Filter to reduce turbulences?

[OzStarGazer]

Is there a filter to reduce turbulences? They are pretty strong at high magnifications... :(

Thanks! :)

[hobbit]

You mean something like adaptive optics

[OzStarGazer]

Yes. For a small refractor.

[Wavytone]

We'd all love such a thing, but no, doesn't exist.

The nearest is adaptive optics, but that's not for small refractors.

The simpler solution is to find an observing site with excellent local conditions ( comparative site survey with some quantified measure of the seeing) and be lucky enough to find a night with the right weather conditions.

[julianh72]

I think your best bet is to try video astro-photography using a modified webcam or a dedicated telescope eyepiece camera, and then stack the frames to filter out the best possible image.

[OzStarGazer]

Thanks, I tried that, but I don't have tracking, so with the image moving so fast across the screen + the turbulences it is difficult to stack the frames... There have been a couple of good days too though. I don't know if it is a coincidence (I assume it is), but during the day it always seems to be much better with the moon.

[OzStarGazer]

I might get some tracking soon.
I have fallen in love with the NexStar 8", but it is so huge for a balcony... It is relatively cheap overseas - well, at least for its specs.

[glend]

The summer brings ths turbulence more often than winter. The ground heats up during the day and emits heat back into the atmosphere at night - these rising heat currents can produce they wobbly probems your hqving and no filter is going to eliminate it. Your best summer strategy is to start very late after the ground has lost its stored heat - like after mid-night. That will mitigate it somewhat. Avoid aiming your scope across heat sources, like house roofs, etc.

[OzStarGazer]

Thanks. The moon was actually either in the middle of the night or about an hour before dawn, but pretty low on the horizon (that's when it is in front of my balcony). Full moon was much earlier though. And it was warm... Unfortunately there are also all sorts of buildings around. And trees too, which might also cause some air movements. :( Not the best place for watching, I know... I hope this will change soon. I hope winter will be better. The weather has been really crazy here though since I bought my camera. Very hot (>30 degrees), then strong winds, then suddenly rain and thunderstorms, then hot again,... REALLY crazy. When it arrived the weather was still normal. I had my first session and it was great. But then.... New equipment curse I guess.

[JB80]

I have seen many people using a red filter on Lunar images recently and when I asked what that was all about cutting through the seeing was mentioned.
I haven't had the chance to try it myself yet and I am sceptical that I will notice any difference at all but I will give it a shot though.
It still wont stop the turbulence so I don't think it is a solution as such.

The other thing is I guess it is better to have a red filter that already has IR cut to it.

[cometcatcher]

Red will help with haze, but not turbulence. In such conditions smaller apertures are better. But if you're already using a small refractor, I don't think there's much you can do, other than stop it down.

[JB80]

Apparently the longer wavelengths are better at handling the turbulence as well, at least according to some.
http://www.visit-the-moon.com/techniques
It seems entirely plausible and from the article you may be better off with something a bit longer.
I'd still wager the effects are going to be minimal.

[OzStarGazer]

Thanks all for the replies.

Yes, I found that article too yesterday evening and intended to read it thoroughly this morning.

Anyway, what I do at the moment is take many pics in a sequence (this is possible in iCap) and then select the best pics for post-processing... It is amazing how the pics can change in quality from one sec to the next.

I have a dark orange filter, which is obviously not the same as red, but could buy a red one too and give it a go. I can use it for something else if it doesn't work. A colour filter is not very expensive anyway.

[Huey]

You could also change your observing time since early mornings are often less turbulent, but this may require careful planning and some objects may not be visible from your site.

[OzStarGazer]

I tried early morning, but there were still turbulences. But I had the Neximage for less than a month. Maybe it was not the right morning.

[ZeroID]

Video is the only real answer. I do 10 second bursts at 30+ frames per second then stack them. 3-400 frames has a chance to resolve some detail. The ASI 120 will do about 60 fps so even better.

[OzStarGazer]

OK, I will try a really short video at the maximum resolution (52 fps) as soon as I have the chance. If the video is really short maybe RegiStax will still be able to do something with it even if I don't have tracking as the movement won't be that bad. Even 5 secs would be 260 frames, which is not bad.
EDIT: I do have a short video of Tycho which is only 6 secs. Maybe I can experiment a bit with it, but I was not using the max resolution, so it was not 52 fps. I will experiment a bit with it tomorrow anyway.

[OzStarGazer]

It seems that a red filter can indeed help. Some say deep orange helps too because red is too dark.

http://www.cloudynights.com/topic/474778-fighting-turbulence-with-red-filter/

I guess if used with the NexImage and iCap I could play around with the settings, so red (as also recommended in the article we discussed yesterday) would also be fine and not too dark.

The OP in the thread above even says that the view was improved "significantly" with a red filter.

[Shiraz]

the effects of turbulence reduce roughly linearly as the wavelength increases, so you get a really significant improvement using a deep red filter (eg a 25a) with a mono CCD - the moon is a really good target for this since it is so bright and the filter throws away a lot of light.

You can use a smaller aperture so that the scope limits resolution rather than the atmosphere. That does not get you more resolution, but means that fluctuations in seeing are not as noticeable as with a big scope - some prefer the consistent lower resolution of a small scope to the occasional glimpses of better resolution with a larger scope.

[OzStarGazer]

Thank. Yes, I also have a really small scope and the turbulences are not so evident (the image is just a bit fuzzy, but not deformed). It is really small, so small that the whole moon fits on the screen... (It is just a small scope for travelling.)
I would have also liked to experiment with my normal refractor and a focal reducer (although the aperture would be the same), but the focal reducer unfortunately doesn't work with my scope and the Neximage (I think because the NexImage adapter/connector is a bit too long). There might be a focal reducer that works with my scope out there, but I don't know exactly where...

[Shiraz]

I think that your neximage is a colour camera, which limits what you can do with filters. If you can possibly remove the built in IR filter, you could probably get good results using a 25a red filter, since the Bayer filters that produce the color all leak light at wavelengths just beyond visible and you will get an image in all colour channels.

[OzStarGazer]

Yes, it is a colour camera, although you can also take videos in black and white in iCap (but it's obviously not the same).
Unfortunately I don't know how to remove the IR fiilter. I have never opened or modified a camera and don't want to ruin it... But filters are pretty cheap so I can give the 23A a go. Hopefully it will improve it at least a bit.
Thanks for the advice!

[OzStarGazer]

Finally!!!! Last night I could not sleep. At some stage I got up and went past the balcony. The moon was up there, so I gave it a go. Finally the turbulences were not so bad. It was actually the first time I could get nice images with the NexImage even with no binning, and this was the closest I have ever been to the moon. The craters were huge. With 2x binning the images were even better (most of them don't even need post-work).
After that I even had a couple of hours of sleep in the morning!
At any rate with no binning the magnification is huge. I think it works as a 5-7mm eyepiece with 2x binning, but without binning it is probably like a 3mm eyepiece???

[OzStarGazer]

A couple of days ago ("first quarter" moon) I wanted to give it a go, but when I opened the door of the balcony it was like feeling a hair drier on my face... It was a hot day, very bright and windy too, and the turbulences were enormous. I decided to test the red filter with the NexImage... It improved the turbulences just a little bit, but at the bottom of the images there is a dark stripe. Transparent, but darker than the rest of the image? Is it normal when using a colour filter with the NexImage? Thanks. :)