Alpy600 Calibration Problem

[Christian Buil]

I agree with Robin comments : blue image can be degraded in some circonstance... if the f/6.3 is respected
(same result with Meade and Celestron f/6.3 - Baader is not really bette concerning chromatism)
In your setup Keith I suspect a focal ratio reduction of f/8 - f/8.5 because the small distance
between FC and slit.

Make a special focal reducer special for spectrography is a future target...

Christian

[Keith Graham]

Hi Christian,

This is very interesting. I did not realize the distance between the focal reducer lens and the Alpy lens would change the f ratio. I just figured I was getting f/6.3 because this is the f/ration of the focal reducer. Is there a formula I can use to determine the actual focal ratio based on distance? I ask this because I want to do some experimentation. I am noticing that ISIS does not appreciate my RR Lyra spectra and has great difficulty in calibrating this 7.1 mag star (very large RMS). ISIS has no problem with nearby 5.4 mag HD 184875. I thought focusing might be the problem, but I do focus on HD184875 before moving to RR Lyr. You mentioned that my UV end was quite good, but I am wondering if this is because the f ratio is around f/8-f/8.5. If I can get that f ratio down to f/6 or f/7. this may give me the additional sensitivity to get a viable RR Lyr spectrum as well as a decent UBV region. However, I can see how lowering the f ratio may also affect the resolution of the UV end. So I want to try some different distances to see if I can get both good UV resolution and an overall spectrum that ISIS will like.

Cheers,

Keith

[Ken Harrison]

Keith,
I have a spreadsheet which calculates the reduction factor for various reducers/ distances.
The "original" Meade x0.63 reducer (marked Japan) had a focal length of 220mm and needed 100 backfocus to achieve f6.3
The more recent versions (marked China) are only 80mm focal length (!) and use around 50mm backfocus.......
(The Celestron versions - if still available - were always 220mm focal length)

[Christian Buil]

Keith,

Never calibrate on a faint star. ISIS is a very stable spectrograph. Find calibration parameters on a bright star (V = 3 to 6) and applied these parameters for process RR Lyr spectra
(by security, select a reference star not very far from the traget objet - here RR Lyr).

Christian

[Keith Graham]

Hi Christian,

Well, this will be another addition to my learning curve. I did not realize that I should not calibrate on such a dim star, and I did not realize I could calibrate on a star different than the target and then apply that calibration to the target. Is this what you are saying to do? If so, I can easily calibrate on the nearby star HD184875 (V=5.34). I could then apply that to RR Lyr if I knew the procedure for doing so and if this is what you mean.

Perhaps I can figure this out and you can tell me if i am correct. If, for example, I use HD 184875 as the calibration star and RR Lyr as the target, would I

1. Enter HD184875 into the Calibration assistant
2. Complete the Calibration Assistant process.
3.Move to the Go tab and press the Go button.

By pressing the Go button, would ISIS then use the information developed in the Calibration Assistant fpr HD 184875 and apply it to RRLyr?

Cheers,

Keith

[Christian Buil]

Yes Keith this is the exact procedure.
ISIS use the latest calibration for the next processing. The use of
ISIS is very simple in fact

The calibration page is now translated in english:

http://www.astrosurf.com/buil/isis/guid ... ration.htm

(your question Keith is adressed in the text...)

(thanks Robin for the translation help!).

Remember also, you recover the calibration parameters after closing ISIS and run
a new session (see "Profile" tab and "Dispersion" button, the polynom coefficients
are the same).

And now, probably one of the most powerful and advanced functions of ISIS : XML files attached to
each processed spectra for recover the specific processing parameters. See ISIS revision 4.3.0
(09/05/2012) section, here:

http://www.astrosurf.com/buil/isis/new/release.html

Christian

[Keith Graham]

Hi Christian,

Thanks for the clarification. I did read (and have printed out) your document on calibration. The description of Method 3(which I use) shows the target star also being the calibration star in the Calibration Assistant. Since a dim target will not work for calibration, I wanted to be certain I could use a brighter reference star in the Calibration Assistant. I must have missed that part in the document so I will re-read it. But, I did try using HD184875 as the reference star to calibrate RR Lyr, and it works great.

BTW-I re-measured that distance between the focal reducer lens and the Alpy objective more carefully and got 95mm, not the 85mm I originally reported. So I am going to leave everything as is for now. I do like the detail in the UV end, and I am getting very nice calibration now that I know I can use a brighter star for the calibration routine.

Again, many thanks for your help in getting me on the right track.

Cheers,

Keith