Dew, frost, acclimatising or collimation issue

[DAT]

Hello, I am back with more problems.

To those who have helped answer my questions in the past, particularly with regards to collimation, thank you. I cannot be sure if my scope (8” Stellalyra dob) is collimated but I have definitely made progress, at least in so far as understanding what is going  on.

I am currently getting quite frustrated as we have been blessed (in London) with clear nights , but my images are anything but clear , except on long focal length eps.

I think I have fixed at least some of my collimation issues, crosshairs of Cheshire are centred on the doughnut as is the dot from the cap. I also no longer need an extension tube to focus 1.25”eps so hopefully the primary is no longer too far up the ota.

I am wandering if the lack of clarity could be the cold weather, I leave my scope out for 2 hours before observing to acclimatise and tonight the secondary is totally obscured (frost/dew?).

I would like to think it is the weather (if so, what can I do?) , but am very conscious that it could be something else I have done wrong (collimation issue).

Thoughts and comments are much appreciated as usual

[Captain Scarlet]

It sounds from your description as though collimation is fine. With Newtonians, yes dew can be a problem. Likely the secondary will haze over first. I’ve used a hairdryer, but it’s a hassle running power out to the rig. Nowadays if I find the secondary dewing up, I call it a day. I also don’t anymore leave it outside to cool. I take the OTA out last thing and use the cooling time for alignment etc. This maximizes my “time to dew” observing time. Many use dew heaters, but I’ve not gone down that road yet.

If your secondary dews up, the eyepiece, including the inner side, will be next. Keep unused eyepieces in your pockets if you’re not using many for the session, they’ll stay warmer for longer, assuaging the dew. If the primary dews up, definitely it’s game over for the session.

Cheers, Magnus

[PeterStudz]

What are the targets that you are trying to observe that aren’t clear? And when they aren’t clear what eyepiece(s) are you using? 

[DAT]

Moon and Jupiter are not too bad , stars are all very blurry.

EPS are all under 18mm (11,9,7).

I  messed up the secondary alignment previously , would like to think it is the cold but I have a  feeling I am trying to blame the mirror for my ugly face. I just let the secondary clear up a bit inside and took the scope out again and same problem. I think it is a collimation issue 

[bosun21]

I use a standard dew shield on my dobsonian and every other dob I have owned. This protects the secondary mirror and also helps cut down stray light.

[wookie1965]

I made a dew shield out of a camping mat and some plastic sheeting glued together I never had a problem with dew.

[PeterStudz]

If dew is the problem you’ll notice it on your finder(s) first. So much so that I can’t see the finder cross-hairs let along the target. For this reason I have a finder dew shield too.

If you think that your secondary has dew shining a light at it, down the focuser, will confirm or deny.

As for collimation… have you done a star test? 

The rapid change in temperature of bringing the OTA back into the house where it’s warmer will quickly create condensation on your mirrors (primary and secondary) and in situations where you haven’t even got dew. You will then need to wait a considerable time, sometimes a couple of hours, for the condensation to evaporate.

Also keep in mind that heat from around your eyes/face can cause condensation to form on the glass of your eyepiece too.

Edited January 20 by PeterStudz

[wookie1965]

I used to leave the front cover and the eyepiece insert off for a couple of hours after I brought the scope in and laid it horizontal just in case as PeterStudz says the warmth produces any dew. 

[DirkSteele]

Where in London are you? If you are able, bring the scope to the next Baker Street Irregular Astronomers star party in Regents Park and we can take a look at the collimation (Nick, one of my fellow committee members is good with Newt collimation). Next meeting is probably last week of February.

 

Find us on Facebook / google to get notified when the next meeting is.

[dweller25]

The issue you are having could be poor seeing/jetsream related ?

[DAT]

1 hour ago, dweller25 said:

The issue you are having could be poor seeing/jetsream related ?

Pretty sure it’s collimation + dew, wish it was though

[DAT]

2 hours ago, DirkSteele said:

Where in London are you? If you are able, bring the scope to the next Baker Street Irregular Astronomers star party in Regents Park and we can take a look at the collimation (Nick, one of my fellow committee members is good with Newt collimation). Next meeting is probably last week of February.

 

Find us on Facebook / google to get notified when the next meeting is.

Thank you, have been meaning to come down for a while but work keeps getting in the way. Live in Wembley so would have to drive down, where would I park?

will look it up on fb, thanks

[DAT]

3 hours ago, PeterStudz said:

If dew is the problem you’ll notice it on your finder(s) first. So much so that I can’t see the finder cross-hairs let along the target. For this reason I have a finder dew shield too.

If you think that your secondary has dew shining a light at it, down the focuser, will confirm or deny.

As for collimation… have you done a star test? 

The rapid change in temperature of bringing the OTA back into the house where it’s warmer will quickly create condensation on your mirrors (primary and secondary) and in situations where you haven’t even got dew. You will then need to wait a considerable time, sometimes a couple of hours, for the condensation to evaporate.

Also keep in mind that heat from around your eyes/face can cause condensation to form on the glass of your eyepiece too.

I think it must be more of a collimation issue otherwise I assume the picture would not be ok on the 30mm ep. At some point I will take up someone’s kind offer of help and get to the bottom of it. For the meantime I just have to keep trying and be patient 

[dweller25]

@DAT

A star test will immediately tell you if your scope is collimated or not.

But good star test or not if it looks like cotton wool and is unstable then your scope is either not cooled down or you have a poor quality sky.

My scope is perfectly collimated and I know when it’s fully cooled, so if the views are poor - which they frequently are for me as I live just north of Manchester, then I know the sky quality is poor.

Edited January 20 by dweller25

[PeterStudz]

1 hour ago, dweller25 said:

The issue you are having could be poor seeing/jetsream related ?

I thought it might be. There has been some very poor seeing the last week for me in Southampton. Monday the exception but still it wasn’t great.

Astronomy requires patience. I can now recognise poor seeing, dew, transparency, collimation issues, stray light, when the OTA hasn’t  cooled enough… Eg for cooling I think it’s useful to look at targets as soon as you get setup and learn to recognise the issue. For me being able to recognise all of the above took about a year, maybe more. 

[PeterStudz]

8 minutes ago, DAT said:

I think it must be more of a collimation issue otherwise I assume the picture would not be ok on the 30mm ep. At some point I will take up someone’s kind offer of help and get to the bottom of it. For the meantime I just have to keep trying and be patient 

Poor seeing will be more obvious as you push the magnification. Eg for me on Thursday Jupiter looked small but tack sharp at x80 to x100. But at x170 and above it was a fuzzy mess.  

[PeterStudz]

10 minutes ago, dweller25 said:

@DAT

A star test will immediately tell you if your scope is collimated or not.

But good star test or not if looks like cotton wool and is unstable then your scope is either not cooled down or you have a poor quality sky.

My scope is perfectly collimated and I know when it’s fully cooled, so if the views are poor - which they frequently are for me as I live just north of Manchester, then I know the sky quality is poor.

This ⬆️. A star test is essential and something you should learn to do.

[DAT]

As a beginner it’s difficult to separate collimation issues from poor seeing. Scope is usually out a couple of hours before I start seeing, so do not think that is the issue. Do the eps need to acclimatise as well?

The sky looked so clear the last two days I could not imagine the seeing conditions to be bad but I assume bad conditions are not always “visible.

Will try and perform a start test again and see, last time I tried I would say it looked like a collimation issue as I got concentric rings but slightly off and very fuzzy.

Thank you all for you help as always.

 

[PeterStudz]

Collimation issues can be seen by doing a star test. A beginner can do that too.

From experience my 8” Dob takes about 50mins to fully cool. I generally allow 1hour but that doesn’t mean I don’t look/observe before hand. For a start it’s useful to be able to learn and recognise tube currents.

A sky looking clear does NOT mean good seeing. You can have a crystal clear sky and very poor seeing. Sometimes you can judge poor seeing by stars “twinkling”. Sometimes not. Poor seeing is often caused by the jetstream which is 5-7 miles above the earth’s surface. It’s invisible naked eye. 

You need a night of good seeing to do a star test. Otherwise it’ll appear “fuzzy/wobbly”. If a star that’s high in the sky is heavily twinkling then don’t do a star test. Or maybe give it a go in order to practice, learn & recognise poor seeing. 

[dweller25]

17 minutes ago, DAT said:

The sky looked so clear the last two days I could not imagine the seeing conditions to be bad but I assume bad conditions are not always “visible.

 

 

I went out to a perfectly clear night on the 18th and the seeing was terrible

[DAT]

Great insights on this thread, learning lots.

[DirkSteele]

52 minutes ago, DAT said:

Thank you, have been meaning to come down for a while but work keeps getting in the way. Live in Wembley so would have to drive down, where would I park?

will look it up on fb, thanks

After 6:30pm (which happens to be when the meetings start), the Outer Circle is free to park. Recommend you be somewhat near London Zoo as the Monkey Gate is the only one that remains open after dark as we have a key.

[dweller25]

Keep at it, you are doing nothing wrong 👍

[Kon]

What's said above but the past few weeks the Jetstream has been a killer. Yes nice transparent skies but awful seeing. DSO observing doesn't suffer as much but planets and moon look all mushy. I check my collimation before every imaging session and it is the bad seeing we are having. Not much dew either the past few weeks either as the air has been dry.

A star test will help to verify if you are collimated or it needs a tweak. Defocus in/out but not all the way and you should see concentric circles.

Keep an eye on Jetstream maps too.

[Spile]

The best way to rule out collimation as a potential issue is to check the alignment of your mirrors. If your secondary is concentric a=b=c=d and e=f and the primary donut is in the middle of the ring of the cap then your good to go.