Skywatcher 190MN - Help please.

[Komet]

Hello ....

I have a 190MN. I bought it new in March but have only recently completed my obs so it has lain dormant for a while. When I first had it setup on a tripod I had odd shaped stars which I had put down to either tracking on a tripod or perhaps a collimation issue. I decided to sort it when I had it on the pier which it now is.

I still have odd shaped stars

This is a 5 second image taken through a QHY IMG2Pro. The star shapes can be clearly seen. They remain the same irrespective of exposure length - it is not a tracking issue.

I have tried so many times to collimate this scope and no matter how good I think I have got it I still get these shape stars.

Now, I have had a few people tell me that there were a few 190MN's with problems earlier this year and that some have been replaced indicating a fault with some scopes. However I have not read or heard anything about this except from a few people who do not have this scope anyway. I have heard nothing on any forum about this.

So is there anybody out there who has first hand knowledge of this apparent fault? What the symptoms were and anything to look out for?

Has anyone had cause to collimate their 190MN? How did it go?

I have to admit to being at my wits end with this. Having spend nigh on a grand on this I was expecting round stars at the very least.

All help and comments gratefully received.

[Bazzaar]

I believe it IS a tracking problem, but not the usual tracking error problem but tracking smoothness. The scope is either vibrating slightly as it moves or overshooting and falling back slightly.

If you were to superimpose the dec and RA axes on this picture would one of them line up with the bulge of the stars?

Regards

Barry

[Komet]

I believe it IS a tracking problem, but not the usual tracking error problem but tracking smoothness. The scope is either vibrating slightly as it moves or overshooting and falling back slightly.

If you were to superimpose the dec and RA axes on this picture would one of them line up with the bulge of the stars?

Regards

Barry

Hi Barry ...

As that is an unguided image then DEC plays no part in it at all.

If the tracking is modulating somehow then wouldn't the star be the same width rather than a teardrop?

I will next time I get the chance remove the guide scope (it is on a dual system at the moment) and clamp the 190MN directly to the EQ6 to see if that changes anything. Its worth a shot.

Cheers

[AndyUK]

Hi Mike - Welcome to SGL...!

Barry could well be on to something there... I guess you normally use your WO ZS66 to guide, but maybe you could test this by swapping it around and using the ZS66 as your imaging scope and guiding with the MN190 to see if you get the same result?

(Just a thought...)

[steppenwolf]

From what I can see, this is an RA tracking error. I cannot verify your polar alignment, of course but I can to the best of my knowledge confirm the following:-

1. The image is from a region in Vulpecula centred on RA 302.06 DEC 24.64

2. Your image is tilted at an angle of 14.76 degrees

This image shows the DEC axis in red:-

[lukebl]

Is it a tracking problem, though? That's only a 5 second exposure, and I wouldn't have thought tracking problems would show up after such a short exposure unless the polar alignment was way out. How does a 1 sec exposusre look like?

[Whippy]

Annother thing to add into the mix is flexture. If there's something not quite bolted down properly, it'll show up in the image as stars like these.

Tony..

[Martin-Devon]

Several folks with 190MNs were complaining about flexure caused by the way the OTA was mounted was a problem- apparently the supplied Skywatcher mounting fixtures were not good quality and several people replaced this with a stronger better quality type that eliminated the flexure and this helped them a lot. Just a thought....

[Whippy]

Several folks with 190MNs were complaining about flexure caused by the way the OTA was mounted was a problem- apparently the supplied Skywatcher mounting fixtures were not good quality and several people replaced this with a stronger better quality type that eliminated the flexure and this helped them a lot. Just a thought....

Indeed, I should have poined out that the supplied dovetails aren't the best and really should be replaced by a better quality one such as the ADM range.

Tony..

[ollypenrice]

Steve has a point. The error seems to lie along RA. I would just try some very short subs on bright stars. One second or so.

The fact that you are not guiding does not eliminate the possibility of backlash. For the short sub test I would load the mount heavy on the east to remove backlash.

I agree with the others who point out that this is a big scope that needs good solid attachment.

Olly

[Komet]

Thanks for all of the input - it is very much appreciated.

When I purchased the scope I also ordered a heavier dovetail as I know what Skywatcher doves are like (had a few in the past).

I have the scope mounted on a heavy duty dual mount with the MN190 side boasting a heavier clamp. For the next time the skies ever clear I will mount the scope on its own on the mount - its a quite well aligned NEQ6Pro on a dead pier btw - to see what happens.

It is mounted scope base heavy and counterweight (East) heavy anyway.

My guiding, when I have bothered , is superb. I use a WO66SD with a Lodestar and get RMS figures of less than 0.05 pix regularly with a 4 second exposure. Its pretty smooth.

I am still however puzzled by the reports I have received of some scopes being exchanged as faulty but no actual evidence from someone who has had theirs exchanged. Odd.

Anyway I will report back with any further findings.

Thanks again

[ollypenrice]

If you are feeling anxious and would like to test it before a clear sky comes, then there is an easy way to do so.

Make an artificial star by gluing a small ballbearing onto a matt black card and illuminating it with a bright beam. Observe this in the scope, which should really be a good way back, though I found I got convincing diffraction rings showing from just 6 metres. Only light from the neaest bit of the ball gets to the scope, making a good point source. You could then image this.

The other obvious thing to do (why didn't I say this first time around??) is to image Polaris for one second with the drive switched off. The field rotation will be negligible.

Olly

[Bazzaar]

You may be able to see the problem directly by watching a bright object at high magnification. But it does look like the RA motion is modulated.

Barry

[steppenwolf]

I have some sympathy with the argument against it being tracking as the exposures are indeed very short BUT it is surely not a coincidence that the error appears to lie so clearly in the RA axis according to my research on this one image?

An exposure of Polaris with no motors running would certainly help as Olly has suggested as would mounting this (rather) heavy telescope on a better dovetail but no point in doing that unless you upgrade the tube rings as well IMHO or you will simply be bolting a small base cast tube ring to a large flat surfaced wider dovetail bar.

[andrew s]

Olly said "I got convincing diffraction rings showing from just 6 meters" indeed you can but a "star" test at this range will show a perfect mirror to have seriously imperfect correction as a parabola is not corrected for such close imaging. It would show up gross non symmetrical errors though. Andrew

[Komet]

Right ..... now listen carefully as this is VERY technical. 8-))))

I have removed the dual bar mount and clamped the OTA of the 190MN directly to the top of the mount. Now this has a solid, heavier dovetail but the standard rings.

I have moved the mount so it is lifting the counterweights and put the OTA flat.

The mount is tracking.

On top of the OTA I have placed a glass of water. If I look at the surface of the water from below I can see any vibrations transmitted to the OTA from the mount.

As an aside I can jump up and down on the observatory floor and not affect the water so any vibrations come purely from the mount.

The mount, an NEQ6 makes the rhythmic squeaky noise plus there is also a cyclic underlying resonance. The surface of the water shows very slight vibrations that appear to follow this underlying sound.

Even though everything is clamped up tight there is a fair degree of flexure of the OTA. I can push it with my finger and there is a definite deflection. Tube rings?

This is raising more questions than answers. How much vibration should a mount transit. If I used very thin rubber between the tube rings and the OTA would this help reduce the vibrations? Are the vibrations relevant (you really have to get the light right to see them)

Aaaaarrgghh!

[ollypenrice]

Thanks to Andrew S for his important caveat. I was collimating a refractor at the time.

I have never done this vibration test, nor have I heard of anyone else doing it. We need to find you a control experiment to see how much is normal!

I'd have thought a vibration would blur symmetrically. If it comes from the RA only along the line of drive (possible in principle) then I'd expect symmetrical blurring along that axis.

But what we see in the image looks like a scope spending most of its time collecting light in the right hand position and less of its time doing so in the left position. This is usually an indicator of backlash with a preference for one side of mesh. A static test would eliminate the optics but my feeling is that the defect is too regular across the chip for that.

Olly

[Whippy]

I've never heard of vibration experiments either, certainly a new one on me! The mounts noises sound about right to me too. I know some people bolt a second dovetail on the top of the rings to combat flexture on newts, might be worth a thought but it might be the backlash too. Balance is critical when you're putting a heavy scope with a long(ish) focal length on it.

Tony..

[Bazzaar]

I would fix a laser pointer to the OTA, project in into a camera as far away as I could and monitor the deflections.

[andrew s]

It seems to me that the worm is too loose. I would guess that the worm pushes the wheel which accelerated away from the worm. It either then falls back due to the imbalance of the mount or the worm catches up and pushes it again. The effect will be the same a vibration in RA but with slightly different period/characteristics. I am not sure it would be symmetrical as viewed via the eye piece as one acceleration is due to gravity and the other due to the worm contacting the wheel.

To test this idea see how much play there is in the wheel & worm.

Hope this helps Andrew.

[andrew s]

Mick - Another thought. How free is the RA axis if the gear is disconnected? If it's stiff the vibration could be due to a stick slip vibration as the motors build up enough force to overcome the friction. The same could happen if the worm is too tight in the wheel. I have had these types of issues before causing vibration

in mounts

Andrew

[Komet]

Well I got out last night for a bit.

I have 3 images for you to peruse. All are crops of the same area around M27. There is a 1 second, 2 second and 5 second exposure.

Right ascension is horizontal and DEC is vertical

1 second ..... note the star shapes. Now at this length exposure there should not be much in the way of tracking issues affecting the image so I would expect the stars to be symmetrical at the very least. Note the star shape

2 second .... the star shape is the same but more clearly defined due to the longer exposure.

5 seconds ...... the stars look rounder here but this is because of the extra exposure.

I would be interested in your thoughts gentlemen.

Thanks

[andrew s]

Mick - As proposed on the PAIG site I would inspect your images visually to see if you can detect any optical issues (I would use a medium to high power and view both inside and outside focus). If you have any asymmetrical errors this should show them up. Or you could image a bright star inside and outside focus with 1sec or less exposure. Or try rotating you tube by 90deg (leaving the CCD as is) and see if the distortion stays with the imaging train or the mount axes.

Your mount should not be vibrating. I have has a Fullerscopes MkIV with AC and stepper drives, a Parallax HD200 with DC motors and currently have Vixen Polaris with stepper drive, AP 900 with DC drive and Paramount ME with DC drive. They have a variety of gearing from low cost gear box to belt drive. I have never detected vibration when imaging or viewing at 1.5m to 4m focal length. I tried you water test on the ME last night and could not see any vibration.

I suspect you have a combination of issues: a) some residual optical errors either on the glass or collimation, and mount vibration which may or may not be adding to it.

I don't think you have tracking errors distorting your stars.

Andrew

[ollypenrice]

Hmmm. Now you have slight elongation along the Dec axis. A crop would be good.

It is beginning to look optical. How about the attachment of the camera and the health of the focuser? Is there any reason to doubt that the chip is dead square to the light cone?

There were some problem examples of this scope but does anyone know what they were? Could they shed any light?

Olly

[steppenwolf]

Using the same test that I carried out before, the star elongation no longer matches the RA axis so that may have been an unfortunate coincidence but, boy was it a convincing one! I don't think a 1 second exposure would not show a tracking error even with the 1000mm focal length of the 190 MN. I would like to see a close-up crop of your star image.

Also, have you tried imaging a de-focussed star to see what the airy disk looks like - again, an image of Polaris would help to remove 'secondary' issues.