Mon, 02/13/2023 - 21:23
Has anyone worked out a solution for exchanging between a camera and a sectrograph without having to rebalance the telescope? I have a Planewave CDK20 on an L-500 mount so balance is very important to correct tracking. I would like to easily change between a spectrograph and a camera without having to rebalance the telescope. As far as the spectrograph, I don't expect to find one with an R = 15,000, but a good one as possible given the limitations of small telescopes?
I don't see how you can avoid the need to rebalance your scope when you make such a change. I keep my StarEx spectroscope mounted all the time, and so it's not an issue for me.
You said you don't expect to find one with an R=15,000. I achieve close to R=20,000 with my spectroscope routinely. It's a homemade StarEx with a 2400 L/mm grating and a 23 um slit. Cost was less than $1,000 including the optics, and I used my 3D printer to make the device. To go with a Planewave, you might want to look into the commercially manufactured "LHires III" by Shelyak, Inc in France. Rick
I have a friend with a CDK-20 on the L-500 mount: a wonderful setup, but it is quite a bit more sensitive to balance than a gear-driven mount. So, any instrument change will almost certainly be accompanied by re-balancing.
One thing that you might consider: Include some moveable counterweights on the upper or lower dovetail. I do that on my Paramount and it seems to work nicely: slide the counterweight to the position that balances whatever instrument is on the ‘scope. Much easier than moving the ‘scope to re-balance.
"One thing that you might consider: Include some moveable counterweights on the upper or lower dovetail. I do that on my Paramount and it seems to work nicely: slide the counterweight to the position that balances whatever instrument is on the ‘scope. Much easier than moving the ‘scope to re-balance."
Yes that is what I do too, a sliding weight on the lower dovetail (which currently is also carrying a camera plus lens) balancing a range of different spectrographs and camera setups on my C11 Eq6 setup
I suggest you determine which is the higher priority activity and use either a sturdy flip mirror, or if the instrument or camera is too heavy a multi port instrument selector such as https://www.baader-planetarium.com/en/instrument-multi-port-imp85.html
The latter are expensive and need more back focus.
I use a flip mirror. Spectroscopy is my main activity, so the spectrograph gets the straight through path while my DSLR gets the reflected port. I have it servo actuated for remote use.
What's your pw cdk focal ratio? That should not exceed the focal ratio of your spectrographs collimator. A shelyak uvex with motorization would give you medium resolution, R depends on gratings selection, and has fully reflective optics for quickly changing wavelength band of interest with no refocusing. But the Czerny turner optical arrangement does sometimes take some effort to get fully adjusted. Peter velez I. Australia is very successful with his on a pw cdk12, and fully automated in terms of set target and go to bed, which is a real accomplishment in spectroscopy.
If higher resolution than the uvex is desired, your 20 inch aperture could support an echelle spectrograph but they are quite expensive, north of 12000euro I think. Even the uvex is circa 4000, haven't checked recently though.
In general, for spectroscopy, what is your objective in terms of both resolution and object magnitude? The aperture defines the number of photons for a given magnitude. Higher resolution means dispersing the finite photon flux over more spectrograph camera pixels. Eventually you run into shot noise limit, or skyglow limit, depending on site. With a good camera, read noise or dark current shot noise may be the last limiting factors.
R~15000 has been commonplace for amateurs since the introduction of the LHIRES III in 2006 but there is a direct trade off between resolution and sensitivity and there is much work to be done at lower resolutions.
Spectrographs scale (in size and weight and cost) with telescope aperture and most commercial spectrographs for the amateur are designed for more modest apertures of 8-12 inch compared with your 20 inch so unless your seeing is exceptional (so the star will fit in the slit) they will underperform on a 20 inch, either compromising throughput or resolution. Matching the focal ratio is also important for efficiency and the f6.8 from your scope would be too fast for a LHIRES for example.
One suggestion which could fit your bill would be a fibre fed echelle eg the Shelyak eShel which would give you high R~10000 resolution across a wide range with only the weight of the guide head to carry on the telescope