PEP Novice Questions

As someone who has done both PEP and CCD photometry, I'd recommend that you also consider CCD photometry.    To me, when using the great tools provided by the AAVSO (ensembles of AAVSO comps, VPhot ) both the observing and data reduction processes are much quicker.   Also, you can observe fainter stars with good precision using a CCD, at least when compared with a photomultiplier tube type PEP, so there are a lot more potential targets.  The standard CCD photometric filters also help out with the light pollution problem.

All that said, I'm sure there is plenty of worthwhile work to be done with PEP on brighter stars which are difficult to observe with CCD's.

Phil

Paul,

I was traveling earlier this week and missed you post, and feel somewhat remiss in my role as the PEP section chair.  Anyway here are a few thoughts, although I will admit to a bit of bias.

1. When I started PEP several years ago I purchased new Optec SSP-3 and SSP-4 photometers which have proved to be reliable instruments.  I am currrently working on getting three older SSP-3 photometers working, two from AAVSO HQ and one I purchased on ebay.  The electronics in the older SSP3's are pretty straightforward and rugged but there are some potential issues.  The first is filters.  You may find that if the photometer was not stored in a dry location the filters have degraded.  You can still purchase sliders and filters new from Optec if this is a problem.  If you do purchase a used photometer either Tom or I can help walk you through observations to determine your transformation coefficients which will tell us if the filters are still in good shape.  Another issue that can come up, again if the photometer is stored in a moist environment is erratic behavior in the sensor assembly which can happen if moisture penetrates the sensor assmebly.  This can sometimes be resolved by placing the photometer inside a sealed bag with a dessicant, or gently heating to drive off moisture.  Once you get a photometer I can help walk you through either of these issues.  Also we can try mixing and matching assemblies from the two SSP3's from HQ or the third one I recently purchased from ebay.

2. In terms of mounts we have a lot of different setups in the PEP section.  The key is to be able to keep the star in the photometric aperature during the 30 to 40 seconds it takes to get three 10 second integrations.  So if you have a very long focus scope the requirements on the mount will be greater than if you have shorter focal length instrument.  The tradeoff is excluding field stars which is easier with the longer focal length.  With PEP we use designated comparison  and check stars so the ability to slew or move to nearby stars is also important.   We do mostly bright stars (most brighter than 7th magnitude, so the stars are relatively easy to find, and your experience as a visual observer probably will make this pretty easy for you.  In my case my focal length is about 140 inches, giving me a 50 arc second aperature.  I would not recommend going smaller than this aperature so you may want to keep your focal length below this.  An 8 inch f/10 would work great for many of our stars.

3.  My view is that PEP has good absolute accuracy observer to observer on the bright stars we observe.  I find there are more bright targets than observing time.  Currently I believe the PEP observing techniques yield more accurate absolute values on our targets than CCDs.  However, it takes about 45 minutes to do a two color measurement on a single star using our techniques, so the focus is on accuracy not throughput.  

I also really enjoy visually looking through the scope when I make observations.  Even though the instrument is making the measurements you are slewing the scope, centering the star and looking through an eyepiece.  As a visual observer I am sure you appreciate the joy of actually looking at the stars you study.  It has the added benefit of allowing you to actually see and observe sky conditions and eliminate problems early with an observing run. 

Hope you join the PEP team.  Feel free to contact me with any questions, or help setting up your system.

Jim Kay

  Thanks for the replies.

  Phil, I may branch out into CCD observing some day, but I'd like to learn PEP first, especially since fewer observers seem to be doing it. But I appreciate your advice.

  Jim, I am now the proud owner of an SSP-3 (Generation 1, I believe). It arrived in the mail yesterday. I've been in touch with Tom Calderwood for several months, and he's also given me a lot of advice on how to test it out. I also very much appreciate all the information that you wrote in the post above. I'll read through the manual that Tom e-mailed to me, and I'll also read the posts in the Peptalk group that Tom sent me a link to. Hopefully I'll either find a way to use it with the telescope I have now, or get a telescope that's better suited to PEP work. This whole field of variable star observing sounds fascinating.

  Paul (ZPA)

Hi Jim K,

I have begun with the help of a KWU student, Tanner Klingensmith, to collect and reduce photometry data with an Optec SSP-3.  The AAVSO procedure for determining Epsilon-V is fairly straightforward using the V band photometric filter. My question is when we collect variable star data for differential photometry; do we need measure both v and b magnitudes with the V band and B band filters, respectively?  I ask specifically, because the transformation equation from v to V includes the term Epsilon-V times delta (B-V).  I learned from 'slight embarrassment' statement on page 191 of Hall, Genet book titled Photoelectric Photometry of Variable Stars (1988); that its ok to use delta (b-v)subscript 0 in lieu of delta (B-V).  We can readily obtain delta (b-v), so I believe our observing program should include both V band and B band measurements.  Please explain why I am in error and we do NOT need to collect B band measurements to calculate V magnitudes.

Thanks,

Jeffrey Kasoff

Salina Astronomy Club

Greetings,

I've been cut off from various discussions for some time due to a variety of reasons - one was a close lightning strike where the electromagnetic pulse fried quite a number of things. But that's in the past.  I noticed some comments from a couple of people new to PEP - Paul Z and Jeff K who look to be starting in this interesting field.  I too have an SSP3 which I should use but don't - it has a complete set of UBVRI filters and I've always wondered if there would be any response in U.  I'll find out one day.

My background is in largely UBV photometry using a custom built photometer which in the late 1960s was as good as anything in the southern hemisphere.  But like Paul age catches up and now I work on analysing brighter stars using other observer's data.  I had a spell doing CCD colour photometry and would support the comment that good PEP is more accurate than most CCD measures.

One of the problems is that many people wish to be 'instant astronomers' and do not take the time to really understand the art.  There are many short-cut methods around which make reductions more simple but an observer needs to know enough about the correct methods to know what can be skipped in each case. 

Jeff mentions V only measures but you cannot transform accurately using only one filter - two are always needed.  The most popular are B and V to give B-V which also provides temperature information if the data are used for analysis. OK, we now have Sloan filters which are better designed but the database of UBVRI measures makes those unattractive to amateurs.  And there are many opportunities with bright stars.  I'm presently setting up a multi-longitude observing project to observe a sixth magnitude eclipsing binary which is probably the most complex system in our galaxy.

So a good plan for beginners is to read a reference which explains why good transformations, including primary and secondary extinction, are necessary.  Then you can decide on suitable comparisons and which steps can be avoided to obtain the necessary accuracy.  There are better sources and books than Hall and Genet.  But there's no real need to become expert in the application of these in many cases.  About 2010 Tom Krajki wrote an interesting article about all of this which I expanded in the November, 2010, Variable Stars South Newsletter.  It's probably on-line at their website.

So B and V are essential if you wish to get good accuracy in V.  After reducing the measures in the form, V, B-V, V-R or V-I it's simple to derive the single filter measures which the AAVSO likes - even if the most analysts prefer the former.  And once you've made the measures it's even better to analyse these to see what they mean.

I'll look forward to hearing about progress.

Regards, Stan

Hi Stan,

Thanks for the quick reply and recommendation to read your article in the Nov 2010 VSS Newsletter.  The article was helpful, but I need to study further. 

I think the only justification for V-Band only differential photometry is when one determines the observer's system minimally deviates from the Standard BV system (i.e., absolute value(Epsilon(v)) < 0.001), then correction to go from v to V would be less than 0.001 mag, assuming that (B-V) difference between target and comp is <= 1.

Initial Epsilon(v) determination from Red-Blue pair is 7E-4 based on truncated four (4) block observing session on 14-Nov-2016. Need to repeat with prescribed seven (7) block dataset for a proper determination before we submit to AAVSO for review.  

We will initially collect both V and  B  band filter data in our VS observing program, just to monitor difference bewteen v, instrument  V-band magnitude, and V, transformed V-band magnitude.

Slightly less confused,

Jeffrey

Hi,

I received the PEP draft manual in my email inbox.  Thanks Tom C.  I skimmed through the manual to see where you identify reliable Blue-Red star pairs for epsilon(V) determination.  I was surprised by Tom C e-mail remark that the Aqr pair underestimates epsilon(V).. The Aqr pair is one of four pairs listed in Dr. Hall 1983 procedure paper. Should I ask how many PEP measurements used this unreliable pair?  I should not ask.  I would recommend that you update the  AAVSO PEP Program home page.  It has link titled 'For New PEP Observers - Instructions for Computing the Epsilon(V)'. . The procedure under this link ( https://www.aavso.org/obtaining-your-pep-epsilonv-coefficient ) has all four pairs from Dr. Hall's procedure plus more pairs.  We will follow Tom C recommendation to use Per, Ori, and/or LMi for reliable Epsilon(V) determination.  Hopefully, more pairs will be vetted and the online Instructions will be updated.  In the vetting process it would be great if uncertainty metrics could be added to V and B magnitudes for the pairs.  The original procedure paper from Dr. Hall appears to show magintudes to the nearest 0.005 mag IMO.

Again, thanks for the quick and detailed responses,

Jeffrey Kasoff

The web page with information on calculating epsilon v using red blue pairs has been updated.  The Aqr and Peg pairs are no longer recommended for epsilon calculations.  The And pair is only recommended if no other pair is available.