|Proposer||(36547) Lauren Herrington (firstname.lastname@example.org) obscode: HLAA|
|Assigned To||(3663) Dirk Terrell|
|Date Submitted||May 24, 2022|
This proposal is designed to help determine the optimal settings for the Star Analyzer gratings on the BSM-TX and BSM-Berry telescopes, as mentioned during the AAVSOnet Management meeting earlier today.
With slitless diffraction gratings such as Star Analyzers, the best focus actually occurs when the sensor is moved slightly closer to the telescope than the "best focus" position for a star. The grating itself also introduces a focus offset to both star and spectrum, similar to photometric filters.
The spectrum is astigmatic, so the spectral strip will cover several pixels vertically when it is at its sharpest (this often confuses people, because the vertical elongation makes it "look" like it's out of focus).
The chosen target star (delta Oph) is an early M type, and should therefore show noticeable spectral features no matter the spectral resolution (at R<100 moderately shallow TiO bands should be seen, and at R >100 strong metallic line blends such as the 5170A Mg triplet can aid with fine focusing). It is also bright, and well-placed for observations from both hemispheres around local midnight this time of year.
Sexagesimal coordinates: 16 14 20.74, -03 41 39.6
|Target||RA (H.HH)||Dec (D.DD)||Magnitude||Telescope||Observation Frequency||Expiration Date||Proprietary Term|
|NSV 7556||16.239094||-3.69433||2.72–2.75||BSM_Berry||—||Nov. 24, 2022||No|
Committed to TX and Berry. Several + and - defocus values added. Check exposure and defocus.
I suspect an A type star with some strong sharp lines would be easier to check for focus? Time will tell.
Thank you! It's true that class A stars are a popular recommendation for focusing with a Star Analyzer; however, I personally found early M-type stars to be more useful when focusing my own Star Analyzer. The issue is that the strong hydrogen Balmer lines in main sequence A stars are naturally very broad, so their sensitivity to focus is reduced. (Before I learned to use M stars, I often fell back to the relatively sharp calcium K line, but that meant optimizing for the UV instead of the visible.) In supergiant A stars, the hydrogen lines are usefully thin and sharp, but they're also weaker, so they might not show up well in a low resolution Star Analyzer spectrum. M stars have strong features at many scales (systems of TiO bands, individual TiO bands, and atomic lines), so no matter the resolution, they have something just on the edge of visibility--I find that to make focusing a bit easier.
I learned something new! I'm getting copies, so I'll try too.
I just checked out the first batch of results from BSM-Berry. Thanks very much!
The best offset was had in the 18th frame "NSV_7556_bsm_berry_GR_220606_155137_hlaa.fts", which has JD = 2459737.1608520835 in the header. Although the 18th frame was the closest, I suspect that the best offset is not represented in the dataset; extrapolating from the 16th ("NSV_7556_bsm_berry_GR_220606_153106_hlaa.fts") and 18th frames, the ideal offset probably occurs somewhere around 1.8 * (18th_frame_offset - 16th_frame_offset). I don't see anything in the header which records the value of the offset, so unfortunately I can't give exact numbers.
Other potential optimizations I noted:
- The header indicates that the image was automatically flatted. This should be disabled for grating spectra, if possible, since flats introduce distortions into spectra taken with slitless gratings. (The spectrum is produced so close to the camera that it itself is unaffected by vignetting, and uneven backgrounds are handled separately as part of standard spectroscopic data reduction procedure.)
- If possible, the grating should be rotated (48.5 degrees clockwise) within its spot in the filter wheel, so that the brightest spectrum is pointing due west (+-2 degrees). That will make it possible to extract the spectral data without rotating the image in software, a procedure which usually introduces artifacts.
- An exposure of 3 seconds is technically too long for a star this bright; 0.8s or less would probably be more appropriate. However, this is not a major issue because with this configuration, the other, fainter spectral orders are always within view, and can be processed in lieu of the primary spectrum (which is what I did to evaluate the focus). In this case I'd say it's better to be overexposed than underexposed.
I changed the exposure from 30s to 8s. Check it on next set. We need a table of mag vs exposure for grating.
Was image 17 between 16 and 18 in terms of focus? I'll send the plan with order.
Sorry for dropping off previously; when summer classes started up, I became overwhelmed, and wasn't able to make time to examine the images before they were deleted from the cloud. If you could please schedule another set of images, I'd like to take another crack at determining focus offsets. (I have better control of my schedule now, so I don't anticipate running into the same lack of time.)
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