26 replies to this topic

[umairasim]

Hi,

 

Please see the attached image.. what am i doing wrong here?

 

Equipment:

Edge 8

Alpy600

Cal Module

533 mono

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[SeymoreStars]

Can you provide a little background information about your situation and the problem encountered?

 

Such as .... Are you using the Shelyak calibration module?

[JonL]

Wow, never saw this. Does it happen also with stars?

[umairasim]

Sorry for being brief earlier.

 

I am using Alpy600 Guide and Cal module. Taking Flats and Calibration lines with the module. Somehow, everything is skewed, and automatic detection never works. It is happening the all the spectrums i am acquiring. 

Attached Thumbnails

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Edited by umairasim, 29 November 2024 - 09:55 AM.

[SeymoreStars]

Sorry for being brief earlier.

 

I am using Alpy600 Guide and Cal module. Taking Flats and Calibration lines with the module. Somehow, everything is skewed, and automatic detection never works. It is happening the all the spectrums i am acquiring. 

Can you share your calibration lamp image with me on a share drive? I will try to evaluate it.

 

Edit: Also display the Geometry settings.

 

Steve

Edited by SeymoreStars, 29 November 2024 - 10:00 AM.

[umairasim]

Here is the link:

 

https://drive.google...?usp=drive_link

 

It is already corrected but here are the settings:

Angle: 1.300

Y0: 889

Radius: 7471.05

[SeymoreStars]

OK you're way under exposed, I recommend a minimum of 3 minutes, 5 may be better.

 

I just created a new calibration frame that went 10 minutes.

[robin_astro]

Here is the link:

 

https://drive.google...?usp=drive_link

 

It is already corrected but here are the settings:

Angle: 1.300

Y0: 889

Radius: 7471.05

Like I did with with "SeymoreStars'" lamp spectrum here

https://www.cloudyni...ted/?p=13823902

I also tried running this one through  ISIS. It struggled a bit. I had to adjust  the effective pixel size a bit  from 7.52 to 7.43 (2xbinned) and it has a larger than usual error on one of the lines (4511A) which throws the calibration out a bit (possibly due to some blending in that line) but still less than 1A everywhere.

 

Here is the fit

 

 

and attached is the calibrated spectrum profile. If you overlay that on that from "SeymoreStars" you will see the fit is OK

 

ISIS reports a rather low resolving power of R = 430 (It should be  ~550 with the ALPY600 and a standard 23um slit) so the focus may be a bit soft. Binning is not recommended with CMOS sensors as Demetra has some processing tricks reduce CMOS "telegraph noise" which needs non binned spectra. Both of these steps might  sharpen  the spectrum a bit and help Demetra (and ISIS) find the lines

 

Cheers

Robin

Attached Files

•  _saturn_calibration_1_g_20241027_850_Leadbeater.txt   253.89KB   4 downloads

[umairasim]

Thank you all!

 

So two things: More exposure and better focus.

 

How do you compare precise focus of the Alpy600 and the Telescope focus? Do i make any sense here?

[SeymoreStars]

Here's my procedure...

 

1) Put the ALPY "on the bench" (see photo #1) The ALPY is attached to an ASI533mm camera, in a ZWO ring, mounted on a camera tripod. Allow Sunlight to reach the ALPY slit.

 

2) Adjust the position (threaded) of the ALPY so that the blue end of the spectrum (on the left) is in focus and has the best definition (see photo #2). Tighten down hex screws.

 

3) Mount the ALPY into the telescope, point it at a patch of blue sky away from the Sun (DO NOT POINT IT AT THE SUN EVER). Stop your mount motors.

 

4) Focus the telescope image.

 

5) After focusing the ALPY in the telescope, point at an object in the distance. Focus your guider camera by moving it in and out of the holder. Make sure the slit is well focused and tighten down the hex screws. (see photo #3)

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Edited by SeymoreStars, 03 December 2024 - 10:49 AM.

[umairasim]

Thanks.. will try that.

So when taking the spectra, do you focus on the spectrum or the star in the guider?

[SeymoreStars]

Yes focus the guider image and put the star on the slit or you will see nothing in the science camera.

Edited by SeymoreStars, 03 December 2024 - 03:34 PM.

[umairasim]

Hello all,

 

So i tried refocusing again and got some light of Pleione:

https://drive.google...iew?usp=sharing

 

I see double line at Hbeta.. is that expected? or is something wrong with my acquisition?

[robin_astro]

 

I see double line at Hbeta.. is that expected? 

Yes Pleione is a Be star so you see emission from the circumstellar disc superimposed on the broad absorption from the B star photosphere. At H alpha the emission dominates and at H gamma and below the absorption dominates but at H beta you see the broad absorption with a central emission making it appear like a double absorption line. At higher resolution the emission line is further resolved into two lines because of the rotation of the disc. You can see it here in this spectrum by Joan Guarro Flo for example (left click and drag with the mouse to zoom in)

https://britastro.or...hp?obs_id=16503

 

Cheers

Robin

[robin_astro]

From your image it looks like you have quite a bit of chromatism in the telescope optics (are you using a focal reducer?) The focus is good at the IR end but is out at the blue end causing the "fish tail" affect. I suggest shifting the telescope focus a bit to give a better average focus over the full wavelength range. This is an example of the effect of changing the focus with my setup (ALPY600 C11+focal reducer) 

 

 

Cheers

Robin

[umairasim]

Thanks Robin!

 

I am not using a FR. I also do notice this Fish Tail effect.. cant understand why this is happening. I am using Edge 8 HD. Checked collimation.. seems good. 

 

Thanks for the info on Be star disk. I need to read a lot on this. I have started 'Key to the stars' .. Please if you have more books on the subject.. Do share.

[robin_astro]

There is this review article on Be stars by Rivinius et al, though it get pretty technical rather fast

https://arxiv.org/abs/1310.3962

It is still not completely understood the mechanism that causes Be stars to throw off material into a disc and they have been a subject of various pro-am collaborations

[Ed Wiley]

Robin's advice is wise. I originally thought to concentrate on good focus of the guide scope on the slit. In taking with a seasoned oibserver, he suggested that I spend some time bringing the spectrum to focus, just like Robin suggests. When you do this both the slit and the guide stars may loose sharp focus, so you then focus the guide FOV as needed. I am still in the process of tuning my UVEX, I am hoping that once I get the guide FOV focused it will stay that way given I have quartz glass and no mirror flop. But I will also slew to a bright star each session and take a test spectrum to insure the the spectra I will be taking are in focus.

 

Ed

 

ps, what I though was good focus using the guide stars was 100 clicks out of focus for the spectra, 1650 versus out of focus 1550.

[Dahu]

I am not using a FR. I also do notice this Fish Tail effect.. cant understand why this is happening. I am using Edge 8 HD.

The Celestron Edge HD "reflectors" embed an optical train in order to reduce various defects (coma and so on). This optical train is made of lenses, and those create some chromatic aberation, hence the 'fish tail' .

[umairasim]

So is there any software based focusing tools specially designed for focusing on spectrums? I only know of one in Rspec "focus Tool". Or should I only reply on my aging eyes with the glasses on?

[robin_astro]

Actually this is quite a complex subject but one that for slit spectographs fortunately does not have that great an influence on the quality of the spectrum at the end of the day as provided the slit width is chosen to match the size of the in focus star image, the resolution is defined by the slit width so apart from some loss of signal  the spectrum is relatively unaffected by the telescope focus. For the ALPY, just make sure you first focus its optics so your calibration lamp lines are nice and sharp (particularly the fainter ones at the blue end) Then with the slit in focus, focus the telescope to give a sharp star image in the guider, then tweak it if necessary to even out any effect of chromatism in the spectrum image.

 

 

To dig into this in a bit more detail though, what we are trying to do when focusing the telescope with a slit spectrograph is to maximise the fraction of the star light which passes through the slit. This is always when the star image is perfectly focussed on the slit. This does not always produce the narrowest spectrum image though as some spectrograph designs have natural astigmatism which means if you focus for the narrowest spectrum the star at the slit will be out of focus and so you lose light. The UVEX is a good example of this.  In that case focusing the slit in the guider image and then focussing the star on the slit should maximise the throughput even though the spectrum image may appear to be wide. 

 

If there is an unexpected significant mismatch between sharp focus of slit and star in the guider compared with the spectrum it can be worthwhile following it up. It may for example be that the spectrograph focus or collimation is the issue. It can also be due to chromatism in the guider optics so for example if you focus on the slit using a halogen light source rich in IR, the focus may be different from that of a hot star. Some people advise using a UV/IR block on the guide camera, though it is not something I use, preferring the extra sensitivity on faint objects.

 

Tuning the focus to maximise the brightness drop of the star in the guider image as it is moved onto the slit is one way of maximising throughput. (for example I keep an eye on the drop in SNR in PHD. If the SNR halves I know I am getting ~70% through the slit which is a good figure) In practise though a small focus error at the telescope may have little effect. For example if the star is slightly out of focus on the slit, the effect on the signal of any small guiding errors is less . It is an interesting experiment to try. Take a longish exposure spectrum of a star (long enough to include the guiding errors) with the spectrum image in and slightly out of focus and compare the absolute counts in the binned spectrum. 

 

Cheers

Robin

Edited by robin_astro, 07 January 2025 - 02:07 PM.

[robin_astro]

 In practise though a small focus error at the telescope may have little effect

What is vitally important though if there is any chromatism in the telescope optics is that there is no change in focus between the target and reference star measurements as this would change the wavelengths sampled by the slit, changing the shape of  the continuum 

[ngc2218]

So is there any software based focusing tools specially designed for focusing on spectrums? I only know of one in Rspec "focus Tool". Or should I only reply on my aging eyes with the glasses on?

If I remember corectly Prism v.11 advanced can do this 

[umairasim]

Thanks for Prism reference.. i think i have it and will give it a go.

[umairasim]

From your image it looks like you have quite a bit of chromatism in the telescope optics (are you using a focal reducer?) The focus is good at the IR end but is out at the blue end causing the "fish tail" affect. I suggest shifting the telescope focus a bit to give a better average focus over the full wavelength range. This is an example of the effect of changing the focus with my setup (ALPY600 C11+focal reducer) 

 

alpy_focus_C11_FR.png

 

Cheers

Robin

Hello Robin, so I have Celestron HD 8 inch and right now it is directly attached with Alpy 600 with guider and calibration module. I do see different focus for blue and red end of the spectrum and right now I am trying to take two sets for both ends and combine the profile later.

My question is: Since I am at native f/10, should I put in Celestron HD reducer which will make it f/7 (for better results)? The price of the reducer probably will show some not so bad results or should I scrap the idea of a reducer and keep using Alpy 600 at f/10. I just cannot change the scope, at least for now.