Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

AZ for photometry

  • Please log in to reply
17 replies to this topic

#1 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 23 February 2021 - 03:47 PM

Hi all

 

I am interested in photometry of variable stars and spectrography

Any chance to achieve decent results with an AZ mount and stacking of abt 30 secs short  exposures  ?



#2 Jim Waters

Jim Waters

    Fly Me to the Moon

  • *****
  • Posts: 5,466
  • Joined: 21 Oct 2007
  • Loc: Phoenix, AZ USA

Posted 23 February 2021 - 04:11 PM

Not likely.


  • Miguelo likes this

#3 GaryShaw

GaryShaw

    Viking 1

  • *****
  • Posts: 925
  • Joined: 28 Nov 2017
  • Loc: Massachusetts / Wyoming

Posted 23 February 2021 - 05:45 PM

Hi all

 

I am interested in photometry of variable stars and spectrography

Any chance to achieve decent results with an AZ mount and stacking of abt 30 secs short  exposures  ?

Should be no problem as long as your mount tracks ok for 10-15 seconds( depends on the variable you’re observing. I’ve been having no difficulties with my iOptron AZ mount Pro. On the variables I’ve been working with so far at V magnitudes of around 10-14 range, I typically take 5 images in a sequence at 10-15 seconds. After dark and flat calibration in Maximdl, I set image #3 as the reference for aligning and stacking the 5 images and things work out fine for uploading to AAVSO’s VPHOT application for magnitude analysis and reporting. If you can plate solve prior to uploading to VPHOT, it’s better. Plate solving in VPHOT can get bogged down by other users. 
 

There are various different processing steps depending on whether you’re using mono camera with filters or a OSC camera and splitting the color planes but I assume you’re on top of all that. So, yes, you can use an AZ mount. 
Good luck

Gary



#4 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 23 February 2021 - 06:44 PM

Hi Gary

Actually I have in mind to purchase an iOptron AZ mount Pro
an SCT 8 f 0.5 or a Newtonian 200/1000
ASI178 MM mono
What SNR value can you obtain at V 10-14 magnitude ?
With an AZ mount you do not have access to a rather large part of the sky , close to zenit
Is it, from your experience , a serious drawback of AZ vs  EQ  mounts ?

 

Hi Jim

 

Why do you think that AZ mounts are not suitable for photometry and spectrography ?

They are quite popular for EAA , it should not be that different I suppose (I hope )

                                                all the best

 
                                                    Alberto



#5 Hubert

Hubert

    Ranger 4

  • -----
  • Posts: 394
  • Joined: 26 Sep 2006
  • Loc: Belgium

Posted 24 February 2021 - 01:38 AM

Yes, it can be done as long you set your standars not to hijgh. I have done it at the eclips of Epsilon Aurigae with a 80mm, EOS camera on a non driven tripod. With fairly good results.



#6 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 24 February 2021 - 06:59 AM

Hi Hubert

 

Can you give me a rough indication of what you mean for not too high standards ?

For example in term of max magnitude of the  stars and /or max  accuracy of the measures

 

                                          bye

 

                             Alberto



#7 Hubert

Hubert

    Ranger 4

  • -----
  • Posts: 394
  • Joined: 26 Sep 2006
  • Loc: Belgium

Posted 24 February 2021 - 09:15 AM

Hi Hubert

 

Can you give me a rough indication of what you mean for not too high standards ?

For example in term of max magnitude of the  stars and /or max  accuracy of the measures

 

                                          bye

 

                             Alberto

In terms of accuracy is was 2 points after de decimal point.  I made about 10 series of 5 frames and stacked these frames. Then I had 10 stacked frames, measured in AIP4WIN manualy the brightnes of Epsilon Aurigae. Than I averaged the 10 brightness measurements and got a green channel measurement.  More info on Epsilon Aurigae:

 

https://www.aavso.org/epsilon-aurigae

 

In this paper is a lightcurve of the star and the RLO points are mine.

 

https://vs-compas.be...article/3-4.pdf



#8 GaryShaw

GaryShaw

    Viking 1

  • *****
  • Posts: 925
  • Joined: 28 Nov 2017
  • Loc: Massachusetts / Wyoming

Posted 24 February 2021 - 09:33 AM

Hi Gary

Actually I have in mind to purchase an iOptron AZ mount Pro
an SCT 8 f 0.5 or a Newtonian 200/1000
ASI178 MM mono
What SNR value can you obtain at V 10-14 magnitude ?
With an AZ mount you do not have access to a rather large part of the sky , close to zenit
Is it, from your experience , a serious drawback of AZ vs  EQ  mounts ?

 

Hi Jim

 

Why do you think that AZ mounts are not suitable for photometry and spectrography ?

They are quite popular for EAA , it should not be that different I suppose (I hope )

                                                all the best

 
                                                    Alberto

I checked some recent submission data to AAVSO and here’s the following:

 

SS Cyg, 11.6, SNR 122

RX And. 12.4, SNR  104

R.  And.  12.4, SNR 334

I feel the limitation on AZ mounts for this purpose is simply - can you track fairly well and long enough on a given star to gather around 1/2 full well depth of adu on your sensor but, of course, not saturate any of the pixels. In my case, I’ve used 5-15 second exposures, mostly 10-12 seconds. If you’re in a tricky area of the sky for rotation you CAN, if you need to, exceed the recommended exposure limits as long as you’re thoughtful about aligning and stacking the image series.  I’ve found I can exceed the recommended exposure and still capture good data for accurate reporting to AAVSO.

 

Should you ever become interested in Exoplanet confirmations, an AZ mount can likely be used only to confirm ‘midpoint’ time of transits. The longer tracking times required to confirm exact elapsed transit times necessitate guided EQ level tracking. Ditto for doing astrometry on NEO which also may need long focal lengths. 
hope this helps. 
Gary



#9 robin_astro

robin_astro

    Apollo

  • -----
  • Posts: 1,224
  • Joined: 18 Dec 2005

Posted 24 February 2021 - 12:13 PM

Hi all

 

I am interested in photometry of variable stars and spectrography

Any chance to achieve decent results with an AZ mount and stacking of abt 30 secs short  exposures  ?

With spectroscopy, field rotation when using an Alt Az mount can give issues but they are not show stoppers. 

 

With slitless spectroscopy in crowded fields, field rotation can cause field stars and their spectra to drift through the spectrum you want which means you may have to reject some exposures.

 

With slit spectrographs you may occasionally find other stars in the field drifting through the slit as the field rotates but this is not a problem provided you still have enough clean regions along the slit to sample the sky background.

 

With slit spectrographs and Alt Az mounts you are forced to guide on the target star. (If you use any other star in the field for guiding, the field rotation would quickly cause the target to drift out of the slit.) Most spectrograph designs these days however have mirror slit guiders which allow this so this is not a problem provided the target is bright enough. This can be a problem for faint targets where there may not be enough light overspilling the slit to guide on. 

 

As an aside, Alt Az mounts actually have one advantage for spectroscopy as the slit can be oriented so it is always perpendicular to the horizon which minimises the effects of atmospheric diffraction

 

Cheers

Robin


Edited by robin_astro, 24 February 2021 - 12:14 PM.


#10 robin_astro

robin_astro

    Apollo

  • -----
  • Posts: 1,224
  • Joined: 18 Dec 2005

Posted 24 February 2021 - 12:32 PM

And of course when imaging for photometry, CMOS cameras have the advantage compared with CCD when it comes to stacking short exposures as there is less of a hit from the lower read noise. This combined with multi star alignment tools to remove the field rotation means you can go fainter by stacking short exposures minimising the problem of trailing due to field rotation

 

Robin


Edited by robin_astro, 24 February 2021 - 12:33 PM.


#11 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 24 February 2021 - 01:10 PM

Hi Gary , Hubert and Robin

 

 

your support was definitely of help, thanks a lot

 

It seems that photometry with AZ, if not the best , is fairly good

The SNR are impressive for 12 magnitude stars, at least, you have got higher values  than I was expecting

Accuracy should be in the range of second decimal point ,as Hubert said ,even with rather short exposures

 

 

Spectrography seems to be more critical in AZ

I read that stacking of short exposures does not work for spectrography or at least it is not that effective

Is it true ?

Robin, is this the reason why you speak of guiding ?

For short exposures, generally speaking,  it should not be necessary

 

                                                                              cheers

                                   Alberto



#12 robin_astro

robin_astro

    Apollo

  • -----
  • Posts: 1,224
  • Joined: 18 Dec 2005

Posted 24 February 2021 - 03:52 PM

 

Spectrography seems to be more critical in AZ

I read that stacking of short exposures does not work for spectrography or at least it is not that effective

Is it true ?

Robin, is this the reason why you speak of guiding ?

For short exposures, generally speaking,  it should not be necessary

 

                                                                            

With simple low resolution slitless spectroscopy (like the Star Analyser for example) where you image the whole field with all its spectra, there is no particular need to guide provided your mount tracks reasonably well.  You can take short unguided exposures and align on the star of interest and stack them and this is the way many people operate.  Either an Alt Az or Equatorial mount can be used, though the Alt Az has the slight disadvantage that  any other stars in the same field could rotate across the path of the spectrum you are measuring during a long run of exposures meaning you may have to reject some individual exposures.

 

Slit spectrographs have to be guided .  You have to be able to place the star precisely on the slit (which is about the same width as the star image), focus it and keep it precisely in the same location without it drifting off for a long time, typically tens of minutes even up to a couple of hours in total. Even high end mounts would struggle with this so it is done using a guide camera looking at the star on the slit. You could sum many shorter exposures but you would still need to guide to keep  the star on the slit so there is no advantage.  (With spectroscopy the star light is spread out very thinly such that even at low resolution the brightness of the spectrum is around 7 magnitudes  and at high resolution as much as 11 magnitudes fainter than the star.)

 

Alt Az or Equatorial mounts  both work fine in principle with a slit spectrograph provided the guiding works well but spectrographs with their imaging and guide cameras can be bulky and heavy instruments with a large overhang which can be restricting with some mount designs.

 

Cheers

Robin


Edited by robin_astro, 24 February 2021 - 03:59 PM.


#13 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 24 February 2021 - 06:48 PM

thanks a lot Robin for your clear and exhaustive explanation

 

                                       bye

 

                                     Alberto



#14 Ed Wiley

Ed Wiley

    Vanguard

  • *****
  • Posts: 2,084
  • Joined: 18 May 2005
  • Loc: Texas, USA

Posted 24 February 2021 - 11:55 PM

Zoltrix,

 

My strong recommendation is to have a look at the AAVSO Photometry Guide and, if you have a DSLR and not a mono CCD or mono CMOS sensor have a look at the AAVSO DSLR guide. These will give you some authoritative information. They are free to download and you do not have to be an AAVSO member to access them.

 

https://www.aavso.or...serving-manuals

 

Ed

 

ps-- I have never used an AZ mount for photometry, so my opinion would not be informed.



#15 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 25 February 2021 - 05:09 PM

Hi Ed

 

I want to buy a CMOS ASI178 MM Mono, I suppose it is a good choice for short exposures photometry and spectrography

can the DSLR guide be of use anyway ?

DSLR are fitted with CMOS sensors

What's the difference ?



#16 Ed Wiley

Ed Wiley

    Vanguard

  • *****
  • Posts: 2,084
  • Joined: 18 May 2005
  • Loc: Texas, USA

Posted 26 February 2021 - 12:06 PM

Zoltrix,

Mono cameras are the usual choice for photometry. I am not familiar with the ASI178, but you can probably get some information if you tune into the photometry forum threads at the AAVSO site. I suspect they work just like my ASI183. The difference is that DSLRs have a Bayer Color matrix while mono cameras do not. Mono cameras are thus more sensitive.

 

A gentle suggestion: I think that before you buy you need to do some research. YOu need to invest some time earning about sensors, camera characteristics, filters and what not. Be informed enough to make good choices. Spot inquires about particular cameras is not a good choice. The guides are a start and the CCD Guide also applies, in most respects, to CMOS cameras. But you also need to explore other information about CMOS cameras and photometry before stepping in and investing. Frankly, I have done CCD for years but CMOS is relatively new to me (as it is to many old hands) and I am also learning as I go along. I a finding that CMOS is not that much different from CCD, but there are differences, even between cameras. The good news it that the ones I am familiar with do good photometry.

 

Ed



#17 zoltrix

zoltrix

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 23 Feb 2021

Posted 26 February 2021 - 01:22 PM

Thanks Ed

I will follow your suggestion

#18 GaryShaw

GaryShaw

    Viking 1

  • *****
  • Posts: 925
  • Joined: 28 Nov 2017
  • Loc: Massachusetts / Wyoming

Posted 26 February 2021 - 03:12 PM

If you want to do serious variable star photometry, I’d recommend that you join AAVSO, and request a mentor ( AAVSO provided this to members ) who is familiar with cmos photometry. They will guide you through the process of using either 1) a DSLR, 2) a one shot color cmos such as ZWO 294 mc or 3) a mono cmos like ZWO 183 or 178mm. 
 

With options 1 and 2, you don’t use any photometric filter and will need to work with the Bayer ‘color planes’ to end up with a ‘tricolor green’ (TG) image to base your magnitude analysis on within asvso’s VPHOT application.  With option 3, you will need a proper photometric filter(s) which seem completely unavailable right now. 
 

I’d urge you to join AAVSO and learn the ropes thoroughly for whichever of the options above you plan to pursue. 
Good luck. 
Gary




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics