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What is so difficult about guiding ?

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#1 bilgebay

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Posted 03 September 2015 - 01:36 PM

What are the problems you are facing ? 

 

This is not a criticism of those who want to pursue unguided imaging.... it's a quest to understand the difficulties about guiding.

 

My opinion is that, if someone is able to image without guiding, he should be able to image with guiding in a heartbeat.

 

Unguided imaging requires a higher spec mount to begin with, which will be a plus for the autoguider's performance and make life much easier for the user.

 

OK, Pandora's box is open :)  let's hear your views on this.



#2 Hobby Astronomer

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Posted 03 September 2015 - 02:36 PM

I think new astronomers are faced with numerous problems from their underlying equipment and the quality of the equipment. People are tricked and sold equipment that will not deliver. It is sad, but many new astronomers are just sold gear. The sales people do it intentionally. 

 

If you are making a banana milk shake for your little child and you put rotten bananas into the blender what are you going to get? A rotten milk shake? Would you give that to your child? 

 

Most new astronomers can only trust the people that sell to them and often get sold junk.

 

Do you see the bait and switch in this video. I will come back to prove the point later today.

 

https://www.youtube....h?v=pp1zjTGRn3c



#3 Jim78154

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Posted 03 September 2015 - 02:44 PM

If you can catch fish with your bare hands, does that make it easier to catch fish with a rod m and reel? You are asking people to compare apples and oranges here. The two are completely different routes to the same destination. The two disciplines do not share a lot of common features. 



#4 Rick J

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Posted 03 September 2015 - 02:54 PM

As someone who does get very slightly better results not guiding than guiding and using a top mount I'll add my experience here. 

 

A top mount does make guiding a ton easier but it can't solve all issues.  Everything must be very rigid.  One second of arc is a quarter at 3 miles.  Since I image at 0.5" per pixel I need to guide with about the accuracy of a quarter at 12 miles.  It doesn't take much for gravity to move something in the guiding system just enough to create a problem.  OAG helps greatly but can't eliminate everything.  Then there's the mechanical issues.  When guiding my mount is moving about 70 kilograms of weight and doing so with a precision equal to that quarter at 12 miles.  It must start, stop and not overshoot or vibrate with that accuracy.  When not guiding motions are always smooth and adjustments very slow and gradual.  This is easily handled compared to the sudden movements when guiding.  Then you can end up chasing seeing when guiding.  Even using 10 second guide exposures to even out seeing I still find at the error level I'm dealing with that I chase seeing to a small extent.  Guiders with random hot pixels can cause a random misguide.  Proper guide parameters limit this but it does happen.  Usually when not guiding I get slightly tighter stars than when I guide for these reasons and probably others I'm not thinking of at this moment.

 

Setting a system for unguided tracking is no trivial task.  Few actually achieve it.  I had to do severe mods to the scope to lock the mirror rigidly enough, the Meade Mirror Lock wasn't at all sufficient for example.  But once you solve these issues it is a pleasure not to need to guide or deal with guiding issues that seem to always crop up with even the best of mounts.  Just you get them rarely.  Even with guiding when outside my Tpoint map (new Paramounts don't require the severe Tpoint map my 12 year old one does) I've only thrown out one frame in nearly 10 years but I've lived with some elongated stars when guiding didn't work as well as I'd have liked.  This happens more often when guiding than when not guiding and then usually in strong winds  For some reason in winds over 40 kph the mount over corrects in RA (both directions) if the wind is from the east or west.  I try to avoid fields needing guiding on such windy nights.  Of course my previous mounts couldn't work in 40 kph winds at all.

 

I'd never recommend a beginner try to set up for unguided exposures.  Guiding is far easier than tackling these issues even with a top mount.  But with a lot of experience those with such mounts should give it a try.  Few do however as it is a daunting task though the newer direct drive systems do make this a bit easier and very expensive.  Still you learn a ton of things you never expected about your system when tracking down all the problems encountered along the journey.

 

Rick



#5 bilgebay

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Posted 03 September 2015 - 03:12 PM

Thank you Rick! Great information! Are you using PEC ?



#6 ElGato

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Posted 03 September 2015 - 04:53 PM

I think if you follow the advice given to beginners in regards to focusing on the mount first, and starting with a short FL scope, then going guided isn't difficult to tackle at all.  I only started guiding about 2 weeks ago and I've actually found that things like PA became much simpler since I was able to use PHD2 to aid the process.  Having read the suggestions of "start unguided to keep things simple,"  I was actually a tad surprised at how easy it was to get guiding going.  I'll admit I watched a few YouTube videos on PHD beforehand so I had an idea as to what each parameter did.  I actually started with an old webcam I had and now have replaced it with a $10 one that's actually better.  Eventually i'll get a proper guide cam but for now things seem to be working.



#7 rmollise

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Posted 03 September 2015 - 05:00 PM

The only hard part is getting set up in the beginning. Getting the balance correct for your mount and the initial values in your guiding software right. Usually that's just "a little east heavy and defaults in PHD."

How tough it is also depends on what you are doing. If, like most of us, your goal is some pretty pictures to show your mates, wife, or girlfriend and you are shooting at 1500mm max, it's just not that bad. Want closeups composed of long intergration subs at 2000mm or more? Then things get a little dicey. :lol:


Edited by rmollise, 03 September 2015 - 05:00 PM.


#8 schmeah

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Posted 03 September 2015 - 09:35 PM

I am now guiding with two hour long subs, round stars, FWHM less than 2 at a FL 2350, image scale 0.65". Can that be done unguided? I would love to be able to do that unguided consistently. I can't image unguided with my mount. Losing a guide star due to clouds with subsequent drift is the only thing preventing me from doing it consistently without wasting any subs ... well perhaps the occasional satellite and plane trail ...

 

Derek



#9 JoeR

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Posted 03 September 2015 - 10:59 PM

For me the hard part is differential flexure. Having everything setup right with PA, balance, and PHD settings can all go to pot with just one tiny movement somewhere in your setup which cannot be isolated. This is the sticky situation I'm in now that prevents me from going really deep with long prime focus exposures. The Hyperstar is my primary method of getting image data because it's very forgiving to slight tracking errors, many objects can be imaged without guiding, and even some narrowband data can be obtained guided before flexure becomes a nuisance. I would like to have an OAG for prime focus but I was told by Optec it would not work with my Lepus telecompressor so everything would have to be f/10. So I continue to troubleshoot one section at a time when clear nights allow it.


Edited by JoeR, 04 September 2015 - 07:27 AM.


#10 Per Frejvall

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Posted 04 September 2015 - 12:49 AM

I had no real issues when I was guiding, but it wasn't trouble free, for sure. Guiding is just one more thing that has to be taken into account, and even though the automation software out there will usually be able to handle it, it is more difficult to automate than is unguided. There is usually a fair number of guiding issues on the support forums for automation software, indicative of it not being a trivial task to get right when automating.

 

There is also the technological thrill of it being possible at all, which for me is something that cannot be ignored ;)

 

I throw more subs for cosmetic and focus reasons than I do for tracking issues. I use sub lengths up to half an hour, mostly around 20 minutes, and I use a 1000 mm scope. Why would I even want to guide under those conditions when it works perfectly well unguided?

 

What I do want to do is to try adaptive optics, say at 10 Hz. It would then be adaptive without guiding, just the AO doing small corrections for atmospheric flicker and whatever other tracking imperfections that may lure.

 

/per



#11 Rick J

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Posted 04 September 2015 - 02:30 AM

Thank you Rick! Great information! Are you using PEC ?

Yes, that's necessary with worm gear drives.  Not so much with direct drives now coming out.  But with the Paramount it starts with less PE than most worm drives have after PEC.  Mine was +/- 1.3 seconds without PEC.  I can't measure it with PEC.  Highly stable.  Each year I remove the old grease and apply new.  That changes the PEC characteristics very slightly to where old PEC gives a very slight error.  So I redo it after cycling the mount from stop to stop about 50 times with the full scope and cameras etc. installed.  That works the new grease in and I get a new PEC table that's good for another year.  The Sky has a routine to do the cycling.  Just tell it how many to do and it does the rest.

 

Rick



#12 freestar8n

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Posted 04 September 2015 - 03:41 AM

I think the main challenge to guiding is that there is so much misinformation about how to do it - and it gets repeated and passed along without a tie to improved results. Another factor is that people who struggle with mid-range equipment and never get good results then move to high end equipment and get good results - but then give guiding advice to people with mid-range equipment, where they haven't actually demonstrated good results with that advice and that equipment.

I have written about these things many times - but the main things I would watch out for are: 1) centroid accuracy is amazing and makes guidescopes as good as oag 2) If you guide with long focal length you will chase the seeing 3) If you make guide corrections every second you will chase the seeing 4) If you have flexure guiding an sct with a guidescope, you have a problem in the guidescope itself 5) PEC and polar alignment are critical to guiding and you should spend a great deal of effort making them as small as possible.

I think there are also fundamental problems with guiding programs - particularly when they work inherently in pixels rather than arc-seconds. I wrote my own guide program many years ago and made it available for use to get some of these points across - and regularly post results with mid-range equipment - so that my advice on methods and software is backed by actual results rather than anecdote.

Frank

#13 schmeah

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Posted 04 September 2015 - 05:09 AM

 

I am now guiding with two hour long subs, round stars, FWHM less than 2 at a FL 2350, image scale 0.65". Can that be done unguided? I would love to be able to do that unguided consistently. I can't image unguided with my mount. Losing a guide star due to clouds with subsequent drift is the only thing preventing me from doing it consistently without wasting any subs ... well perhaps the occasional satellite and plane trail ...

Derek


Would really like to see a raw sub at that integration, fwhm and FL.

 

 

Here is a screen shot. Uncalibrated 2 hour sub on 9.25 Edge at F/10. I can upload the original fits file if you like. The FWHM was similar in Maxim.

 

Derek

Attached Thumbnails

  • Screenshot (521).jpg


#14 bilgebay

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Posted 04 September 2015 - 05:37 AM



I can upload the original fits file if you like. The FWHM was similar in Maxim.

 

Derek

 

 

Thank you for sharing this sub with us Derek. Please share the original with us so that we can pixel peep it :)

 

Let me share a 30 minute and a 60 minute guided sub, shot with my current setup. These are the leanest stars I've ever achieved with this setup at my location. I am planning to shoot unguided ( with PEC and Sky model) and see the difference. You can see all the details on Astrobin pages.

 

get.jpg

 

get.jpg



#15 bilgebay

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Posted 04 September 2015 - 05:41 AM

 4) If you have flexure guiding an sct with a guidescope, you have a problem in the guidescope itself 

 

All good points, thank you Frank. Can you elaborate on item 4 please ? 



#16 neptun2

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Posted 04 September 2015 - 05:45 AM

Well interesting discussion here. I think that almost everybody starts with guiding because this is the only way to get decent results with middle or low range equipment that many of us use to start in this hobby. I personally started with heq5 pro mount and 8" f5 newtonian combined with DSLR. My heq5 was the old type with the slow dual axis motors which i later upgraded to the heq 5 pro synscan and had very serious periodic error. I did not measure it directly but was at least 20 arcse peak to peak because this mount required correctins every second or otherwise i got seriously elongated stars.  My polar alignment also was not good enough at that time. Initially for guiding i used piggybacked 80mm f5 achromat and later switched to off-axis guider which improved the situation a bit but was still not perfect. Offaxis was sometimes difficult to find proper guide star. Using 1 second exposures also in many cases made me to chase seeing. To sum it up - with low range or even sometimes with middle range equipment guiding is not so easy because there are many components contributing to the tracking errors like the periodic error of the mount, flex, not good enough balance (especially with the mass produced chinese newtonians used by many people including me till soon), not good enough orthogonality between guide scope and main scope if you use second scoep for guiding, difficulties to find star with off-axis guider, cable drag and so on and so on. Having such mix of components contributing to the tracking errors makes it difficult to narrow down and fix the problems in your setup. So guiding with such equipment (and it is mostly needed exactly with this type of equipment) will never be simple or easily repeatable task. Of course the level of even low range equipment is constantly improving in the last several years so these problems will be minimzed in future but definitely will not be completely gone.

 

On the other hand guiding with middle o high range mounts/systems is much easier and usually problem free in general. My current system with the CEM60-EC mount and my 120ED refractor together with off-axis guider is much easier to guide. Thanks to the RA high precision encoder and the much better polar scope now i get much better stars than with my old heq5 and corrections are usually sent by phd in 10 to 15% of the time only. Compare that with the 100% of the time with my old setup. Now i can even use longer exposures - even 5 seconds so that i don't guide on seeing. The refractor is also much more stable compared with my old newtonian.

 

To sum it up - yes if you have good mount and stable OTA guiding can be a breeze. But with such system and mount which supprots modeling you can also try to further tune the system and go unguided. In fact this is a matter of choice - either to go guided and lets guiding eliminate the remaining errors in the system or to try to model them and go completely unguided. 

 

I also saw mentioned the so called adaptive optics units sold by SBIG and other companies. In fact these units do not bring too much to the table in our setups. Yes they can correct several times per second but what tells them how to correct? The same guide camera working with 1 second or even longer exposures. So you can correct several times per second but you get input for correction at most 1 per second. This may help only for mounts with significant backlash and other mechanical problems but guess what - people imaging with such mounts do not have th money for AO units. So for me these units are just a hype for our purpose. 

 

And one side note - i saw people mentioned discarding exposures due to planes or satellite trails - just use sigma klip as combine method and such exposures are perfectly usable. :)



#17 freestar8n

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Posted 04 September 2015 - 06:16 AM

4) If you have flexure guiding an sct with a guidescope, you have a problem in the guidescope itself

 
All good points, thank you Frank. Can you elaborate on item 4 please ?


Just to be clear - the things I listed are examples of things to avoid or ignore.

The reason I listed #4 is that many people think you can guide an sct with a guidescope as long as you make the guidescope rigid enough. But at the long focal length of an sct, any flexure will show - including flexure within the sct itself. So people get frustrated with oblong stars and first focus on guiding problems - then rigidity of the guidescope - and it just isn't a fruitful approach.

For beginners I recommend guiding a refractor with a guidescope - and if they want to get small round stars with an sct, use oag.

Frank

#18 bilgebay

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Posted 04 September 2015 - 06:21 AM

Ok, now clear.... and I fully agree with your refractor approach.

 

The worst thing a beginner can do is to buy an 8"-10" SCT and start imaging at that huge focal length.... and this is what usually happens...



#19 freestar8n

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Posted 04 September 2015 - 06:35 AM

Yes - that is why I liked the recent image by antimorris in the ccd section. She used a big cpc sct - and on top she had a refractor guided by its own oag. The sct played no role at all except as something to ride on. The result was very good - and it still leveraged the convenience of a cpc on a wedge.

So for people starting imaging, who already have an sct - a good approach may be a separate imaging refractor with a guidescope attached to it. And for even better results - put oag on the refractor and gain familiarity with oag technique. Then for high res small galaxy work - put the oag on the sct and image with it. This allows a step by step process where the focus can be on guiding and basic technique - without flexure limiting the results and causing frustration.

Frank

Edited by freestar8n, 04 September 2015 - 06:40 AM.


#20 schmeah

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Posted 04 September 2015 - 07:36 PM

 



I can upload the original fits file if you like. The FWHM was similar in Maxim.

 

Derek

 

 

Thank you for sharing this sub with us Derek. Please share the original with us so that we can pixel peep it :)

 

 

Here you go:

 

https://www.dropbox....-003Ha.fit?dl=0

 

Derek



#21 Raginar

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Posted 05 September 2015 - 03:26 AM

I think most people don't take the time to learn their equipment or appreciate how complex it can be.  They don't work to center both their telescopes... or they don't get a sensitive enough guide camera... or polar align the mount well enough to make it happen.

 

So, they get frustrated with guiding and think that unguided is the way to go.

 

Personally, I haven't had any problems that weren't due to equipment failure (cable drags mostly) or user error in about 2 years.  Since I swapped to OAG, it's become very simple.  The SBIG OAG is a pleasure to use and the Orion TOAG was fine as well.  I had some growing pains on how to use it (getting spacers right and understanding how it works), but once I figured that out it was a really good investment.

 

Oh, and some of the problems are software driven.  MaximDL has had the 'star lost' problem that the developers refused to acknowledge for years.  Apparently, they've finally recognized the issue and are working to fix it.  I think this was the root cause to many 'noobs' getting frustrated as well.

 

 



#22 Denimsky

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Posted 05 September 2015 - 06:58 AM

Personally, I haven't had any problems that weren't due to equipment failure (cable drags mostly) or user error in about 2 years.  Since I swapped to OAG, it's become very simple.  The SBIG OAG is a pleasure to use and the Orion TOAG was fine as well.  I had some growing pains on how to use it (getting spacers right and understanding how it works), but once I figured that out it was a really good investment.

 

 

When people say 'cable drag', what exactly are they referring to?

Does it mean that the cables are dragging on the ground?

 

Thank you.



#23 tazer

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Posted 05 September 2015 - 07:07 AM

Cable drag is usually a case of having cables that are long and inefficiently routed. They can hang off the end of a scope causing resistance in one direction but not another. You might see this in a guide graph as constant corrections in one direction. Cable drag might also include stiff cables in the winter. Cables can become very rigid and cause the same problem as just hanging off the scope. You're not likely to detect either form of cable drag on a short focal length rig, but the longer you go the more apparent it becomes.

 

Mark



#24 Ian Robinson

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Posted 05 September 2015 - 07:57 AM

What do people expect when they go into a shop or online shop and want huge aperture, but don't want to spend much on a mount ? and get sold a much bigger telescope on a mount that is mass produced by monkeys and grossly underdesigned and undersized capacity wize with secondrate gearing and not so good motors (even it's goto)  by know nothing **** behind the counter who is only interested in meeting a quota and getting a commission and could care less about if the buyer will be happy with what he/she has bought .

 

Then they decide to try imaging. And suddenly find out he's been sold a lemon of mount.

 

But all is not lost - out he goes and buys a SSAG or similar and a laptop and all will be good. Well maybe not.

 

Really imagers now have it sweet, no more exposures of 15m, 30m, 1hr, 2hrs and even more, and they get to see if they recorded some photons instantly (via the little screen on the back of the camera or by looking at it real big like on the laptop screen (no more waiting for someone to process your roll of film and print it for you), and he doesn't even have to at the telescope/camera to image the stars or what ever he's interested in and TO MANUALLY guide the camera / telescope on a target star (my either making small adjustments with the slow motions (remember them) or by hitting the 2x or 4x rate E or W or N or S to make corrections while being superglued the guidescope or oag and the reticle eyepiece (remember them) .... oh the cramps ! oh the boredom - NOW THAT TOOK TRUE DEDICATION !!!

 

Invest in a good quality mount with more capacity than needed to handle your telescope/imaging setup or camera and biggest (longest - heaviest lens) , level the tripod or pedestal properly, polar align as good as you can, and you should have no problems imaging unguided, of cause then if there are minor imperfections in the machining you'll see it in the image  so here comes cleaver code to compensate for these and autoguiding will help make those stars nice and circular spots (as good as you can expect for square pixcels anyway).

 

Rant over.



#25 tazer

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Posted 05 September 2015 - 09:56 AM

What are the problems you are facing ? 

 

The current problem I'm dealing, and have been for a while, is an elusive polar alignment. Well, the problem manifests as that but it's sure to be something else. What I'm experiencing is that when performing a drift align the drift plot show a persistent motion in one direction, as expected. The problem is that the drift isn't consisten. Initially it drifts rapidly then settles down for a while. It'll then level out and sometimes reverse direction. This is on night when the air is perfectly still. The only things I can think of that may be causing the problem are 1) the grass/soil I'm setup on is compressing, 2) the OTA is still changing thermally, 3) the primary mirror isn't as fixed as I had hoped and is slowly compressing one of the 3 rubber clips holding it.

 

 

My opinion is that, if someone is able to image without guiding, he should be able to image with guiding in a heartbeat.

 

How do you define "in a heartbeat?" If you mean it'll be simple or easy, then I'd disagree. Guiding reveals every flaw in your mount and scope (e.g., backlash, stiction),while introducing a number of potential new flaws (e.g., cable drag, differential flexure.) That's with a guidescope but if you go OAG then you've got a slew of new issues to deal with (e.g., spacing of the guidecam with respect to the imaging cam, finding a guide star, off-axis aberrations, inconsistent centroiding due to coma.) With an OAG you'll be able to really see just how bad the atmosphere is. As Frank mentioned, you may end up chasing the seeing.

 

 

Unguided imaging requires a higher spec mount to begin with, which will be a plus for the autoguider's performance and make life much easier for the user.

 

Few people start out with a higher spec mount though. There'd be a lot fewer requests for assistance in the imaging forums if everyone started out with a Tak, SB, or AP mount. Alas, that's not generally what folks do (myself included.)

 

 

OK, Pandora's box is open :)  let's hear your views on this.

 

I agree with your initial statement in principal but in practice guiding is a step up in complexity and difficulty for most.

 

Mark


Edited by tazer, 05 September 2015 - 09:56 AM.



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