A Cost Effective way to increase SA Dispersion

[sheeny (Al)]

After doing some initial trials on my C11, I realised that I would not improve dispersion or even resolution over my old C8.

Then I hit on a cost effective idea to push the SA up the baffle tube of the C11. ATM Andrews are selling GSO 25mm extension tubes for $9. For $45 I can push the SA 125mm further inside the C11 than before which should allow me more dispersion without running out of focus travel.

The GSO 0.5 focal reducer is another experiment to combine with it to reduce the star image size and improve resolution (provided the optics are up to it).

I had an adapter turned up at the local machine shop, with a 1 1/8" x 42TPI thread for the extension tubes and a 48 x 0.6mm thread for the 1.25 to 2" adapter, but these extension tubes will work straight through a 1.25" EP holder provided the bore of the EP holder is clear. BTW if you are thinking of doing this, check the filter thread on the 1.25 to 2" adapter... my research says it should be 48mm x 0.75mm pitch, but mine was 48 x 0.6! ...and take the adapters to the machine shop so they can make sure they fit.

I doubt I'll get a chance to give this it's first light before next year... but I'm looking forward to seeing what I can achieve with this. I'd like to get close to 20A resolution at 4" seeing .

This should also be suitable for refractors, but would result in an obstruction in a newt.

Al.

[Merlin66 (Ken)]

Al,
A couple of guys over here have used old barlow tubes as extension tubes for the SA100...
We haven't found any advantage in going beyond 80mm distance from the grating to the CCD chip. Remember the deviation angle is about 3.2 degrees so unless you add a Grism ( recommended!) you'll have off axis issues.
I've just been given a Baader Afocal camera adaptor which clamps on the focuser and allows you position the camera behind an eyepiece. I'm going to put the SA in a 25mm extension tube and use the DSLR positioned at the 3 degrees from the optical axis see what happens...

Only problem in the UK is that spectroscopes still dont penetrate clouds!!

[sheeny (Al)]

Yeah, I am a fan of using the barlow bodies too. I used to use the body of my short 2x barlow to get about 6.4A/pixel on the C8, and the long 3x Orion barlow body gave me 3.4A/pixel with the ED80 but we was still quite manageable IMO...

I tried the filter wheel as well, but the loss of focus distance meant I couldn't achieve the dispersion/resolution that I wanted.

I'm interested to see how it goes anyway. Being in 25mm segments it's reasonably adjustable! I guess I'll see if the offset becomes a problem...

Al.

[theodog (Jeff)]

Great work Al.
I haven't even looked into the obs for weeks now. Too busy with work, but thats now come to an end for a few weeks so I may get back into it after the festivities.

[sheeny (Al)]

Thanks Jeff.

All I need is to be able to line up my spare time with gaps in the family program, Christmas program, holidays and good weather...

Al.

[sheeny (Al)]

Last night I got an opportunity to get outside and sort some photons.

Attached is the first spectrum from this rig in my C11. The arrangement was exactly as shown in the photo in the first post, and the dispersion achieved was 4.78A/pixel.

Compared with previous spectra of Sirius that I'd collected with the C8, there is a significant improvement in resolution. There was a visible artifact in the original AVI, however, which was still there in subsequent sequences of stills (yet to be processed). It manifests itself as a faint trumpet shaped halo broadening toward the blue end of the spectrum, but I'm pretty sure it's not focus. I think it might be an artifact from the focal reducer, so I'll do some experiments to see what I can do about it... but the end result suggests it is not a big problem.

Measured Planck Temperature for this spectrum is 12000K. I must check what the official temp is...

Al.