Situation. I am using an f/6.8 refracting telescope and an ASI533MM-PRO camera for photometry. Have the Optolong B,V,R,I filters installed. The gain I am using is 0 with a calculated offset of 30. This last in order to have “full well capacity”.
The issue that I have with bright stars is to avoid their saturation, and I have not been able to expose them properly… unless I use sub-second individual exposures (for P Cyg, to mention a famous one, have to expose at 0.1 s with B and V, for example.
I want to start a proyect with a V ~ 2.0 star, which is ten time brighter as P Cyg, with a similar B-V index. So, what do you do in these cases? Other problem is to find similar magnitude comparison stars in such a “small” FOV (~ 55.51’ x 55.51’), so my worry is to have appropriate comp stars that will not fade away.
This is also relevant for me, just in case I am able to capture T CrB outburst in the immediate future (hopefuly between March and September of next year). The estimated V mag for this star is about 2 as well (during outburst, of course).
If you want to work with such bright stars you are really best off with a photometer such as the AAVSO “PEP” group uses. We would gladly loan you a unit to use. Admittedly, you would need to remove your camera from the telescope, but I assure you that you could get really good data with an Optec SSP-3 photometer and they are easy to install and operate. If you don’t want to swap instruments on your scope, perhaps you could team up with a friend having a refractor or cassegrain.
I would love to get a photometer placed in Mexico. Feel free to contact me.
Since it has a CMOS active pixel sensor, really short exposures should be possible with that camera. I don’t know how accurate it’s timing is.
I’m aware of some researchers using neutral density filters in a second, cascaded filter wheel. They allow bright star work by swinging in the ND filter in front of the photometric filter.
Good luck!
Colin
Some suggestions : for bright stars, you could drastically stop down the optic (to 50mm, even 25mm), defocuse the images so as the stars appears as large as a few dozen pixels in diameter.
More over, if you use a small focal lenght lense, you will have more comp stars.
I am new to photometry and working on the 10 star tutorial. I am facing the same issue as Enrique presented with del Cep. Though not as bright, it still presents the problem of a several magnitudes between the target and available comp and check stars.
I have found that a 0.025 second exposure gives a high SNR for the target but a low SNR for the comp and check stars available in that sequence.
I have no idea about the photometer. Maybe it solves the problem, but my rig uses a CMOS camera.
These ideas; neutral density filter, stopping down the aperture, and defocusing do not solve the problem of the lack of high SNR for the comp and check stars. I also face the issue of the VPhot plate solver, pinpoint I think, is unable to solve an image with a very bright target and few other stars visible in the field.
Since I’m a beginner here, I am looking for advice on the “canonical” way to solve this problem.
You have to makes some compromises.
In your case, the way is to extend your FOV, and find other comps stars.
Anyway, this is always a good idea to adapt and choose your comps to the target : they should always be as close as possible in magnitude to avoid problems.
Hi Enrique;
Just wanted to add to this conversation to let you know that you are not alone in this struggle. I was previously using a DSLR camera, and became comfortable with doing a quick guess of the exposure times for stars of a certain magnitude and was successful in not saturating them.I had a list of 5-10 SNEWS candidates viewable from my backyard I was tracking. I also purchased the ASI533MM-PRO camera. I was also not prepared for the massive jump in sensitivity. Another challenge is doing automated plate solving alignment to allow for the comp stars to be in the field of view. I am going to try this again weather permitting, with newly purchased optolong filters and a older Rokinon 135mm lens. Stepping down the aperture might be a good compromise to get above shot noise, and get a decent SNR? I tried with a F5.3 MN190 at 1000mm, and a 80mm refractor at F6.1 reduced to 480mm focal length. I might also try with a 70-200mm lens, as finding enough “separation” of stars for the definintion of the radius of the star and sky background can be limited by choice with certain software. I don’t know any other way of doing this apart from using up clear sky time to do lots of manual processing on stars to try and figure out a spreadsheet of times for certain magnitudes.
I have a 200 mm f/2.0 telescope which I use for bright star photometry. To avoid saturation, I take really short exposures (about 10 milliseconds for a V~2 star). To reduce the impact of scintillation, I stack the exposures (using some custom software that makes 128 bit FITS images) to yield 10 second exposures. The disk space requirements for this process are outrageous so I purge the original exposures within a day.
Is a fast lens the one that your’re using. So, I will try to capture del Cep (in order to train me for T CrB) with shorter than a second exposures and see what I get from my rig (103 mm / 700.4 mm, f/6.8 scope - much “slower”) and take special care of the comp stars available. Let’s see how many subs do I get, and the stacking amount that is enough to get a good (or decent) SNR. If I get something possitive (I should, I believe) will share with all of you immediately. Thanks for sharing!
One issue I face when taking a non-saturated image of a bright star is that the PinPoint plate solver used by VPhot does not find the WCS solution for the image. There is basically one bright object in the center of the field and nothing else for the plate solver to use.
I have found that I can bracket the photometry image with longer exposures to be used for plate solving, and then use the VPhot “Update WCS” option to copy the WCS from the longer exposure to the shorter exposure.
I hope this helps other beginners solve one of the problems I have encountered.
Fortunately, when taking really short (< 1 second) exposures, there really isn’t enough time for alignment issues to crop up. You can often just stack the images pixel-for-pixel and get decent results. Even if you have an AltAz mount, a stack of ~10 seconds will only show slight trailing.