Skywatcher 190MN - Help please.

[Komet]

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?

Hello Olly ....

Attached a blow up of the 1 second image. To all intents and purposes it looks like movement along both axis - but seperately .... and all in a second?

The focuser is to the best of my knowledge square. It is new and although not a Moonlite, good imo for a Skywatcher. The camera is mounted squarely with the focuser (a direct connection would better but there isn't one)

[Komet]

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.

Hello Andrew ....

Unfortunately I do not possess any high power eyepieces to try this. Stars - even big ones are points of twinkly light.

My "vibration test" - the vibrations were so small you really had to get the light right to see them and then even an intense scrute was needed. (how else can you scrutinise intensely? 8-)) ) I wasn't actually looking for vibrations but actual ripples to see if the OTA was moving about whilst tracking. Seemed like a sensible way to do it.

[barkis]

Why not try an exposure with your drive switched off, then examine the blown up startrails. At least it will eliminate any drive anomaly that might be present.

Ron.

[Komet]

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.

Hi Steve ... see post above fpr a crop of the 1 second image.

Here is a 20 second airy disk. The 10 second and the 5 second was similar. if there were tracking issues then shouldn't they show up onthis?

To me this, for the most part looks OK with the exception of the disturbance at 8 o'clock. There does appear to be a ripple running right across from outside to in.

There apperas to be nothing protruding into the light path that I can see.

[steppenwolf]

I've got to say, I don't like that star shape at all - I'd still like to see an out of focus disk.

[steppenwolf]

I don't believe it, we doubled!!!!! Interestingly, that is a better out of focus disk than I got with mine although I am curious about the 8 o'clock ripple - thanks for posting.

[Whippy]

That's not the focuser tube intruding into the light path is it?

Tony..

[George]

Any vibration caused by the mount tracking is likely down to the transfer gear being meshed too tightly that would explain your squeaks. If you have no backlash I would try slackening the steppers off slightly and reseating them. Its unlikely to be the worm as it moves to slowly.

[Komet]

That's not the focuser tube intruding into the light path is it?

Tony..

I don't believe it is. Looking down the angry end of the scope the focuse is well tucked out of sight and can not be seen at all.

[Whippy]

I thought as much Mick, but I thought I'd ask as I've seen similar before. Taking a seond look, there's a slight flattening around the 12 o clock reigon too.

Tony..

[barkis]

The 12 O'Clock dip corresponds to one of the Mirror clips, as does the bad one at 8 O'Clock.

[Komet]

I thought as much Mick, but I thought I'd ask as I've seen similar before. Taking a seond look, there's a slight flattening around the 12 o clock reigon too.

Tony..

.... and the left hand upper part is softer than that on the right hand lower. That might well be a collimation issue - don't know. Having collimated it so many times I always end up with these misshapen stars irrespective of how good I think it is collimated. I can make it worse but I cannot get round stars.

[andrew s]

Mick - The out of focus image looks quite good to me. The distortion could be tube currents but as long as the disk remains circular as you approach focus I would say you don't have a major optical issue. If you are too far out of focus then it can mask issues. To confirm all is ok I would do a set of images from on focus to this position.

I would also do a star trail with the motors off and with a delay to allow you to be well away so you can't introduce any vibrations. Then turn on dec guiding only and repeat, then RA only etc. (If you deliberately offset the polar alignment you will get a trail which is simpler to examine.) This might help track the source down.

Do you have a video camera you can use to take high speed images of a bright star this again might help reveal what is going on?

I don't think you should see any vibration on the mount as the optical leaver in your scope will amplify it. This is not a problem for refractors as small movement don't move the image much but it is amplified by the mirror in your SN.

I really feel for you lots of advice and not much progress.

Regards Andrew

[ollypenrice]

What about my idea of an undriven shot of Polaris? That would eliminate the mount entirely.

My hunch is now optics. Mention of the mirror clips is interesting (how many are there? Three?)... but also triangular stars are associated with pinched optics. I can't really decide whether your stars are triangular or not.

Diffraction pattern and aberrations

Olly

[steppenwolf]

Mention of the mirror clips is interesting (how many are there? Three?)... but also triangular stars are associated with pinched optics. I can't really decide whether your stars are triangular or not.

There are indeed three clips, Olly. I can't decide on the shape of these stars either, there are two indentions and a third 'flat' and with a little imagination these *could* be at 120 degrees from one another.

With regard to pinching, I guess that the front meniscus could be pinched or it could be the primary mirror - on mine, it certainly wasn't pinching on the primary mirror (mirror clips were loose) unless there was some kind of stress imparted by the primary cell which I doubt.

[barkis]

What is the collimation adjustment in the cell. Are they a push pull arrangement, and if so, is it possible some cell distortion could impart to the mirror itself via the retaining clamps, should too much tension exist in those adjustment screws. Probably less likely if Bobs Knobs have replaced the originals.

Ron.

[PNJ]

Hi

I have a MN190 and had similar star shapes. My MN190 was mounted on a EM400 with a astro developments plate which is very sturdy. I still had the same probs with star shapes. I then decided to upgrade my focuser to a moonlite and also used a cheshire eyepiece to centralise the focuser. This did cure my problems.

I would suggest you check the cenralisation of your focuser and then re-collimate using a cheshire eyepiece. i did use a laser collimater and this was giving me all kinds of results.

Using a cheshire eyepiece, i use the orion one, you can see if your focuser is square on in the hole whewre the focuser fits onto.

Since upgrading the focuser and making sure the focuser is central and collimating i have no more probs now, and i use the supplied tube rings.

hope this helps

paul j

[PNJ]

Hi

I have a MN190 and had similar star shapes. My MN190 was mounted on a EM400 with a astro developments plate which is very sturdy. I still had the same probs with star shapes. I then decided to upgrade my focuser to a moonlite and also used a cheshire eyepiece to centralise the focuser. This did cure my problems.

I would suggest you check the cenralisation of your focuser and then re-collimate using a cheshire eyepiece. i did use a laser collimater and this was giving me all kinds of results.

Using a cheshire eyepiece, i use the orion one, you can see if your focuser is square on in the hole whewre the focuser fits onto.

Since upgrading the focuser and making sure the focuser is central and collimating i have no more probs now, and i use the supplied tube rings.

hope this helps

paul j

[Komet]

Hi

I have a MN190 and had similar star shapes.

That is the really interesting thing.

It shows that these scopes, despite all the glowing reviews, do have an inherent problem somewhere I feel. Not all the units obviously as the review ones were good it seems. In any case I have seen some great images out of them.

Do you still have the scope or have you sold it on? I ask because I have had contact with someone with an MN190 with a Moonlite focuser that has bad stars still.

[Deneb]

I don't believe it, we doubled!!!!! Interestingly, that is a better out of focus disk than I got with mine although I am curious about the 8 o'clock ripple - thanks for posting.

Nibbles & also slightly @ 12 'o clock

Nadeem

[Komet]

So tell me Nadeem .... what are the stars like on yours?

What was your MN190 like out of the box?

[Deneb]

Actually Mick, I had no artifacts/nibbles showing on my optics, unfortunatly I don't have a airy disk to show you unless I can dig one out from somewhere in image library...

Nadeem.

[Komet]

Actually Mick, I had no artifacts/nibbles showing on my optics, unfortunatly I don't have a airy disk to show you unless I can dig one out from somewhere in image library...

Nadeem.

So I take it you have nice round stars then?

[Deneb]

Apart from my guiding errors - no problems, even with the Bobs Knobs fitted...

Nadeem

[Freff]

A while ago I was getting similar issues with mine. I collimated it till I was blue in the face.

My problem was a out of square focuser, resulting in the removal of the focuser and corector plate to realign.

My 190MN also responds better to a Cheshire collimation rather than a laser.

HTH