does dew cause graineness image now added

[iwols]

hi all had a go at the horses head last night with my ed80 and 414ex camera and filter wheel,a quick process in pixinsight and result was a slightly grainy image could this be caused by dew,it was late last night when i did the quick process and will try again today and post the image for your trusty thoughts cheers guys

[cjdawson]

Dew will degrade any image.  Grain isn't how I'd describe it.  More like fogged up screen.  But I suppose if it's thing enough it could look like grain.

If you suspect dew, it's worth checking your optics between frames to make sure that everything is dew free.  What you are looking for is that tell tale moister layer like you get on car windows at night.  Any hint of it at all and you need to clear the it.

 

In my experience there are 3 ways to deal with dew and these can all be used in combination.

1. Dew gun - aka hair dryer.  You can 12v hair dryers.  These will at a pinch, blow heated air that will clear the dew.  Watch out when using this one, as the scope's optics will be heater, so will need to reach a stable temp again.

2. Dew shield - This is a tube (can be made of anything) that you slip over the optics.  It needs to pretude beyond the end of the focal length of the scope.  it needs to be 1.5x the apeture of the scope in length.    For example for an 4" refractor you'd need a 6" sheild, for an 8" SCT you'd need a 12" shield.  For a 16" newt, a 24" shield would work.    note: it does not matter if your scope if a refractor, SCT or newtonian.    Adding a dew shield can also help to cut down on stray light entering the optics, so helping to increase contract a little.

3. Dew heater - This device will put heat into the optics of your scope.   Doing this constantly will prevent the dew from forming in the process.

 

I said this can be used in combination.  Here's how I do it with my scope.

I have a dew shield on each of my scopes.  This slowed the heat loss from the scope, and can extend my activities from 30 mins (on a bad night) to a couple of hours before things start to dew up.  In addition to the shields, I have a dew heater, in my case  a copper pipe with insulated ni-chrome wire threaded through. The wire gets hot, heats the pipe which then radiates the heat directly in front of the optics.  This means that after leaving it for a few mins, the heat from the heater is replacing the heat that is lost from the optics.  As the dew shields are in place, I do not need to have the heaters on at full power.  In fact I normally only need about 1/4 of their power on a bad night, and less on a good night.    The extra power can be used if the scope does start to dew up, to clear the dew!   Yep, my heaters are that powerful    but that shouldn't be needed.

Finally, just in case I mess up and my scope does get dewed up, I have a 12v hair dryer that I can use to manually clear the dew from the scope.   This is a very power hungry way to do it, so better avoided. But it can save me from a night going terribly wrong because I messed up and let my scope fog up.

[iwols]

thanks for that quick processed image added below and yes i know about the dust bunny cheers guys

 

[ollypenrice]

The graininess, I suspect, comes from lack of signal. You're nicely in focus but how many and how long were the subs?

Olly

[iwols]

thanks olly 5 x 2 min subs per channel

[ollypenrice]

27 minutes ago, iwols said:

thanks olly 5 x 2 min subs per channel

Not enough. There's your problem. Everything else is right. I can't comment on your skyglow because that is site depedent, but 20 mins per colour will be full of noise from anywhere in the world. My own version of this had over 2 hours per colour in 10 minute subs from a very dark site. This is good news, though: all you need is more. (This is a very well known song in astrophotograhy!!!)

Olly