Funky looking stars

[masjstovel]

Hi, 

I noticed most/all of my frames last night on NGC 6888 are looking funny, and i cant explain why. Can anyone help/guide me in the right direction?
Imaging with what is shown in my signature below.

Perhaps useful info:
- I was guiding at <1.00 arcsec on the frames i kept. 
- The screenshot is from Ha-filter
- Stars have not been looking like this before
- I have added Bob's knobs since last session - the knob-heads look a bit big initially, but when try to look in the primary mirror directly behind the secondary i dont see them sticking outside the mirror.
- I did collimate the day before, and havent touched the scope since then.
- These starshapes are not local - they appear all over the images.

 

[Eddie Jones]

No expert here, but would guess at vibration through the mount or wind ?

 

[masjstovel]

I agree it looks like vibration, but as mentioned in my post, these frames guided <1.00 arcsec, and its also consistently pointing diagonally top left to right bottom through the frames

[ChrisWhite]

Something is definitely moving during the exposure.  Essentially most of the time stars stay put, hence the bright central star spots, however at some point during the exposure the mount moves in either RA or DEC and causes this.  It would be helpful to see the guide graph/log to see if it corresponds to a movement.  A cause could be cable drag.  How is your cable management?  Are they dangling off the scope/camera, or are they bundled neatly and supported?  Could also be something sticking in the mount.  I see that your mount is modded.  Did you do this yourself?  It's possible that your backlash is adjusted a little too tightly causing some binding.   Could also be that your exposure is starting before the mount has fully settled (i.e. after a dither).  Tough to tell the cause of this from a single screen grab, but something is definitely moving during the exposure. 

[masjstovel]

@ChrisWhite I agree it looks like vibration, but all the frames i kept is underneath 1.00 arcsec RMS, and that should be enough on my setup. Its ~70x200s frames and they all look the same characteristics wize (direction, length, size of "spikes"). I did check for snag during session, but couldnt find any. I am not denying it could be what youre saying, but with the info here i dont understand how, especially the consistency of it through the frames.  

It would be interesting to see frames up against the time of guiding, how do i do that?

Only thing I can think of is - I do 2sec intervals in guiding. And i dont know how the interiors of PHD2 works, but there was slight wind (up to 5m/s, but probably less where the rig is), and if I aim at lets say a pixel, and windgusts push the scope away and back again to that pixel during the 2 sec exposure, will PHD2 know that it has moved? Silly thought maybe, I don't know.

I think its something optical myself, but cannot figure what. 

Edited October 11, 2020 by masjstovel

[ChrisWhite]

I doubt it's optical.  What you are seeing is a streak from the star moving during the exposure. 

 

What is your capture software?  Are you dithering between frames?  Are you only dithering in one axis by any chance?

[ChrisWhite]

Also, you can view your logs on a PHD log viewer and take a look at the time during your exposure to see if anything jumps out. 

 

One more question:  Are you guiding with an OAG or with a guidescope?  If you are using a guidescope, it's possible you are seeing some sort of differential flex.  Make sure all connections are rock solid and the guidescope is orthogonal to the imaging OTA.

Edited October 11, 2020 by ChrisWhite

[masjstovel]

@ChrisWhiteYou were right! Found 2 frames which the stars look better:
 

Had some sudden "jumps" in both DEC and RA from time to time, but that was maximum 2arcsec jumps, and didnt happen all the time, so wasnt enough to mess up the RMS mean (which was <1.00).
Im thinking i might be too well balanced, so that the backlash made it "free float" in that area of the sky and/or wind. 

[ChrisWhite]

Nice!  Is the elongation in the same orientation as declination?  If so, you could be right that it bobbles a bit.  Try balancing Declination to be camera heavy just a little bit.  You can also try to snug up your backlash a little bit.  Before tinkering with the mount worth trying to see if you can fix it with balance. 

[ollypenrice]

10 hours ago, ChrisWhite said:

 Try balancing Declination to be camera heavy just a little bit. 

Unfortunately this will not replicate the well-known technique of running the mount east-heavy to avoid oscillation across drive backlash. When the mount is pointing at the zenith the effects of being camera-heavy reduce to zero because the heavy end is directly below the light end. You'd need a weight off to one side. There is an alternative way to reduce Dec backlash, though, if you're unable to tune it out: run the mount slightly out of polar alignment and disable Dec guiding on one axis (the axis not involved in correction for polar misalignment.) You have to identify this by trial and error but it will always be the same thereafter unless you do a meridian flip, when the other axis should be activated. At least, with this method, the guider will not spend its time nudging the mount back and forth across the backlash. I've used the technique myself on a troublesome mount and it does bring a big improvement, though fixing the backlash is the best thing.

To the OP: when analysing tracking problems it is vital to know the orientation of RA and Dec on the image.  It's good practice to align your camera along RA and Dec when framing allows it, as it usually does. Align the camera roughly by eye to start with and then take a sub of a few seconds while slewing slowly on one axis. This will produce a star trail at the angle of your camera. Rotate till the trails are horizontal or vertical. Convention has it that the long side of the chip along RA is called 'landscape' and the long side along DEC is called 'portrait.' Keeping the camera orthogonal also makes it a hundred times easier to add more data at a later date.

Olly

PS It isn't necessary for guidescope and imaging scope to be parallel. Guide scope rings were not invented to let you get your two scopes parallel. Quite the opposite: their purpose was to allow the guidescope to be moved off axis in search of a guidestar when guiders were very insensitive. These days its best not to use them and bolt the guidescope down hard, off axis or not.

Edited October 12, 2020 by ollypenrice

[masjstovel]

Thanks guys. Silly me forgot to mention one thing, which might be very important. I'll get to that.

Well long story short: The infamous answer "I've tried everything" to get rid of the backlash. That means supertuning, changing all bearings to SKF, belt modding, and 8 months of pulling the rest of my hair off while adjusting the 2 setscrews back and forth. The result is: Gears grind "before" backlash is tightened up and gone. The only thing I have left to try is to add a teflon washer on top of the brassgear, as it MIGHT be in contact with the housing on the top in some poistions.

So i learnt to live with it and has backlash-compensated in PHD2 (DEC) to the point of great satisfactory (usually). DEC alone has been down to 0.20 arcsec, and usually lays between 0.30-0.50 arcsec. (I Live in Bergen area which is known for its rain and clouds, so when i say usually, that means 5-7 sessions or ~35-50 hours) 

I had some oscillating issue with RA which turned out to be a to loose belt. So the last two sessions (~15-18hours) It has performed at 0.65-0.80 RMS total (Ra+DEC), which is very satisfying to me, at least after all the struggling. 

@ollypenrice; I believe what you are saying about the offset in PA, but because of the explanation above I am very afraid to adjust what I'm doing atm since i have been guided good lately (in my oponion) 

I have my camera Portrait oriented then, or chip aligned with the longside of  the scope. 


So to the thing i forgot to mention:
I have a new guidescope, which i used first time this session. Silly not to mention, but i couldnt get to think of it.
I upgraded from Orion50mm to EvoGuide 50. It is attached in a clamp-shell so it sits very tight i believe. I doublecheck that the camera sits tight as well, but i believe so.

To the point:
Because of all the settings I was too afraid to make a new profile in PHD2 when setting up the EvoGuide, so all i did was change the focal length from 162 (orion) to 242 (evoguide) in the existing profile. Then i did a new calibration. I did not use Guiding Assistant. Its a helpful tool, but the recommended settings that come out of it hasnt really worked for me to be honest, so i have made my way by trying different settings and believe because of my latest guiding that its good enough.

Is there something else I need to change here? But are these settings "optical" or mechanical sort of speak?  Sorry for not mentioning the guidescope before.
 

[ollypenrice]

10 minutes ago, masjstovel said:

 

@ollypenrice; I believe what you are saying about the offset in PA, but because of the explanation above I am very afraid to adjust what I'm doing atm since i have been guided good lately (in my oponion) 

I have my camera Portrait oriented then, or chip aligned with the longside of  the scope. 
 

The problem is that your guide graph is looking OK but your picture isn't. It's the picture you hang on the wall! If those short trails either side of the star are from a Dec oscillation, we're not seeing the usual pattern which arises from this. The usual result is that the mount spends most of its time in one of two locations, either one side of the backlash or the other. It spends little time flipping between. The result is that you get two stellar images, one offset from the other. It can look like an 8, for instance or like o-o.  What you've got is -o- which means that the mount is spending most of its time in the middle, as it should, but is having brief out-and back excursions in one axis. This is very unusual and makes me wonder about backlash as the cause. It might in principle arise from rapid spurious commands to nudge one way and then the other on occasion.

Remember that most of the guide graph is an interpolation. The only real information the software has is where the mount was at the point the sub was taken. It will then draw a smooth curve between those points but, in real life, the mount could do a jig between those points and the software wouldn't know.

The two good subs you found: were they at the beginning or end of the sequence? I'm just wondering if you might have spent some time shooting through a washing line or power lines etc. It wouldn't be the first time...

Olly

[masjstovel]

3 hours ago, ollypenrice said:

The problem is that your guide graph is looking OK but your picture isn't. It's the picture you hang on the wall! If those short trails either side of the star are from a Dec oscillation, we're not seeing the usual pattern which arises from this. The usual result is that the mount spends most of its time in one of two locations, either one side of the backlash or the other. It spends little time flipping between. The result is that you get two stellar images, one offset from the other. It can look like an 8, for instance or like o-o.  What you've got is -o- which means that the mount is spending most of its time in the middle, as it should, but is having brief out-and back excursions in one axis. This is very unusual and makes me wonder about backlash as the cause. It might in principle arise from rapid spurious commands to nudge one way and then the other on occasion.

Remember that most of the guide graph is an interpolation. The only real information the software has is where the mount was at the point the sub was taken. It will then draw a smooth curve between those points but, in real life, the mount could do a jig between those points and the software wouldn't know.

The two good subs you found: were they at the beginning or end of the sequence? I'm just wondering if you might have spent some time shooting through a washing line or power lines etc. It wouldn't be the first time...

Olly

I probably wasnt clear about it, but this is the first time i see this result. I havent seen these starshapes on my other sessions, except bad frames with vibration (just thought the consistancy of angle and size and level of luminance this time through so many frames was odd).

I understand what you mean with o-o and -o-, but could this be RA backlash? If RA is "too well balanced", or would it be more o- or o-- then since it would mainly "rest" in position? 

Its supposed to be clear skies tonight, and i think i will try to have DEC slightly off balance and see for some frames, and then maybe do the same with RA(?). Could also try a couple of targets in very different positions, ie close to horizon vs close to zenith etc. 

The method i did in PHD2 with new guidescope is ok then? 

 

Edit: the good subs was about in the middle of the session. Can check for obstructions in the line of view when i get home, but dont think so in that direction

Edited October 12, 2020 by masjstovel

[masjstovel]

Update: so had a session last night again, and same issue as before. Was some peaks now and then in guiding, so couldnt tell 100% sure it was not guiding related. Then the guuding was as good as it never has been before at 0.54arcsec for 4x200sec exposures, and STILL the same issue. Then tried 60sec exposure and stars was fine, so problem was mechanical for sure.

Set my guiding exposures from 2s to 1s to see if PHD2 ould sort out better the "invisible excursions", but problem was still there.

With help from a friend from here at the forums he had a good idea and I slewed the scope in slow speed in every direction from EQMOD while taking short exposures to determine which axis the error was in, and it turned out to be DEC (no surprise there)

Then made scope back-end heavier - quite much heavier ("a little off balance" can mean different things to different people) and then the stars was cute little round things again. So probably dec is making nudges and because of freefloating it goes haywire so fast PHD2 doesnt notice it before its back in "resting position" again.

So that was the sollution in the end like you guys said. Thanks again!

Too bad i found out in the end after 5 hours and 41 subs thrown away:D

 

Edited October 13, 2020 by masjstovel