Very bright stars sub exposure times

Science Photometry
1

Hello everybody!

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).

Cheers!
Enrique Boeneker (BETB)

2

Enrique:

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.

Tom Calderwood PEP section head

1 Like
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Tom,

Thanks so much for the offer!!! I will certainly send you an email.

Cheers!
Enrique Boeneker (BETB)

4

Enrique, here’s a couple of other ideas:

  1. 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.
  2. 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
1 Like
5

Hello Enrique,

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.

Chris

2 Likes
6

Hello Colin,

That’s an excelent tip, thank you!

Enrique (BETB)

7

Hey Chris,

I forgot totally about defocusing… thanks for reminding me! I’ll try that too.

Enrique (BETB)