Is the dew heater causing blurring?

[Moonshed]

This may seem a strange question and I would not be surprised if it’s a really dumb one, but I’m prepared to risk it.

Although I have been peering through telescopes since a boy, I’m now retired, I am new to imaging planets and new to using a ZWO ASI 224MC, having only started this month, so it’s been a steep learning curve.

I have always used a dew heater as it’s pretty much essential when using an SCT, and for observing purposes it’s proved great, it’s also been great when imaging galaxies and nebulae with my Canon. 

The focussing is good having using the Bhatinov mask on a star just before and the tracking and PA very good as well, Mars stays in the centre of the screen for minutes on end.

Now here comes the ‘but’. I find that when looking at Mars on the FireCapture screen, although staying in position, it is always squishing in and out, and I am wondering if the seeing conditions are causing this or perhaps the dew heater, or a combination of the two? I can’t help wondering if the gentle heating around the top of the scope builds up over a couple of hours and causes large enough air currents to create distortion. 

Any thoughts on this? 

Edited November 29, 2020 by Moonshed

[Freddie]

The biggest impact will come from the general seeing conditions but a dew heater definitely contributes. The heater should therefore be used with a dew shield (also useful for keeping stray light out) and the heater set to the absolute minimum that will just keeps any dew off.

[Moonshed]

11 minutes ago, Freddie said:

The biggest impact will come from the general seeing conditions but a dew heater definitely contributes. The heater should therefore be used with a dew shield (also useful for keeping stray light out) and the heater set to the absolute minimum that will just keeps any dew off.

Hi Freddie, thanks for that advice. I should have said I always fit a dew shield  because as you say it reduces stray light as well as helping with keeping dew to a minimum. Regarding the settings of the dew heater I do not have any, I just run it off a Meade 12v power supply. Is it worthwhile to invest in the type you use and which one do you have and recommend?

EDIT. I should add that if I place my hand at the top of the scope after an hour or two I can feel it is definitely warmer than at the bottom. Is this how it should be or is it too warm do you think?

Keith

Edited November 29, 2020 by Moonshed

[Moonshed]

I have checked Telescope House and they have a Revelation Pro dew heater for £21 for my 8”SCT , I figure no harm in getting a new one, and a Revelation Pro dual channel dew heater for £46.

The prices look very reasonable and I have long traded with Telescope House, but have other members any experience with this particular make? Or are others better although dearer and are to be preferred? I always consider the old adage buy cheap buy twice.

[Mr Spock]

I never use a dew heater for just that reason. Damien Peach confirmed the same in a lecture he gave a year or so ago.

If you must use heat to get rid of dew, then a hair dryer is the best way. It only takes a few seconds for the scope to cool again.

[Moonshed]

4 minutes ago, Mr Spock said:

I never use a dew heater for just that reason. Damien Peach confirmed the same in a lecture he gave a year or so ago.

If you must use heat to get rid of dew, then a hair dryer is the best way. It only takes a few seconds for the scope to cool again.

It does make sense not to use a dew heater because I can’t help thinking it simply has to cause air currents as it’s applying heat.
I tried using my trusty hair dryer, as I have done so many times over the years while observing, but after around 20 seconds of using it I returned to the screen and was amazed to see Mars jumping about so much it was actually forming stretched blurred cylinders of itself as it danced around. I was amazed at how much disturbance the dryer had created, that was my imaging over for 30 minutes before it once again settled down to its usual  gentle bobbing about. It was this extreme heating that got me thinking about the dew heater.

[Moonshed]

After giving the matter some more thought I have decided to hold off purchasing a new dew heater system, instead  I’m going to do a test run without switching my dew heater on, something I have never done in the 28 years I have had the Celestron. I will take my scope  out of the shed and place it on the concrete base and just leave it there for an hour or so to cool down to ambient temperature, with dew shield on and tilted down a little.

When ready I will fire up the laptop, do the PA, focus with the Bhatinov, no need to bother with star alignment, and straight away run some recordings of Mars. From start to finish, ignoring the standing time, the whole thing should only take a max of 10 min before I start recording so no chance of any misting forming on the corrector plate. I will probably take 4 or 5 recordings of 1 or 2 minutes and see how it goes.

I am quite hopeful that I will get a better result than I have been getting with my on/off dew heater. 
I will let you know how it goes.

Keith