Skywatcher 150pds - ? light leaks and focuser misalignment ?

[Altair8389]

Hi,

I am a beginner with imaging with the Skywatcher 150pds.

I noticed when collimating that there could be a problem with the focuser at the appropriate alignment with the primary mirror axis. I noticed that after collimating with a cheshire eyepiece, then when I did the Laser collimator, the laser did not line up with the doughnut on the primary mirror in the centre.

 

I found that using the laser, I had to readjust the secondary mirror. When this kept on repeating, I stopped using the laser and used the Cheshire instead to align both the secondary and primary mirrors.

1) Is there a quick way to work out if my focuser is not orthogonal (90 degree to the primary optical axis).

Is this focuser mis alignment a common problem for (cheap) reflectors ?

2) Another issue, is there a light leak problem with this reflector ? I noticed when getting a dark frame, the dark frame was NOT DARK. In fact doing a dark in the daylight led to a white out in the frame. I notice maybe you need to cover the primary mirror cell, maybe the focuser tube or front of the scope. Is there a common thing you have to do ?

 

(I have not managed to work out where the light leak is yet) .

I had no idea imaging was so difficult and so many problems....focuser not orthogonal ? Collimation and coma correctors and their use/setup !!!! [cloudy nights link to GSO coma corrector] https://www.cloudynights.com/topic/462985-setting-up-the-gso-coma-corrector/page-2?hl= gso coma corrector for imaging

 

[after sorting out balance and focusing issues)

Thanks everyone!

Magnus

 

[Rallemikken]

34 minutes ago, Altair8389 said:

1) Is there a quick way to work out if my focuser is not orthogonal (90 degree to the primary optical axis).

Just a thaught: Empty the focuser tube, wind it all the way out. Set the mount so that the telescope tube is aligned horizontally with the ground. Use a spirit level. Twist the tube in the rings, so that the focuser point upwards. Use the spider as guide, not the focuser, use the spiritlevel again. Now the telescope itself is perfectly aligned in both ways. Put the spirit level horizontally over the focuser tube, and rotate it. If all is OK, it should be leveled all the way around. I belive that one of the holes for the spider alignes with the big hole for the focuser on this scope. If it's difficult to measure, maybe put a couple of magneto mini-spirit-levels on the vaines?

[Rallemikken]

47 minutes ago, Altair8389 said:

(I have not managed to work out where the light leak is yet) .

Mainly in the bottom. I've made a drawstring bag of double dark cloth when i image. Handy to put a dew heater inside, too. And I never take darks in daylight. Almost impossible to get everything tight.

[wimvb]

1 hour ago, Altair8389 said:

2) Another issue, is there a light leak problem with this reflector ? I noticed when getting a dark frame, the dark frame was NOT DARK. In fact doing a dark in the daylight led to a white out in the frame. I notice maybe you need to cover the primary mirror cell, maybe the focuser tube or front of the scope. Is there a common thing you have to do ?

I don’t think there is any scope where I would take darks with the camera in place. The best way is to remove the camera and take darks with a metal cover for the sensor. Do this in a dark room preferrably at night. Note that most plastic covers are not opaque enough to allow taking darks during daytime.

[Stefan73]

1 hour ago, Altair8389 said:

1) Is there a quick way to work out if my focuser is not orthogonal (90 degree to the primary optical axis).

Is this focuser mis alignment a common problem for (cheap) reflectors ?

I suspect it is.  My 130 PDS seems to collimate in 3 different ways depending on if I use a laser, a cheshire or collimation cap.  All of them seemed OK with a star test but the laser and cheshire seem to throw the illumination off centre in a picture or flat.  I decided life was too short to go fiddling with focuser tilt though!

1 hour ago, Altair8389 said:

2) Another issue, is there a light leak problem with this reflector ? I noticed when getting a dark frame, the dark frame was NOT DARK. In fact doing a dark in the daylight led to a white out in the frame. I notice maybe you need to cover the primary mirror cell, maybe the focuser tube or front of the scope. Is there a common thing you have to do ?

Almost certainly leaking light; many people have black shower caps or similar around the base to stop light getting in when imaging.  I take darks with the camera out of the telescope with its dust cap on.  Unless you're using a DSLR to image in which case I wouldn't bother with darks.

[Enceladus Dan]

1 hour ago, Altair8389 said:

2) Another issue, is there a light leak problem with this reflector ? I noticed when getting a dark frame, the dark frame was NOT DARK. In fact doing a dark in the daylight led to a white out in the frame. I notice maybe you need to cover the primary mirror cell, maybe the focuser tube or front of the scope. Is there a common thing you have to do ?

If you have a DSLR, Something I didn't realise for a while, is DSLR's leak light in from the viewfinder. I took off the cover and slid in some folded over black duct tape, before putting the cover back on. 

Danny

[Padraic M]

4 hours ago, Altair8389 said:

2) Another issue, is there a light leak problem with this reflector ? I noticed when getting a dark frame, the dark frame was NOT DARK. In fact doing a dark in the daylight led to a white out in the frame. I notice maybe you need to cover the primary mirror cell, maybe the focuser tube or front of the scope. Is there a common thing you have to do ?

Oh yes there is indeed! The 150PDS is a nice scope to image with, but it took quite a while to get all of the niggles sorted out up front, including the light leaks.

Three places to seal when taking darks:

1. Cover the front of the OTA (obviously!). The standard cover should be enough but I usually throw a sheet or a black sack over it too.
2. The primary cell 'rear' end of the OTA. People recommend stretching a black swimming cap over the end. I use a woolly had because I don't have a swimming cap! I just leave it in place permanently, as if it's leaking light during the daytime, it stands to reason that it also leaks light, no matter how little, at night.
3. Most importantly, cover the focuser draw-tube. Best bet is to wrap it with tinfoil; enough to stop light getting in between the drawtube and the holder. Again, I leave this in place permanently. Wrap it well, but make sure that it can still move freely for focusing.

The best way to test is to put your capture software on loop, set the scope up as you would for taking darks, and shine a really bright flashlight all over the OTA. You will see the leaks quite clearly in the captured frames.

I now never have a problem taking darks with the camera in situ, which I like to do so that I'm not impacting alignment or rotation by removing it.

[ONIKKINEN]

5 hours ago, Altair8389 said:

I noticed when collimating that there could be a problem with the focuser at the appropriate alignment with the primary mirror axis. I noticed that after collimating with a cheshire eyepiece, then when I did the Laser collimator, the laser did not line up with the doughnut on the primary mirror in the centre.

 

I found that using the laser, I had to readjust the secondary mirror. When this kept on repeating, I stopped using the laser and used the Cheshire instead to align both the secondary and primary mirrors.

1) Is there a quick way to work out if my focuser is not orthogonal (90 degree to the primary optical axis).

Is this focuser mis alignment a common problem for (cheap) reflectors ?

There is a slight problem here, how can you be sure that the cheshire is the correct tool out of the 2 you have used? It could be decentered in the focuser too but in a different way. You can check the focuser orthogonality by eyeballing it or spirit leveling it, but i doubt the issue is there, this sounds more like an inconsistent tool being used possibly because the tool itself is decentered or the way you connect the tool to the focuser is not repeatable accurately. If you haven't collimated the laser yet, do it at this point (they dont come well aligned from the factory). The best way to decide what tool to use is real world results, collimate using one and check corner aberrations in an image. The one that landed you on the better stars is the more correct one, but still the unreliability issue should be investigated and solved.

And little issues like these are very common in even not so cheap reflectors, just part of the charm.

5 hours ago, Altair8389 said:

2) Another issue, is there a light leak problem with this reflector ? I noticed when getting a dark frame, the dark frame was NOT DARK. In fact doing a dark in the daylight led to a white out in the frame. I notice maybe you need to cover the primary mirror cell, maybe the focuser tube or front of the scope. Is there a common thing you have to do ?

Yes, newtonians are open on both ends and have focuser leakage, which is unavoidable because the focuser has to have a gap between the drawtube and the OTA or it would not be able to move. There is no way you can completely plug all the gaps and still have good ventilation of the tube (very important if you want to get sharp images due to thermal issues with a front open tube that is closed at the back). You need to take the camera off the scope, plug it completely and take darks in a dark room so that not a single photon of light gets to enter the camera. I recommend placing the camera in the fridge, that way its closer to outdoor temperatures already and will have no ambient light to leak into the sensor and so darks are true darks. Still worth it to keep in mind the light leak issues when shooting, if you're under a full Moon or under light pollution you need to plug the back of the scope somehow to prevent light leaks and weird gradients in the image. I recommend something made of a dark fabric rather than a solid cover so that air can still pass through.

[Altair8389]

Thanks everyone who submitted advice. I will take note of all this. I had no idea this would be needed for inaging

and taking dark frames! Especially covering the parts of the scope when imaging!

 

Magnus

[Ole Larsen]

I found out that a lazer is no go for first kollimation, try to turn it around in the focuser, and se how the "dot" is turning around. Se this.

https://collimator.altervista.org/collimator-net-english/

[Altair8389]

15 hours ago, Ole Larsen said:

I found out that a lazer is no go for first kollimation, try to turn it around in the focuser, and se how the "dot" is turning around. Se this.

https://collimator.altervista.org/collimator-net-english/

Hi Ole,

Thank you for that tip. Yes that is excellent idea. I have found a cheap laser collimator did not work properly and collimation of the laser was difficult. I got a Bresser Messier laser collimator and that is excellent.

 

Update: My Skywatcher 150 PDS has been collimated ok now. My secondary mirror seemed slightly rotated down too much and I have corrected it. Still not perfect but I realised my focuser tube is slightly out of alignment by a few degrees. I will think about adjusting this another time.

My 8 inch Bresser Messier dobsonian is correctly collimated by both Cheshire collimators x2/Laser Bresser/Collimation cap almost perfectly.

 

The Skywatcher 150pds: Cheshire collimates it well (I was using it wrong before !!).

I will have to star test the system next and think about correcting the light leaks and also the focuser position....

 

Many thanks for all who helped and made comments and advice.

 

Magnus

[Ole Larsen]

Yes, Sec. mirror can be a little  beast to kollimate. You can se the video on my homepage, what i done with it.

My side is in danish, but i think google can translate.

The video is in English, and he has a lot of videos from the first setup, to go deeper in.

Write "astronomyshed" in you tube, i have learned a lot from him.

When you have the sec. mirror in place, you can use the laser to adjust the prim. mirror.

Edited February 15, 2023 by Ole Larsen

[Altair8389]

Hi all,

I estimated the misalignment of the focuser caused about 1 degree deviation of the laser from the centre of the mirror. Would a 1 degree deviation be serious ?

I would have thought it was not so serious for imaging. What do you think ?

I did a trigonometry, estimated the position of the laser on the mirror and divided this distance by about 560 mm which is the approximate distance of the primary to the secondary mirror.

Not sure if it would be easy to correct this entirely by making adjustments to the screws which attach the focuser to the OTA tube.

Any ideas anyone ?

 

Thanks a lot.

 

Magnus