Any ideas ? Dew perhaps ?

[Astro Noodles]

I have been having an issue in the last couple of weeks. I set my equipment up, set the intervalometer and come back at the end of the session. Just lately I have only got useable images from the start of the session. For example, on Wednesday I took 1200 13 sec exposures but could only stack 169 of them. Yesterday I took 400 20 sec exposures, but only the first 17 were stacked. The tracking is ok and the camera still has power at the end of the session.

Am I experiencing the effects of dew?

[Budgie1]

What are you using, scope or camera lens, and does it have a mist across the front optics at the end of the session? If it does then there's a good chance it's dew.

Why are the other images unusable? If you could post a JPG version of a good and bad exposure it may help diagnose the issue.

[Astro Noodles]

Hi Martin

Thanks for your reply

I am using a DSLR with a 200mm lens. I have noticed that the lens is fogged after bringing it indoors but I'm not sure whether that is because I have just brought it in from the cold outside. 

Stupidly I have deleted the unusable subs. I am using Deep Sky Stacker and have noticed the star count drop whilst registering the image. The first sub was about 50 stars but it very quickly dropped to 1 or 0.

[Shimrod]

1 minute ago, Astro Noodles said:

Hi Martin

Thanks for your reply

I am using a DSLR with a 200mm lens. I have noticed that the lens is fogged after bringing it indoors but I'm not sure whether that is because I have just brought it in from the cold outside. 

Stupidly I have deleted the unusable subs. I am using Deep Sky Stacker and have noticed the star count drop whilst registering the image. The first sub was about 50 stars but it very quickly dropped to 1 or 0.

How long are you spending setting up before you start imaging, and are you moving the camera straight from a warm environment to a cold environment? Your 'time to dew' seems very short so if it is dew it is likely to be already forming at the start of your imaging session. Can you get a cheap lens hood for the lens - it will cut out some stray light and give you a bit more time before dew forms.

Try giving your camera sometime to acclimatise before you start using it outside. Ideally you could get a small dew heater, but I know some people use a small battery powered hair dryer to remove dew that has already formed.

 

[Astro Noodles]

Hi Shimrod.

The lens and camera had been outside for a coupe of hours before the imaging session. The camera on the mount and the lens inside a backpack. I haven't experienced this problem before, just other problems like faulty polar alignment, poor focus and being unable to find the target. I have ordered a dew heater to see if that helps.

I have noticed lately that the temperature is dropping by about 5-10degC over the course of o couple of hours and it is pretty cold by midnight. Would this make dew more likely to form?

[Shimrod]

8 hours ago, Astro Noodles said:

Hi Shimrod.

The lens and camera had been outside for a coupe of hours before the imaging session. The camera on the mount and the lens inside a backpack. I haven't experienced this problem before, just other problems like faulty polar alignment, poor focus and being unable to find the target. I have ordered a dew heater to see if that helps.

I have noticed lately that the temperature is dropping by about 5-10degC over the course of o couple of hours and it is pretty cold by midnight. Would this make dew more likely to form?

If you have an app like accuweather on your phone, it can tell you what the dew point is. If you are experiencing large temperature drops then in all likelihood dew is the culprit.

[Astro Noodles]

So I now have a lens heater and have just set off 399 x 20 sec subs on M101. I am keen to see if the heater fixes this problem.

[rickwayne]

The other issue with temperatures dropping during the night is the thermal contraction of bits in your equipment, especially any metal ones. Many of us refocus our telescopes every hour, or even oftener, or use software to monitor a sharpness figure of merit to trigger refocusing. For my 336 mm f/5 refractor, the zone of critical focus is all of 74 microns deep. Doesn't take a lot of contraction to exceed that.

[Astro Noodles]

I had another go last night with a dew heater. It was a little bit more successful but the issue still persists.

I have attached two subs below. The first if from 21:23. DSS detects 131 stars. The second is from 23:08. DSS detects 2 stars.

Would it be possible to take a look to see what the issue is?

IMG_6907.CR2 IMG_7105.CR2