This past year, we have made some upgrades to the Illig system - primarily the Nitecrawler rotator+focuser and mount maintenance. To further capitalize on these foundational upgrades would be to next take a look at the imaging system to increase the flexibility of the system and to take advantage of recent CMOS sensor developments.
An upgraded imaging system should allow for flexibility in both the individual imaging project and public outreach realms. No single system can accommodate both with perfection, but I think it is possible to get close and allow the Illig system to cover a fair range of interests and object types.
To recap the current Illig configuration, it is a Takahashi TOA-150 triplet refractor, with a native focal length of 1050mm at f/7. It is fitted with a Takahashi 67 1.0x field flattener. This combination creates a corrected field that is circa 92mm in diameter, 106.2mm in back of the flattener. This means we have a 106.2mm gap to fill with equipment between the flattener and where the camera sensor would be.
With that in mind, I propose the following. This list is available, with pricing, at this Google Sheet.
- Replace the existing ZWO ASI071MC-Pro camera with a ZWO ASI2600MC-Pro
- The sensor in each camera is the same APS-C format, 32mm on the diagonal
- The 2600MC's sensor is a color Sony IMX571. 16 bit per pixel output, 26.8 megapixels. 3.76um pixel size
- Importantly, the IMX571 sensor is a back-illuminated architecture which allows for Quantum Efficiencies into the high 80% in red.
- The dark current of the IMX571 is extremely faint, with no notable gains in effect when cooling below -10C
- Unlike previous CMOS generations, there is no amp glow effect
- Read noise is 1.0 to 1.2 e-
- Add a ZWO 7x2" filter wheel
- Typically for color cameras, one uses a filter cartridge system to manually swap out various filters. We don't want to bother with that, so we will use a filter wheel.
- The 7 slots in this filter wheel will allow us to stock a variety of 2" filters that are useful on color astro cameras
- The 7 slots will also allow us to keep the filter wheel and reconfigure it with LRGB and narrowband filters should we decide to switch to a full frame or other monochrome camera in the future.
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Add a ZWO Off-Axis Guider (OAG)
- Obviates an external guide scope
- Appropriate at 1m focal length
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Add a ZWO ASI174MM-mini guide camera
- Would be fitted to the OAG
- Large chip size (for a CMOS guide camera) for a larger guiding FOV
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Add Astro-Physics Command Center (APCC) Pro
- Mount management software that does point and tracking modeling for unguided imaging
- With NINA and using a plugin I wrote, one can create dec-arc models for high accuracy tracking directly from with the sequencer on a per-target basis
- Can replace OR complement the OAG-based guide system, based on user's preference and familiarity
Since we are continuing to use a color "OSC" astro camera, the filters we would stock in the filter wheel should be useful and compelling for both imaging and public outreach. Unlike the ASI071MC, the ASI2600MC camera comes equipped with a built-in UV/IR cut filter. While it's practically required to have a UV/IR cut filter in front of a color astro camera, it's not common for it to be fitted directly to the camera. I think that ZWO did this to help users simplify their setups and to remove the need for extra filter cartridges and the like. But, in my envisioned use case, it's an impediment as it cuts out frequencies longer than 680nm. I propose that we replace the stock UV/IR filter on the ASI2600MC with a clear anti-reflection coated glass. This item is available from TS-Optic. The stock UV/IR cut filter would be retained in storage. The reason for this modification is given below:
Proposed filter complement:
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Optolong 2" UV/IR cut filter. We already have one of these - I added it to the existing ASI071MC this past summer to get rid of the star bloat as the ASI071MC has an AR window, not a UV/IR cut window. This filter would be reused in the proposed 7x2" filter wheel.
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Optolong 2" L-eXtreme dual-band narrowband filter. This filter passes both Ha and OIII frequencies, each at a 7nm bandwidth. This will allow for extraordinary imaging of H-alpha and OIII emission nebulas and other objects.
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Optolong 2" IR685 IR-pass filter. This might be perhaps the most controversial of the proposed filters, and is the reason why I also proposed replacing the UV/IR front window of the 2600MC camera with a clear AR window. The IR685 filter passes wavelengths longer than 685nm, which is the beginning of the Near Infra-Red chunk of spectrum. This will allow us to do great moon and Jupiter imaging as these longer NIR wavelengths are not as affected by seeing as higher frequencies of light are. The CFA (color filter array) of the sensor is essentially transparent at these frequencies. To get an idea of the improvement in lunar imaging there is by using this technique, you can reference the Baader comparison for their equivalent filter. The improvement in image quality is remarkable as you progress from the higher blue frequencies into the infra red.
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Baader Solar Continuum filter - David and Leona provided a white light solar filter for the front of the TOA-150. To further the quality of solar white light viewing, we ought to use a Solar Continuum filter. The one from Baader is popular for this purpose.
Farther outside the realm of "typical" are other filter options. The stellar spectroscopy grating filter and software from R-Spec comes to mind, which would apply to public outreach and citizen science activities.
- Custom Tak 67 to ZWO M68 spacer+adatper from Precise Parts. The camera, filter wheel, and OAG consume 68mm of the 106.2mm total backfocus of the system. This means we must have a custom adapter that does two roles:
- Adapts the M92 rear of the Tak 67 flattener to the ZWO M68 tilt plate fitted to the front of the ZWO OAG-L
- Fills the 43.2mm of unused bakfocus distance to complete the 106.2mm of space between rear of Tak 67 flattener and the camera sensor.
- This is potentially a long lead-time item. How fast we get it from Precise Parts depends on how slammed they are with orders. I might be able to get an equivalent adapter from an alternate source.
