[Aavso-photometry] Aavso-photometry Digest, Vol 47, Issue 12

Thu Oct 18 20:11:06 EDT 2007

Jeff,

That's great information you have for someone like me, who is really clueless as to next steps.  Not only did I gain confidence in the use of the DSI for CCD-VSO, but I also learned, in a nutshell, about the basics of CCD (and PEP and PMT) photometry.  In fact, I just made a deal for a DSI on Astromart!  $105 to my doorstep.  Better yet, the camera mods will add more of that "DIY" look to my gear.  How cool is that?  DIY science on a budget just can't be beat.

Thanks!
Sal

----- Original Message ----
From: Jeff Hopkins <phxjeff at hposoft.com>
To: Sal Giambruno <salgiambruno at yahoo.com>
Sent: Thursday, October 18, 2007 8:03:09 AM
Subject: Re: [Aavso-photometry] Aavso-photometry Digest, Vol 47, Issue 12

Hello Sal,

I have been doing photometry for some 25 years. For UBV work I use a 
photon counting system I built. For infrared work I use a SSP-4 (JH 
band). I couple of years ago I got a DSI Pro (monochrome version) 
added a TEC cooler, filter wheel with BVRI filters and have been 
using it for BVRI photometry.

See  http://www.hposoft.com/Astro/AutoStar/DSI/DSI.html

Note the DSI and DSI II are color cameras and not suitable for 
photometry. The Pro versions are monochrome and work well.

I also started a mentor program for those getting started with CCD 
photometry, particularly those using a DSI Pro or DSI Pro II. See 
http://www.hposoft.com/SZHER/Mentor.html

The recent price reduction for the DSI Pro ($129) is a no brainer. 
For that price the DSI Pro is an extremely cost effective way to do 
CCD photometry. The DSI Pro II is better, but no price reduction and 
at $600 is considerably more expensive. Meade has just announced a 
new DSI Pro III for $1,300 which has a much larger chip. This may be 
great for pretty pictures, but not too much advantage for photometry. 
This may mean a future price reduction of the DSI Pro II, but no 
guarantees that will happen.

I did a BVRI calibration to determine the transformation coefficient 
and the results are reasonable. As for accuracy, there are many 
factors. I always take three images in each band. Then obtain three 
values of the program star's magnitude in each filter. I average 
those values and take a standard deviation. I have found the best I 
can do is around 0.02 magnitudes standard deviation. I think the 
biggest problem is to get good flats. This is with 11 magnitude stars 
and a 12" SCT with focal reducer for an F/3.3 system. I have also 
used it at F/10 with similar results.

While the Phoenix, Arizona area have very clear skies, it is severely 
light polluted. This does not seem to hurt the photometry, however. 
As long as you can find the stars, you can get photometry data on 
them. Using a focal reducer allows you to see a bigger field which 
means easier identification of the star field and better chance of 
having good comparison stars in the same field.

The DSI series are not the ultimate CCD cameras for photometry, but 
work well at a very reasonable price. In addition to the camera you 
will need a filter wheel and filters. At a minimum that can add 
another $500 to the system cost, but those can be used with any CCD 
camera. I recommend the cooling mod as it significantly decreases the 
hot pixels when the ambient temperature is above 60 degrees F. It 
also increases the chip's sensitivity. If you observer where typical 
temperatures are below 60 degrees F then cooling is not as important.

I hope that helps. Let me know if you have any other questions.

Good luck with your observing.

Jeff

At 05:33 -0700 10/18/2007, Sal Giambruno wrote:
Hi Jeff,

Thanks for your offer to enlighten!  In fact, and come to think of 
it, I do have a few questions I hope you might address.

Given as you have been successfully, I assume, conducting photometric 
data gathering for some years now, I am led to the conclusion that a 
camera even as basic as a DSI-I  must have inherent value as a tool 
for VSO.  In other words, your DSI-I has stood the test of time, 
which, at the very least, speaks for its durability.

Assumptions of durability aside, can you tell me about the DSI-1's 
inherent limit at resolving stellar magnitude?  When coupled with an 
ideal matching optical system, or in this case, with your telescope, 
to what precision can a measurement be made (e.g. to 0.1 magnitude, 
0.01 magnitude, etc)?  I guess this would be directly dependent on 
the CCD's well depth?

Is the camera's spectral response "adequate" for measurements in the 
visual spectrum (as defined by convention)?  Is it useful at any 
other spectral ranges?

Given an ideal (or merely a satisfactory matching) optical system, 
does the camera yield a tolerance for a reasonable variety of seeing 
conditions?

What would be the ideal optical system and seeing conditions for a 
CCD having the physical specifications (pixel shape, size, count) of 
the DSI-I?

If the cost of buying an alternate camera, say a DSI-II, or III or 
Pro, was only marginally painful to my pocketbook, would it be worth 
it, or even wise, for me to choose to do so?  Would the benefits 
(convenience, ease of use, reliability, precision, accuracy) greatly, 
marginally or not at all outweigh the added cost?

I hope you don't take my line of questioning as derogatory or 
frivolous, as I am posing these questions based partly on the 
assumption that the DSI-I is not the ultimate in photometric devices, 
and in an effort to gain an understanding of where its limits lie and 
partly on my experience that inexpensive equipment often turns out to 
be equipment of limited or no practical use for a given application.

Sal Giambruno
Observations by GSA.  As rare as The Ether itself . . .

----- Original Message ----
From: Jeff Hopkins <phxjeff at hposoft.com>
To: Sal Giambruno <salgiambruno at yahoo.com>; aavso-photometry at mira.aavso.org
Sent: Monday, October 15, 2007 4:59:12 PM
Subject: Re: [Aavso-photometry] Aavso-photometry Digest, Vol 47, Issue 12

Hello Sal,

I have been using the DSI Pro I for a couple years now. It works well
for CCD photometry. If you have some specific questions I'd be happy
to try to answer them for you.

Jeff

At 16:41 -0700 10/15/2007, Sal Giambruno wrote:
>Hi Bob,
>
>Yes, congratulations on your new entry into CCD VSO!  I am excited
>about taking that first step myself.
>
>Could you provide some insight into your experience using the DSI-1?
>It seems like a most affordable way of getting into CCD, but I was
>curious as to its possible limitations for use in CCD VSO, if any.
>
>Thanks,
>Sal Giambruno (GSA)

--
Jeff Hopkins
HPO SOFT
Counting Photons
<http://www.hposoft.com/Astro/astro.html>http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
(623)849-5889
(623) 247-1190 (Fax)
<http://www.hposoft.com/>www.hposoft.com

-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
(623)849-5889
(623) 247-1190 (Fax)
www.hposoft.com