Dew or seeing?

[vernmid]

Hi All,

Had a bit of a play last night to test Coma Corrector spacing etc and to practice with guiding etc. I chose M31 as a test target.

130PDS/EQ5

9x50 finder+ SPC900NC + PHD2

BYE + 350d

I took 3 300s subs and they steadily deteriorated! See attached sequence

Could this be dew? or seeing? or cloud? Just trying to figure out what problem to attack next 

It looked pretty clear to my aging eyes!!!

Many thanks

Vern

 

[newbie alert]

Moon even?

[vlaiv]

Dew or clouds / haze / transparency issues.

You will need torch to diagnose which one it is. Shine torch up - if you see "light saber" kind of effect, and if it is getting worse - its local humidity/fog - transparency issue.

Shine torch down the tube and look for signs of fogging on mirrors - primary is easily checked, secondary - look at the reflection in primary if you don't want to remove camera to check thru focuser (and I guess that you don't want that).

[vernmid]

3 minutes ago, newbie alert said:

Moon even?

Moon hadn't quite risen :-)

3 minutes ago, vlaiv said:

You will need torch to diagnose which one it is. Shine torch up - if you see "light saber" kind of effect, and if it is getting worse - its local humidity/fog - transparency issue.

Good idea. I'll try it. Thanks. I live about 200 meters from a river so suspect that local fog may be an issue

 

[vernmid]

Out of interest, if this was caused by river mist, what are my options? I'm assuming pack up and go to the pub?

 

[vlaiv]

1 hour ago, vernmid said:

Out of interest, if this was caused by river mist, what are my options? I'm assuming pack up and go to the pub?

 

Either that, or just accept that some of your subs will be of low quality, which you can either:

- discard

- stack as usual (using average method) - this will produce worse stack than if your subs were equally good

- stack with uber/mighty/magic stacking method (that I'm currently developing ) that will be able to properly "weigh" subs according to SNR and give better results than option 2 (straight average), so it won't be time completely lost under stars (it might be time lost in the pub, but that really depends on how keen you are to spend time in the pub )

[vernmid]

Thanks Vlaiv,

Lets see what happens over the weekend :-)

 

[alacant]

20 hours ago, vernmid said:

dew? or seeing? or cloud?

Hi. In my limited experience it nearly always gets better as the night goes on. I think you should still go to the bar but leave the telescope tracking and snapping away. Best of both? Cheers.

[Stu]

I would expect to see some haloes around the brighter stars if it were dew so perhaps it was iust the transparency going off due to high hazy cloud? It can be hard to see visually, but you just see fewer stars although it looks clear.

[wimvb]

If you have problems with dew, the secondary is most likely to fog up first. A small dew heater (easy to diy) around the secondary's support will solve this.

Another option that works for me is attaching a small fan to the back of the scope, blowing air into the tube. I haven't had dew problems since I installed one this summer. (Otoh, I haven't had my scope out much either since this summer. You'll need biiiig fans to get rid of fog this way.)