Proposal #722

Comments

Dirk Terrell — Aug. 24, 2026, 4:05 p.m.

A new system dedicated to time series observations like these will be coming online about the time of the requested observations, DBO40 (Desert Blooms Observatory 16-inch). It would probably be a better use of resources than MPO61. It will have BVRI so would I (Cousins) be ok or is Sloan i required?

Marvin Kohls — Aug. 27, 2026, 7:12 a.m.

Thanks for the feedback! DBO40 would also be okay.

Marvin Kohls — Aug. 27, 2026, 7:12 a.m.

Thanks for the feedback! DBO40 would also be okay.

Dirk Terrell — Aug. 28, 2026, 4:53 p.m.

Ok. DBO40 should be fine at this brightness. Ken will handle the scheduling. I'd suggest using MPO61 if DBO40 is not yet online by the time your suggested transits arrive. Once it is online, use it for this project.

Kenneth Menzies — Aug. 28, 2026, 8:43 p.m.

Committed to mpo61. At Dec=-26, seems too low to me? Check test image magnitude coming soon?

Marvin Kohls — Sept. 2, 2026, 7:56 a.m.

Thanks Ken, and thanks for committing it.

On the declination: I checked all seven windows in the Swarthmore Transit Finder using MPO61's coordinates, and the target stays above 30°. It culminates around 32°, so we're working at roughly airmass 1.9 at best and closer to 2.0 at the window edges. Not ideal, I agree — but the transit is deep (14.88 ppt) and the star is V = 13.7, so I think it's workable if the scatter cooperates.

Yes please to the test image. Once it's available I'll measure the target and a few comparison stars and report back the SNR per 60 s frame, and we can decide from there. If the photometric precision won't support a 15 ppt depth at that airmass, I'm happy to drop MPO61 and gor for an alternative — Cousins I is fine for this, Sloan i′ is not required.

Marvin Kohls — Sept. 3, 2026, 9:55 a.m.

Test frames came through on 2 Sep — thanks Kenneth. Two 45 s SI exposures, both plate-solved cleanly (173 stars matched, 0.11" residual). Here's what I measure.

Target is correctly identified and cleanly isolated: it sits 0.57" from the catalogue position with no other detection within 19", so no dilution to worry about. Seeing was good, FWHM 1.55" at airmass 2.19.

SNR on the target is 33.6 per 45 s frame at r = 5.2 px, i.e. 3.0 % per-frame photometric precision. Scaling to the 60 s I promised: SNR ≈ 42, or 2.4 % per frame. The comparison stars are all brighter than the target — the best, at [4108, 1506], reaches SNR 221 — so differential photometry adds essentially nothing to the noise budget. The target alone sets the error.

With T14 = 59 min (b = 0.86) that gives roughly 3.5 sigma on a 14.88 ppt depth at 60 s cadence with twice the out-of-transit baseline. Marginal. But the noise budget is read-noise dominated — I measure about 10.3 e-/px read+dark against only 7.0 e-/px sky shot noise — so longer subs pay off directly. At 180 s it improves to about 4.5 sigma, and there's ample headroom: the target peaks at 158 ADU and the brightest comparison at 1857 ADU against a 60000 limit.

So my recommendation is MPO61 at 180 s rather than 60 s. That still leaves ~18 points across the transit, enough to constrain the shape.

On the declination concern: I don't think altitude is what's limiting us here. Extinction in Sloan i costs roughly 0.05 mag/airmass, so going from airmass 2.2 to zenith would buy about 6 % in flux — negligible next to the exposure-time effect. The test frames were actually taken at 27.1 degrees, slightly below our window minimum, and still gave usable photometry.

DBO40 at 16 inches collects about 43 % of what MPO61's 24 inches does, so per-frame SNR there would be roughly a third to a half lower. If DBO40 is still the preference I'd want to plan on ~300 s subs there rather than 60 s. Happy to defer to your read on it.

So I would be happy to continue with MPO61 at 180s if possible, DBO40 would probably need 300s for the same quality...

Dirk Terrell — Sept. 3, 2026, 4:40 p.m.

I don't have any objection to using MPO61 as long as the MPO people are ok with it.

Kenneth Menzies — Sept. 4, 2026, 5:50 p.m.

Marvin: My main concern is that a 180s exposure makes the cadence too slow to provide an adequate number of images to adequately resolve the short transit. It's the reason I used 45s and not 60s exposures in the test. I will do what you want since it is your data!

Marvin Kohls — Sept. 10, 2026, 6:28 a.m.

Hi Kenneth,

I will look at the data as soon as it is in. I will inform you should it be problematic, so that one could cancel the second round and switch to another scope.

Thanks for all the effort!

Comments on this proposal are closed.