eShel III: An Improved Version of eShel II
By Olivier Garde
The eShel II spectroscope features an 85-mm focal-length camera lens that is no longer manufactured; an alternative had to be found, so Shelyak Instruments designed its own 122-mm focal-length Petzval-type lens. This “custom” design has made it possible to optimize the overall quality of the spectra produced:
– More consistent focus across the entire spectrum
– More orders available in the near-UV and, especially, the near-IR
The successor to the eShel II: the eShel III is now available with this new lens, either with manual focus or with motorized focus that can be controlled remotely.
The lenses, available in both manual and motorized versions, are available for upgrading your eShel II. The motorized version includes a PrimaLuce Lab focuser. Changing the lens is easy to do, but it absolutely requires the use of a sensor larger than that of the Atik 460ex camera, which is typically used with this instrument. Therefore, we recommend using a SONY IMX 571 sensor, specifically the ASI 2600 MM Pro, which is directly compatible with the eShel III.
Superior Performance
The first advantage that becomes apparent when comparing a raw spectrum from the eShel II with that of the eShel III is a spectrum that is much better focused at the extreme orders of the spectrum and, in general, a more uniform spectrum from one order to the next and thus a more uniform resolution as well.
Next, we see that we can process many more orders with this new approach:
• With the eShel II, it was possible to cover the range from order 31 to 58, inclusive. The raw spectrum shown here illustrates the spectral range obtained using an ATIK 460ex camera and an 85mm lens.
• With the eShel III, the range of orders that can be processed using a thorium/argon lamp is from 25 to 60, inclusive. The raw image shown here was captured using an ASI 2600 MM Pro and a Petzval lens with a focal length of 122 mm.
On the right are details on the extreme orders in the near-UV and near-IR for the eShel II and eShel III versions.
Example of a result on Vega
Here is the spectrum of Vega, obtained using the eShel III on a 406-mm-diameter Ritchey-Chrétien telescope.
9 exposures of 120 seconds each taken with an ASI 2600 MM Pro (gain 101, cooled to -10°C) using 2×2 binning, processed with ISIS software by modifying the line.lst file to include lines in the near-UV and near-IR. (The original file did not go beyond 4,000 Å and 7,500 Å.)
The spectral range is from approximately 3730 Å to 9100 Å (orders 60 through 25, inclusive)
The high resolution of certain spectra for the Balmer series up to H13 and the Paschen series in the near-infrared allows us to observe the last lines in these series (Pa 13, among others).
The spectrum of the star Be 48 Lib
The spectrum was obtained using the same configuration as for Vega on this Be star of magnitude V=4.87, with 6 exposures of 600 seconds each.
On the right, the spliced spectrum of orders #25 through #60; below are the graphs for orders #60 and #34 (H Alpha).
Conclusion
The eShel III version significantly improves upon the eShel spectroscope by producing more consistent spectra from one order to the next and expanding the spectral range covered.
The manual version uses the same focusing mechanism as the eShel II (by turning the focus ring); it is more precise than the one on the original 85mm lens, and you can also lock the ring in place once you’ve found the right setting (which wasn’t possible with the previous model).
The motorized version will be ideal for use in remote-controlled setups to fine-tune the focus of the eShel III. The focuser can be controlled using, among other things, an ASCOM or INDI driver provided with the focuser.
For those who already own an eShel II and would like to upgrade their spectrograph to this new version, they can purchase either of the two upgrade options (manual or motorized). The upgrade instructions explain how to remove the old lens and easily install the new one.
