This is the first time I have imaged a faint(ish) object - I know mag 6 is hardly faint but I have only taken images of bright object to now. Still fighting DEc problems with the mount so I was plased to get this image of this active galaxy...13 million light years from my back yard this is one of the brightest radio source is the sky. My wife described it as looking spooky - I guess that is right an awesome display of the power of gravity...
Hello all, mind if I winge a bit, I seem to have hit a plateau and am not getting the results I would like as I push to longer exposures.
I got all set up on Sat night and did a run of 17*4 min exposures on Cent A but DEC runaway ruined all but 3 of those. I am really getting tired of this problem with the mount (dec backlask as measured by Guidemaster is 17.5s!). I think I need to get much better at polar alignment to minimise the need for DEC corrections. My Current proceedure is approximate as I have not been able to make sense of drift aligning yet... FWIW latest effort attached. F9 and ISO 1600 btw.
A few questions on the image - I have had to push quite hard (not enough exposure obviously gived the above fail rate) and I notice I have some artifacts appearing - coloured pixels in little groups, are these hot pixels? Should they have been compensated for by the dark frames? AIs the pattern due to misaligned frames? I cropped the image to reduce size and put the galaxy more central but on the original I have a purple patch appearing on the lower right on the frame - is this amp glow? And if yes should this also have been removed by dark frame subtraction?
Maybe I can learn something from this night of frustration. Oh an by the way Sunday was worse - my remote relase cable palyed up an the shutter triggered as soon as it was connected - arghhhh, no moon, clear skies had to happen didn't it.
From my limited experience with the Nikon, They are hot pixels and it sounds like amp glow. I find lassoing the area of ampglow and playing with it's curves prior to playing with the pictures curves a bit of a solution, but then I'm happy if i get a piccy even close to the quality of yours
My understanding is ( and it's prolly wrong ) that if you take a dark frame through a dslr your still going to get amp glow on the darkframe. I've only tried longer exposures a few times with mine and it was wholly unsuccessful, so I'm interested to follow this thread !
Keep up the great work, I think your results are excellent, for the hassles youv'e outlined with your tracking, the stars came up pretty good with only minimal distortion. Bravo !
I will try and answer some of your questions and, I hope you don't mind, I will be slightly critical on one aspect. My only criticism is that the focus is slightly soft, ie, could be a little sharper, over the period of a night, focus changes with colder temps, not sure on the Vixen but it definately happens with an SCT.
As far the quiding is concerned, will GuideMaster allow you to Calibrate the drive. I use an ST-4, so whilst procedures are different, the basics are the same. Before taking any shots, I allow the guider to go through about 5 minutes of guiding corrections to settle down, doing a drive calibration is a critical step to successful guiding. I have my mount drift aligned so the star stays on the crosshairs for at least 1/2 hour without moving, the better you can get that the better your guiding will be, if you can eliminate most of the DEC corrections your guiding errors will be reduced significantly.
I don't know what software you are using but, I use ImagesPlus 2.75. I use it to automaticaly combine my Flat field, Light, Dark and Bias frames, when these are combined and the Darks and Bias removed, the Amp Glow, at the lower right hand corner, and other hot pixels etc are removed and averaged out. This is shooting with RAW's.
As far as the camera triggering immediately, this is caused by the camera being on when the long exposure cable is plugged in, leave the camera off until the cable is attached to the computer then turn the camera on.
JohnG, I use IP2.75 also and I have not seens these artifacts in previous images - I must have stuffed up the processing somehow on this occasion.
For autoguiding I am using Guidemaster v 1.3x, and yes, I can calibrate OK - that's how I measured my DEC backlash at a whopping 17.5seconds.
I guess I must just bite the bullet and get good at drift alignment....I do get confused with left, right, north south with the use of a reflector and refactor...I have read the instructions many times but always get confused when I am under the stars...was hoping a rough polar and guiding would be good enough. Guess I was wrong.
Interesting, I am asuming that you have taken a number of Darks the same length as your light exposures, a similar number of Bias on your shortest exposure. Sounds like you may have missed a step somewhere.
Your DEC backlash is large although I would have thought that you could manually import a number of seconds to calibrate, with the G-11 I have to set my backlash compensation at 12 seconds to get the mount to move the required 9 pixels, it is nothing unusual to have this setting up around 20 - 25 seconds. Is there a way to adjust the Aggressiveness, on the ST-4, I have this set at 3-5 depending on the seeing.
And, yes, good alignment helps, if you want, send me a PM with your email address and I will send you a PDF file with Southern Hemishere Drift Aligning explained.
Interesting, I am asuming that you have taken a number of Darks the same length as your light exposures, a similar number of Bias on your shortest exposure. Sounds like you may have missed a step somewhere.
Your DEC backlash is large although I would have thought that you could manually import a number of seconds to calibrate, with the G-11 I have to set my backlash compensation at 12 seconds to get the mount to move the required 9 pixels, it is nothing unusual to have this setting up around 20 - 25 seconds. Is there a way to adjust the Aggressiveness, on the ST-4, I have this set at 3-5 depending on the seeing.
And, yes, good alignment helps, if you want, send me a PM with your email address and I will send you a PDF file with Southern Hemishere Drift Aligning explained.
JohnG
I think I must have missed a step in processing, not sure how I could have done it as I used the auto image set processing option as normal...
17s is not excessive DEC backlash? Really? I was freaking out about this as this is at full siderial rate, does that put it in context? Should I just accept this then.
I'm going to work on drift alignment tonight (it the wx gods play nice) using the LPI....
I think I was misunderstood, the DEC backlash you have is large, what I am saying is that at times I have had to set the Backlash Compensation as high as 20 - 25 seconds depending on seeing, I don't know if you have tried a higher number or not, might be worthwhile giving it a go, you have to use this number in conjunction with the Aggressiveness setting. The majority of times my setting is 12 seconds, the Autoguider has no problem with this and I get perfectly round star images.
John that is a great capture, I have not had the sucess you enjoy with the Hambuger. I dont think it is an easy target and you did very well to produce a fine image.
alex
Kind words, I hope to get better at this - gee - its a big step up in complexity this autoguiding lark...
My latest effort was based on a good polar alignment - better than I have had before anyway - star did not move on the LPI screen for more than a pixel or two over a 2 min test.
Switched on guiding and went for 4 min shots - 100% failed with DEC runaways, grr. Looks like the mount is going back to the shop -again....
I should probably take this thread elsewhere but FWIW the shop is claiming that the DEC backlash at 17s is GOOD perfomance and that if I have a problem with it the answer is better polar alignment.
The second part may be true but what about the first part? I have no experiance with other GEMs and autoguiding is DEC correction not worth the trouble, must I precision polar (ie drift) align every time?
What is the performance like on a EQ6 or Losmandy. The shop claim the DEC axis has a synch motor not a stepper so there is spool up and down time that cannot be eliminated. Is this normal?
I have read it is best to unbalance DEC so the motor is always under load and backlash is no longer an issue - how much unbalance is ok? If I do that how will I calibrate Guidemaster?
I am confused, the mount is going back for an other attempt at adjustment but I do not expect much improvement as they claim performance is already the best they have seen for this model....
I need some clarification here, with the DEC backlash is it: 17 arc seconds on the screen or is 17 seconds from the time the motor stops until it restarts. If it is the latter it is way, way too excesive. You will never control that with an Autoguider.
The aim of a Drift Alignment is to eliminate as much DEC correction as possible, you should aim to maintain a star on a crosshair eyepiece for at least 20 - 30 mins. You don't adjust for RA, that is the role of the autoguider, you adjust DEC (North & South) only, the less work it has to do in DEC the better guiding will be.
I don't know where you got to unbalance in DEC, if you do this, the motor will take up instantly in one direction and will take ages to catch up in the other direction, in the end the autoguider will be chasing it's tail and never be able to catch up. You should unbalance SLIGHTLY in RA so the weight is against the worm, if you are balanced perfectly, the RA worm will rock back and forth causing a slight trailing.
This behaviour of this runaway is bizzare, if it is doing that, to me the problem is in either the electronics or the motor itself.
Switched on guiding and went for 4 min shots - 100% failed with DEC runaways, grr. Looks like the mount is going back to the shop -again....[/quote]
I have reread this a couple of times, I am wondering, are you hot plugging the autoguider cable? that could cause a runaway, do you need to put the controller into, for example, photo mode or similar? are there additional steps that need to happen when autoguiding?
These are only guesses, I am trying to troubleshoot as I would with the Gemini system.
I also did some Googling on the Sphinx mount, the specs say the mount has Servo motors, I asume that you have Firmware version 1.2 build 24 or above, I note that it introduced backlash compensation and autoguider port control. There is also a Yahoo Group http://groups.yahoo.com/group/Vixen_Sphinx that you might want to post the question to.
Firstly it is 17 seconds (time) DEC backlash as measured by guidemaster with the LPI fitted to my 66mm/388mm guidescope. I got a bigger (!) value with the LPI fitted to the main scope (200mm/1800mm). Visually it took 45 sec for a star to reverse movement in DEC after a button press with a 9mm ep in the VC200L...
Drift Alignment - Oh boy 20-30 mins no movement I am nowhere near that - at what magnification is that? I use the LPI and drop cursors on it in Autostarsuite to measure drift rather than mk 1 eyball and a reticle ep - it is more sensitive - but I've not be able to freeze a star completely - movement is apparent after 3-5 mins...I figured a small error would guide out.
The tip I about unbalancing in DEC also suggests you apply DEC corrections in one direction only, it is even suggested that you offset alignment a little to introduce slow drift that can be worked against. That prevents the oscillation you refer to.
There is also something called stiction that can cause DEC issues?
Runaway may have been a misleading term to use - to be clear I do not use the autoguiding port on the starbook, rather I use the LPI/Guidemaster/ASCOM and the ethernet connection to guide. What I observe is a period of control in both axes +- 2 arc secs or so then a step change in DEC - 12-15 arc secs and a gradual recovery. This happens
I am on firmware version 28 - the latest -I have the backlash compensation option activated but set to 0 (it makes no difference at the max zoom setting required for guiding) and yes I have been on the yahoo group and reported the issue there too, it seems to be a fairly common gripe - with a simple fix (allegedly) that seems to be beyond the comprehension of the vendor here in Sydney.
Well the mount is off to the workshops again tomorrow ....stay tuned I'll let you know if I get anywhere.
17/45 seconds to get the DEC motor to make a movement, way, way to much time, you would lose the star from a 9mm eyepiece.
I don't know the mechanics of the Sphinx mount but it sure sounds like something is slipping badly, there must be a hell of a lot of slop in DEC to get those sort of times, they are not normal.
With my G-11, a star stays on the crosshair of a 12mm eyepiece for well over 30 minutes, the only movement I get is a + or - 5/6 arcsecond PE cycle in RA, DEC stays on the line as it should.
That stiction you mentioned is something that is peculiar to Losmandy mounts, it is caused by the Nylon clutch pad sticking when you push the mount in either RA or DEC, it will let go suddenly and you overshoot, with GO-TO it is a bonus.
A movement of + or - 2/5 arc seconds is quite normal for any mount but the jump in DEC of 12 - 15 arcseconds is not, some mounts exhibit this sort of jump in RA but not DEC, there is no PE in DEC because it should not be constantly driven.
The pole offset you mentioned, I do know about this but it is not really recommended, a slight offset at the pole will lead to a fairly significant offset at the equator which in turn leads to field rotation in your shots.
I had to show the disbelieving staffer in the shop the fault, they were insisting there was no problem. It turns out there are stepper motors on both axes as you would expect (so much for the spooling up theory - odd the dealer did not know that!) and when tested we could see the stepper on DEC moving the encoder after a button press but this motion was not transmitted to the drive gears for 10+ secs.
We tried the same test on another Sphinx and, though there was lag, it was much less, 2-3s, large but manageable I think.
I truely hope they can fix this but I must say I am really disappointed with the service...its the third time the mount has been in for adjustment and I have had a fight each time to get any action at all. This issue was originally reported to the dealer in Feb...
....stay tuned I'll let you know if I get anywhere.
