Best targets for 115mm APO

American Association of Variable Star Observers (AAVSO)
Fri, 07/31/2020 - 06:37

Please excuse the rather *newb* question but I am having some difficulty understanding which types of variables are best suited for my imaging equipment. I've recently purchased Chroma B and V filters to get started in photometry. I would like to contribute to exoplanet transits as well but is a larger aperture required for better SNR of transits? Are cataclysmic-variables suitable for this size of scope?

My observatory setup:

  • Atlas Pro EQ6 mount
  • Altair Astro Wave Series 115mm APO-triplet
  • ZWO ASI1600mm-cool
  • f/5.5 and 636 FL
  • FOV 1.59° x 1.21°
  • 1.23" /pixel
  • Bortle 5 zone
Variable Stars South (VSS)
From my experience, the best

From my experience, the best targets for your combination of telescope and camera will be relatively bright variable stars. I have been using the same make and model camera with V and B (mostly V) filters now for over a year, with an 80mm f/7.5 refractor and a 120mm f/7.5 refractor. My refractors have ED doublet objectives, but that is not important.

"Relatively bright" in the above means down to about 11th mag in V, or maybe a bit fainter if you average multiple exposures. I recommend that you browse the topic "CMOS cameras for photometry" in the Photometry forum. I contributed to that, and you will find that some of my posts there have detailed descriptions of tests done on the camera with stars of various magnitudes. The limiting magnitude you can usefully study will also depend on what data you plan to collect. If you plan to do time series photometry on variables with short periods, you should definitely stay with brighter stars for better precision. If you plan to study longer period variables, you can average multiple exposures, and thus achieve good precision for fainter stars.

I recommend also that you experiment with defocussed images, rather than images that are well-focussed. From my experience, you should find that using defocussed images will improve photometric precision. The reason is that defocussing allows you to capture more photons before the sensor saturates (you only have 12 bits to play with), and more photons means a better signal/noise ratio. I routinely use defocussed images, after having experimeted also with images that are well-focussed.

The above does not answer your question (what are the best targets ...), but knowing that your camera+ telescope setup is not optimal for the faintest stars, but should achieve high precision with brighter variables, should get you off to a start.


American Association of Variable Star Observers (AAVSO)
regarding targets

I too place myself in the newbie category still after about a year and half of pursuing photometry, so I totally understand your desire to find suitable targets. The advice I found when deciding which targets to begin with was to seek out long period variables (LPVs). The logic here is that...for the most part...I can compare my results with other observer's observations to confirm my technique is on par. LPVs tend to follow established patterns and those stars are the "bread and butter" of the AAVSO. As Roy says relatively bright stars are a great place to start. Many LPVs fall into that range. My equipment is a 102mm f/7 ED refractor and Atik 314L+ mono which works great for stars between about 9-14 magnitude. I measure in V and B wavelengths with an established set of exposures from 15 to 300 seconds. Stacking as Roy mentions is a regular part of the regime. My goal with LPV targets is to add to the data base, and to learn how to do quality photometry...other targets will come later.

Good Luck and Clear Skies!!


American Association of Variable Star Observers (AAVSO)
Brighter LPVs--good

I'm going to agree with Roy that brighter Long Period Variables, especially Mira-like very red stars, are especially good targets for high-quality, small-aperture scopes. Larger scopes can have a difficult time with these, especially in I filter when the star in I filter reaches brighter than about magnitude 6. For just this reason, I have started deleting from my roster all LPVs with I maxima brighter than mag 6 (Stacking+CMOS might reverse this in 2021.)

Very red stars do require that observations be transformed. Also, the exposure times in B vs V (or V vs I) will be very different, and should be recalculated every couple of weeks or so. So if you want to get started without those challenges, perhaps less red LPVs would be better to begin, then possibly work toward very red stars like Miras.


British Astronomical Association, Variable Star Section (BAA-VSS)

I used a Takahashi FS102 for a few years on my unfiltered CCD CV program. Mainly patrolling for outbursts and then time series photometry on the brighter ones. So if you are interested in CVs, there's plenty you could do.