Short Tube Refractor and Long Exposures?

[zimzimma]

Hi all,

I have a question regarding taking long exposures on a short tube refractor using a cannon 10d.

The other night I thought I would take a photograph of the Pleiades but rather than use my main scope i thought I would take a wide field shot using my Opticstar short tube refractor. I took numerous images but I couldnt seem to get back focus, I gave up for the evening and tried again another night.

Last night I added an extension tube onto the back of the refractor and sure enough I could go in and out of focus. My first target was Mirfak which I thought I would get in focus and then slew to the Pleiades later. However I had no Joy, no matter what I did I seem to get a blue halo around the stars even though the in the viewfinder of the cannon it seemed to be correct. I adjusted the focus in very slight stages and still no joy. Capella was up at this point and being brighter I thought I would try this. Again, still no joy. I'm really confused by this as I can get sharp images of stars via webcam but not DSLR. I took a 4min exposure of the Pleiades and its just an over-exposed image, seemingly just flat white. I have attatched a couple of the images to show what I mean. This all works perfectly on the 200pds but its the first time i have tried to image with the cannon on the refractor, not sure whether I can, whether i'm doing something wrong or whether i'm missing something. I know I cant take an image with the 'L' adapter attached so my camera was connected via a t adapter to 1.25" converter, which was then attached to an extender for focus. Hope someone can help, thanks guys.

Rich T

[GlassWalker]

On the halos, that's a feature of refractors. Light refracts different amounts depending on its wavelength, and designers will have to get as much in focus at the same time as possible. Better refractors correct better, and you're less likely to get those effects. Pure reflectors don't have that problem, and will focus light to a point whatever the wavelength. For cheap refractors, the only workaround I have would suggest is to only do narrowband imaging with them.

For your over-exposed image, erm... expose less! From the Pleiades shot, there's obviously a lot of light pollution being picked up there so you might try a light pollution filter.

[zimzimma]

Thankyou,

I thought it may have been something to do with the assembly of the reflector, just wasnt taking the same shot as I could see through viewfinder. Ive spoke to the guy at Opticstar and he reckons the ED Apochromatic wouldnt be as much of a problem, I just have the standard version but being 90mm f/5 it does pick up light quite well, just doesnt seem to deal with it well. It does however say on the website it can be used for Astrophotography which im a little confused on.

Is there an eyepiece which acts totally the opposite of a barlow that I could put in my main scope that would give me a wider FOV? I really would like to take shots of a wider view and not entirely sold on the afocal side of things.

I did reduce the exposure on the shots and did see some form of clouds in the constellation its just a shame the stars are round really, kind of wrecks the shot. Would I have the same problem with objects that are not so bright?

Thankyou for your help.

RichT

[GlassWalker]

It's a relatively low cost scope (I have the same scope different branded) and you get what you pay for. Similar size scopes can go into the thousands, which would of course offer the better image. To deal with the colour effects causing the halos, basically you can find out what colours they are, and filter them out before the camera. Going further with that, you can do narrowband imaging which looks at specific wavelengths.

The opposite of a barlow is a focal reducer. Just be aware they reduce the focal point also, so you may run into lack of focus range. Also they shrink the scope's image circle, and what's left may not completely cover a SLR sized sensor. I haven't tried on this scope...

[Capricorn]

You have a short achromatic refractor, what you are getting is simply the limitations of a short achromatic lens.

The blue surround is simply the classic Chromatic Aberration, adjust focus and you will focus the blue in more but the other end then comes out and the surround is yellow/red.

The circular stars will be a combination of CA - simply a spread of focal lengths for the different wavelengths - and I would guess some Spherical Aberration - this is where not even the same wavelengths come to the same focus caused by the diameter of the front objective.

Concerning the statement by Opticstar that it is suitable for astrophotography, well it is, you have the images to prove it. There are limitiations to the image produced, but that is a consequence of the scope.

You have the 90mm Opticstar, cost is presently £189, I have an 80mm WO GT-81 cost was close to £700, and I got charged for postage.

If you were expecting the Opticstar to perform the same then it ain't going to happen.

Before you get an ED be aware that they also show some CA, not a lot but it is there.

[sologuitarist61]

Isn't the Baader APO filter meant to cure achromat refractors of this problem of colour fringing?

[zimzimma]

Thanks peeps, and those filters seem to have rave reviews. Certainly going to give it some thought as it certainly seems to make images much more natural looking unlike some others. Thankyou.

RichT