle the rotation issue, which you negated in your first post. The 4SE has it, and while my son has a 4SE with a wedge, I've never tried it for anything.

Thirdly, the "correct" solution, if there is a "correct" solution is to fix it by getting rid of the Alt-Az motion and going to an EQ mount which solves the problem entirely.

 

As noted above, you seem like you are trying to hack a solution that is already solved initially with the built-in tracking, a wedge, or an EQ mount. Your solution might work, but as you've seen, most people facing the problem have gone with the three standard solutions.

 

You seem like you already have a solution you want to do and you're going to force fit that solution, no matter what alternative anyone suggests...You might as well go out, spend the money, and come back and report your results! :)

 

PolyWogg



#9 Don W

Don W

    Founding Member

  • *****
  • Posts: 24,546
  • Joined: 19 May 2003
  • Loc: Wisconsin, USA

Posted 16 September 2020 - 07:47 PM

Thanks Don, that certainly gives me hope.  I don't mean to bug you but do you have any EAA links on autoguiding the 6SE?  I really did search for information last spring and the last couple of days and simply didn't find much of anything useful around how to do it on the 6SE without a wedge and what equipment was used.  Probably just missing some key word but I thought things like centering, autoguiding, 6SE, SE mounts would have found what I was looking for.

I do not. There are a number of people in the EAA section using the SE mounts. Keep in mind that the longer the focal length, the more difficult it becomes. Using a 6.3 reducer will help a lot.



#10 Jeff Lee

Jeff Lee

    Vanguard

  • *****
  • Posts: 2,084
  • Joined: 17 Sep 2006

Posted 16 September 2020 - 08:00 PM

You don't autoguide the SE in EAA. You use a program like SharpCap that automatically takes care of the rotation with its stacking algorithm. If you stack for a long time (10-20 minutes in EAA terms), you just have to crop the edges that show rotation. 



#11 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 08:03 PM

I'm a BIG fan of reducers and a Barlow.  So far since I started the hobby early this year I've imaged things at about f/25 down to about f/2.5 with the 2.5x Barlow, two f/6.3s, and one 0.5x reducer I have.  I've uploaded a bunch of the things I've looked at in "my gallery" and I think most are labeled with what f-ratio I was working at.

 

For a variety of reasons that el cheapo 0.5x reducer is actually my overall favorite for FOV, low cost (~$25) shorter exposures, and being able to hit the zenith with less glass in the way then when using stacked f/6.3s.  You just need to be using a smaller sensor, like my 385mc, or be willing to crop out the vignetting (or do a flat correction) and increasing coma at the edges that show up more grossly with anything larger.



#12 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 08:19 PM

Hi Jeff Lee.  That's the way I'm currently doing it in SharpCap but the object can still drift out of the FOV before enough stacked images are obtained for a decent image.  A lot of this issue also depends on what f-ratio I'm operating at and what the size, shape, and brightness is of the object I'm trying to image is; and certainly having enough stars in the image so SharpCap can livestack.

 

I love SharpCap and if the object would always stay nearer to the center of the FOV I'd get a lot more decent images as a final result.  Hence the whole reason for starting this topic.

 

When doing the short exposure image captures over a 30 - 45 minute time period I'll often enough have to pause the image acquisition and then manually recenter the image and restart the acquisition, but depending on how well behaved the mount is that night I may have to do this recenter quite a bit.  And, of course while pausing and recentering, the field rotation just continues marching on.  It would be nice to have a way to nudge the mount to move the object being imaged back to near the center automatically while acquiring those images instead of having to baby sit the mount so to speak.


Edited by rnyboy, 16 September 2020 - 08:23 PM.


#13 speedster

speedster

    Messenger

  • *****
  • Posts: 400
  • Joined: 13 Aug 2018
  • Loc: Abilene, Texas

Posted 16 September 2020 - 08:24 PM

You might want to look at the alignment issues you are having first.  The SE can keep objects in the FOV for an hour with no nudging at all if the alignment is right.  An eyepiece with cross-hairs improves accuracy when aligning and always finish slews with the same buttons (up button and right button for my situation).  Then, add more alignment points. 



#14 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 09:00 PM

Hi PolyWogg.  I missed your post after seeing Don Ws's last one.  No, it's tracking.  I use Starsense for the alignment and I've never had Saturn stay properly in view for more than about 10 maybe 15 minutes at ~f/25.  More typically it will drift out of view in about 5 minutes.  But that ~f/25 is about the equivalent of a 3ish mm EP!.    A 17.4mm EP in my current 6SE setup would have a wider FOV than using a f/6.3 reducer, actually closer to my 0.5x reducer when used with my 385mc imager.  The FOV with only the 385mc in the optical train is close to a 9mm EP.  I would certainly have no problem keeping a planet size object in the FOV for a long time with a 17.4mm EP but I don't think it necessarily would always be close to the center.  It could kind of drift around in circles more or less around the center but stay in the FOV or it could just eventually drift out of the FOV.

 

Try keeping much of the Trifid nebula centered in the FOV at f/5 where it pretty much takes up somewhat more than the whole FOV when the interesting bits are pretty much spread across the whole sensor face.  You can live stack a pretty nice image of the Trifid at f/5 after about 30 minutes of 10s or so short exposures but more often than not around 25% of it will eventually drift out of the FOV to ruin most of those imaging sessions.  That's why I'm trying to find a way to keep nudging things back to nearer the center.  Think of a big round object taking up that whole FOV in the 17.4 EP and wanting all of that round object to stay in view while imaging it for around 45 minutes.  In my scope that would be asking a lot and probably couldn't be done.  Yours could simply be better at tracking than mine.  I don't know.


Edited by rnyboy, 16 September 2020 - 09:42 PM.


#15 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 16 September 2020 - 09:15 PM

Hi speedster.  Really depends on what FOV you're talking about and I do a 10 pt StarSense alignment that usually gives less than a 4' rms error in the final model so the starting out alignment appears to be quite good.  For calibrating the StarSense with the scope I used the virtual reticle in SharpCap to center the star right in the middle of my sensor at f/20.  So it should be very well calibrated.  A goto on Jupiter or Saturn at ~f/25 may not have the planet in the FOV but it will be just outside the FOV and f/25 is about the equivalent of looking through a 3ish mm EP.  I think that's a pretty good goto centering.

 

Keeping small, relative to the FOV, objects in the FOV isn't really the issue here if the FOV is large enough.  In this case it's more about trying to keep a nebula, trifid is a really good example in this case, where the interesting flame like structures will pretty much be just inside the FOV of my 385mc at f/5 with the rest of the faint reddish nebula being larger and outside my FOV.  My wish is to keep all of the interesting flame like structures in the image while live stacking for 20 to 45 minutes.  The couple of times I tried in when the trifid was visible about 25% or so of the interesting stuff would eventually drift out of FOV and be lost to stacking.  Sometimes it drifts more or less up and out of the FOV and sometimes more or less down and out of the FOV.

 

I know it seems odd to have a lot of drift in a larger FOV but also I don't have any reason to think the alignment is off.  If there's any better way to get that drifting minimized more without anything like autoguiding I'd love to know how to do it.

 

I did mention above that most of my imaging is across the meridian because of the way my property lies and trees obstructing to the east and west.  So I'm wondering if that switch from tracking right and up to right and down may be part of my issue?

 

As far as I can tell the scope is tracking fine given it's an SE mount.  I'm just trying to see if there is an inexpensive way to use software and/or hardware to help keep an object more centered in the FOV for better stacking.  Maybe there isn't but I'm not asking for AP quality autoguiding here either.  Just some method where the software says "Hey, the object image has shifted 10% out to the left and down of the FOV and I'll tell the mount to move the equivalent distance up and right to get the image back to near the starting point again.".  I really don't care about the field rotation bits lost because that's a given and not related to mount performance but simply the fact it's an alt/az mount.


Edited by rnyboy, 16 September 2020 - 09:53 PM.


#16 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 17 September 2020 - 10:20 AM

I just checked Astrospheric for Friday night and the prediction is now for clouds, and average seeing and transparency so tomorrow night looks like a nogo instead of goto. 



#17 Jeff Lee

Jeff Lee

    Vanguard

  • *****
  • Posts: 2,084
  • Joined: 17 Sep 2006

Posted 17 September 2020 - 12:20 PM

I think you are pushing your SE beyond its limits, however since you are using SharpCap try this (if you have plate solving):

 

Stack for say 10-20 minutes, then pause stacking platesolve, restart stacking as many times as you need.


  • rnyboy likes this

#18 rnyboy

rnyboy

    Ranger 4

  • -----
  • topic starter
  • Posts: 392
  • Joined: 29 Oct 2019
  • Loc: Rochester, NY (Bortle 7/8)

Posted 17 September 2020 - 04:28 PM

Hi Jeff.  Good idea with the pause/plate solve.  Thanks.



#19 audioengr

audioengr

    Ranger 4

  • *****
  • Posts: 391
  • Joined: 09 Aug 2020

Posted 17 September 2020 - 05:09 PM

You don't autoguide the SE in EAA. You use a program like SharpCap that automatically takes care of the rotation with its stacking algorithm. If you stack for a long time (10-20 minutes in EAA terms), you just have to crop the edges that show rotation. 

 

I didn't realize that SharpCap actually compensated for rotation.  I thought it was only X-Y.  You sure about that?



#20 mikenoname

mikenoname

    Viking 1

  • *****
  • Posts: 748
  • Joined: 29 Jan 2015
  • Loc: Death Valley Region (Bortle 2 - SQM 21.9+)

Posted 18 September 2020 - 12:03 AM

Yes, SC aligns each sub using background stars before adding it in the stack. It corrects for both translation and rotation. Otherwise you couldn't use an AltAz mount with SC.


Edited by mikenoname, 18 September 2020 - 12:04 AM.


#21 Don W

Don W

    Founding Member

  • *****
  • Posts: 24,546
  • Joined: 19 May 2003
  • Loc: Wisconsin, USA

Posted 18 September 2020 - 05:01 PM

You CAN autoguide in alt/az. Go to the EAA section and find out for yourselves. You can't just sit here and say it won't work. I know for a fact that people are doing it.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics