Eggy stars to the North, but guiding looks good

[Zakalwe]

I'm struggling a bit with this one. I was imaging NGC3718 at 600seconds the other night and was getting eggy stars.

it's the same again tonight. The guiding looks OK-isn (I'm monitoring it via PHDLab)

The drift is all in one direction (DEC, I think?).

I've altered the balance in RA to be east heavy. I've varied the DEC balance from camera-heavy to objective-heavy and am still getting this drift. Guiding to the south is fine...it just seems to be the North.

Ive also tried guiding in North only, as well as Auto, with not much difference.

Any ideas? Is it DEC backlash? Something obvious that I'm missing?

Imaging scope: Altair Triplet 115mm with Photoline .79 reducer

Guiding with OAG and QHY5L-II

[Vince1963]

Well Zakalwe... I'm afraid i dont see anything wrong with your image, in fact its brill!... I do see a circle above NGC3718.. NGC 3729 looks real good.. not that i have a clue what i'm talking abouy LOL... but it looks great... sorry i cant help.. but its a great image from what i can see

 

[Zakalwe]

Well Zakalwe... I'm afraid i dont see anything wrong with your image, in fact its brill!... I do see a circle above NGC3718.. NGC 3729 looks real good.. not that i have a clue what i'm talking abouy LOL... but it looks great... sorry i cant help.. but its a great image from what i can see

 

Thanks for that. Im not bothered about the vignetting or the dust bunny as flats will sort that.

The stars are a mess though, with obvious elongation

[ollypenrice]

If you're near the zentith then being camera or objective end heavy doesn't sort backlash because the OTA is close to vertical so the imbalance is ineffective. How high were you?

I hesitate to mention this because I want to stay well clear of any wrangles (     ) but could the mount have  Dec backlash on one part of the wheel and not on the other, as mine did? On the face of it this would seem to accord closely with your present situation.

Olly

[Owmuchonomy]

Looks like a backlash oscillation in one axis. If you could align your guide cam with the axes or use the PHD axis read out you can tell if it's Dec or RA. You are probably right it is likely to be Dec. Could you also get this effect with flexure between guide cam and main OTA if it is one plane?

[Zakalwe]

If you're near the zentith then being camera or objective end heavy doesn't sort backlash because the OTA is close to vertical so the imbalance is ineffective. How high were you?

I hesitate to mention this because I want to stay well clear of any wrangles (     ) but could the mount have  Dec backlash on one part of the wheel and not on the other, as mine did? On the face of it this would seem to accord closely with your present situation.

Olly

Thanks Olly

I only seem to get it when pointing in the North, which is a bit odd?

NGC3718 was at about 53 degrees. If it is DEC backlash, then it can be tweaked out. I haven't noticed it before though. I could flip the OTA in the saddle and try that. That would put the DEC worm on the other side of the wheel. Food for thought.

Looks like a backlash oscillation in one axis. If you could align your guide cam with the axes or use the PHD axis read out you can tell if it's Dec or RA. You are probably right it is likely to be Dec. Could you also get this effect with flexure between guide cam and main OTA if it is one plane?

Thanks Chris,

Regarding flexure....I am using an OAG, but I did have the guide camera off a while back. I'll check that its sitting tightly in the OAG (its in a clamp, rather than being screwed onto the OAG.

What's "PHD axis readout"?

[Vince1963]

Zakalwe.. could you chech the backlash of gears with one of these...http://www.ebay.co.uk/itm/like/191270156510?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&chn=ps&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=108&ff19=0..

They are very accurate

[Zakalwe]

Zakalwe.. could you chech the backlash of gears with one of these...http://www.ebay.co.uk/itm/like/191270156510?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&chn=ps&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=108&ff19=0..

They are very accurate

Ive tested the backlash, as per the Skywatcher instructions, and there is nothing to write home about. Even using a long dovetail there is the tiniest of movements in DEC, so I don't think that that is the problem, to be honest.

[Vince1963]

The movement is very minuscule looking at your image and not wanting to sound condescending, There as to be a grub screw on the gears lose or wear in the gears or an hiccup in the motor stopping for a brief moment  or software  program used for tracking having some sort of lag, or some sort of  very slight movement in the mount... I would love to know what the problem is once you've resolved it.

Hope you get it sorted

[Starlight 1]

So if you do 53 degrees South its ok . only north then its got to be mount side as every thing will be the same on top .

[Zakalwe]

Right.

I've swapped the OTA end-to-end and then ran the Synscan polar alignment routine. The polar alignment was well out. I can't think of why, other than ground shift (the ground that my house is built on is a near as dammit to swampland, so ground shift isn't unexpected.)

I've redone the polar alignment and am trying again. Its a bit better, but not perfect. I think that I need to work on OTA balance next.

I think that it's going to be a process of elimination. PITA!

[Starlight 1]

 Do it take less time to more round  Polaris  at 53 degrees then something 52 degrees East to west ?

[Zakalwe]

 Do it take less time to more round  Polaris  at 53 degrees then something 52 degrees East to west ?

Sorry, I don't understand?

[Starlight 1]

Sorry, I don't understand?

Sorry Zakalwe I think of a way to put  it.

[Tinker1947]

Its not surprising PA had something to do with it, very nearly every eggy star post ends up with a PA adjustment, plus a bit of something else......

[Zakalwe]

Its not surprising PA had something to do with it, very nearly every eggy star post ends up with a PA adjustment, plus a bit of something else......

You would think that the guiding would sort it out, wouldn't you?

I now remember having this problem once before and the PA had drifted. I'm going to fine-tune the PA again and get it better. I'm just surprised that it is so noticeable at 635mm focal length.

I found another problem last night. My obsy PC has no speakers and last night I was remotely monitoring things. PHD kept randomly beeping at me as the guide star SNR was getting low. I could hear it as the monitoring PC has speakers. Now the conditions were the same last night as the previous nights...lots of heavy moisture and haze in the air. It's possible that that could be adding to the problem (it certainly won't be helping!).

So the plan now is to:

• Fine tune the PA
• Use a better night for testing!
• Check over the OAG. The star shapes are pretty horrible, so I might be able to improve on this.
• Do all the usuals- balance, cable drag.

At this stage, I am sure that it's not the mount. There's no discernible DEC backlash (though I can see some when planetary imaging at 7metres FL. The DEC axis is a little slower to respond to direction changes than RA. 7 metres is pretty unforgiving!). plus I have rotated the DEC wheel 180 degrees by swapping the OTA end to end. If the DEC wheel had a loose spot then this would have highlighted it.

Never a dull moment in this gig!

Now, for something completely different. I had a bash at star trails last night. I have never tried this type of photography before, but I found it pretty interesting. One to play with, I think.

The horrible gradient is from a neighbours insistence on leaving his conservatory lights on 24/7 allied with the above mentioned moisture in the air that totally fogged the objective on the Samyang 14mm f2.8

First attempt at star trails

https://youtu.be/grNovJwUgsg

[Tinker1947]

I use the NEQ6 handset to do PA, 2 star align, the the PA routine pops up, pick a star that's easy to see and use my guide scope and the PHD window to centre the star, the guide scope and camera sensor are aligned perfectly, run this twice on different stars and a different PA star results in a flat graph for a couple of months, my Piers being 8" X 8" dryed Oak maybe moves a tiny bit, the ground is soft silty stuff bone dry, with the Oak in it around 4' deep held in place with pea shingle, so it might move a bit, haven't managed to get to the water table with one hole going down around 5' if your water table is higher that could alter your pier.....

[Zakalwe]

An update on this thread.

I picked up a cheap ST80 and mounted it as an external guidescope. I also installed the latest release of Sequence Generator Pro, which now longer supports PHD Guiding (not a sensible decision IMHO) and insists on a beta version of PHD2. I was reluctant to install this as I have seen various reports of problems. True enough, when I tried to calibrate the guiding it wasn't moving the mount. The mount was selected and connected, everything else was working but PHD2 wouldn't calibrate. I eventually sussed it out by clicking on the "configure" button in equipment connection box. No other ASCOM software seems to need to do this...once the mount is connected them everything else connects. Not PHD2 in my case...it wanted the EQASCOM config page opening and connecting from there. Then started a round of messing about with the various Brain settings to change over from the OAG to an external guidescope. To be fair to PHD2, it does seem to make the configuration easier by providing feedback on the settings, whereas PHD had you delving into the log files. That seems to be a big improvement.

So, after losing about an hour swearing at PHD2, I managed to run off some subs of M51. This is 6 x 10 minutes, stacked in DSS. No calibration files and a very basic stretch in PS. It doesn't look too bad (from a guiding point of view). It's not perfect, but a big improvement.

6 x 10 min lum

[Tinker1947]

They all look to be eggy the same way, but only a pixel nothing more.....

[Zakalwe]

Yeah, thats what I thought. An improvement, but not spot on.

[Carina Lass]

Your real pa and not corrected pa is the problem.

As the ota is pointing north, the calibration if pa is not perfect at x will change to x-ra+dec and that will change every click the field rotates.

If your pa is perfect the correction wont be needed an stars will be round.

[Zakalwe]

Your rwal pa and not corrected pa is the problem.

As the ota is pointing north, the calibration if pa is not perfect at x will change to x-ra+dec and that will change every click the field rotates

Im suspecting that. I did PA using the Synscan routing. I am going to redo it using PHD Drift Align tonight.

[Thalestris24]

Hi

These sort of problems always perplex me! I've always thought that slight PA error shouldn't be a significant cause of problems especially when imaging at shortish focal lengths. In any case, in theory it should manifest itself as field rotation in long exposures rather than eggy stars in one direction? I have a feeling that eggy stars have a different root cause. There are some question marks regarding PHD2 - some people just don't seem to get on with it! On the other hand, others seem to manage with it fine.

Just musing.

Louise

[Zakalwe]

Hi

These sort of problems always perplex me! I've always thought that slight PA error shouldn't be a significant cause of problems especially when imaging at shortish focal lengths. In any case, in theory it should manifest itself as field rotation in long exposures rather than eggy stars in one direction? I have a feeling that eggy stars have a different root cause. There are some question marks regarding PHD2 - some people just don't seem to get on with it! On the other hand, others seem to manage with it fine.

Just musing.

Louise

I thought that too (regarding PA) but reading Singlin's thread is making me reconsider. The ground that I am on is very poor, so I am not surprised that the PA does drift a bit.

[Thalestris24]

I thought that too (regarding PA) but reading Singlin's thread is making me reconsider. The ground that I am on is very poor, so I am not surprised that the PA does drift a bit.

Yeah, I've followed that too but I'm still not convinced it was a PA problem. But I'm not really knowledgeable / experienced enough to say anything definitive!

Louise