Alpy 600 and other slit based spectroscopes

[riklaunim]

Anyone here used Alpy 600 or something similar? I'm looking into slit based spectroscopes and I don't see much options (sanely priced). I used Star Analyser before and now I want something with a slit to target nebulae but also planets, like specific features in Jupiter atmosphere or like sodium emission around Io and so on (and comets if they show up).

[vlaiv]

Do you have access to 3d printer?

Look up low spec spectroscope. It is open source, mostly 3d printed spectroscope. There are several optical components that you need to source (grating, slit and such) - but housing is 3d printed and it is fairly cheap compared to commercial offerings.

[robin_astro]

In the same 3D printed vein, Christian Buil's Starex which can also double as a spectroheliograph in Solex mode (lots of builds of this going on currently supported through forums)

http://www.astrosurf.com/solex/sol-ex-stars-en.html

Also his Uvex which is a superb wide spectral range all reflective design (now also offered commercially by Shelyak), though it can be tricky to align.

https://spectro-uvex.tech/?p=1362&lang=en

http://www.astrosurf.com/buil/UVEX_project_us/

Kits of the special optical parts for these are available from Shelyak

3D printed designs do tend to suffer from poorer stability though compared with the very stable ALPY

Edited April 8, 2022 by robin_astro
added a second uvex link

[robin_astro]

A big chunk of the cost of the ALPY is the guider module (essential) but  the 3D printed module used in the Uvex can be used instead with the critical core module part  from Shelyak

https://spectro-uvex.tech/?p=1820

 

[riklaunim]

Self-made is likely not an option Not that skilled nor a 3D printer.

[Kepler 3]

I have an Alpy 600 with the guiding module and the calibration module. It is low resolution and is a sturdy, good quality piece of equipment but not cheap. I use it with it's 23 micron slit. I use Demetra, which was designed for the Alpy, to do the data reduction. Attaced is a profile of type Be star- 60 Cyg- that I got with the Alpy set up and a 10' Meade SCT with a f/6.4 reducer-Dick

[SteveBz]

On 07/04/2022 at 21:08, riklaunim said:

Anyone here used Alpy 600 or something similar? I'm looking into slit based spectroscopes and I don't see much options (sanely priced). I used Star Analyser before and now I want something with a slit to target nebulae but also planets, like specific features in Jupiter atmosphere or like sodium emission around Io and so on (and comets if they show up).

I agree about the 'Sanely-priced' point.  In fact even with the APLY 600, clocking in at about £2k with the calibration and guiding modules.  If you don't want to tear down your main rig, you have to allow another £1,000 for new cameras and stuff.  I bought a side-by-side bar and mounted a small Newtonian on it sharing the EQ6 that seemed to work, but I only have an SA100, and like you I want to move to a slit-based system.

My SA100 + f/5 system has R<200.  The ALPY 600 is, um, 600 I believe, but I'd like something more like R = 5-10k.  So I was thinking of the Baader Dados which also weighs in at about £2k.  The advantage is that it doesn't absolutely require a guide-cam as it comes with a guiding eyepiece.  I can imagine that might become a bit tiring quite quickly, but it would be a start. 

I also have a small Meade ETX90 and I wondered if I could use this with the Dados.  It has quite a long focal length so it's f/13.8 I think.  The Dados seems to be designed for an f-number of greater than f/10, so I wondered if this would be a good solution.  What do people think?

Kind regards,

Steve.   

[Gasman]

I used my Alpy quite extensively a while back but never had the calibration module as I just used a Relco starter bulb mounted in a frame at the end of my Esprit 120 refractor and it served me well. I gather it's the same bulb used in the calibration module!. I bought my Alpy 600 just before the BAA were offering grants for it so missed out there🤔.

Steve

[Graham Beamson]

Hi,

I purchased an Alpy 600 just recently and have been setting it up, focussing the various parts etc. I have the base module and guide module, but not the calibration module. It has been rather slow because of the short nights and cloud. I was out last night and was able to put a star on the entrance slit (as seen in the output of the guide camera - a ZWO ASI178) and obtain reasonable-looking spectra of Vega, Deneb and Altair. 

However, I’m concerned about two things:

1. My spectra are not centered on the sensor of the spectroscopy camera - they seem to be off-centre by about 1 mm in the non-dispersive direction. The spectroscopy camera is a Lodestar X2.

2. My focus, as seen by the guide camera, seems quite bad and strangely behaved. When I adjust the telescope focus (an 80 mm, f6 refractor) one way I can see a star get bigger as I go away from focus, but going the other way the star gets fainter and disappears. Also, at the best focus I can get the shape of the star is very strange - there is lots of flaring so that it looks a bit like a comet.

I must be doing something wrong - can anyone advise me ?

Thanks, Graham.

[robin_astro]

Hi Graham,

Regarding (2) The procedure is to first adjust the guider system to produce  a sharp image of the slit.  The star should then be brought to focus in the guider. The ALPY 600 guider image shows a lot of coma off axis, particularly with fast telescopes but the star image should be round and tight around  the central region where the slit is. Also, although the focus of the guide camera can be adjusted, for best image quality the camera should also be at the correct focal plane (It is optimised for 17.5mm C mount back focus so best used with a 5mm spacer with 12.5mm back focus cameras for example).

Check that the core module is fully inserted into the guider module. (It should butt up positively against a shoulder) and tighten the 6 screws evenly and progressively.  Is the core module rotated correctly relative to the  guider module?  You need to consider both the orientation of the core module and the guide camera. The  image of the slit can look square in the guide camera but one misalignment can be compensating for  the other.  (The clue if this the the problem is that the guide camera is not square to spectrograph when the slit is square in the image) The instruction manual describes the right orientation.

If you still have a problem feel free to contact me off list.

Cheers

Robin

 

Edited July 3, 2022 by robin_astro
typo

[robin_astro]

3 hours ago, Graham Beamson said:

I can see a star get bigger as I go away from focus, but going the other way the star gets fainter and disappears. Also, at the best focus I can get the shape of the star is very strange - there is lots of flaring so that it looks a bit like a comet.

I assume this is with the star away from the slit ?   With the star on the slit the star will get fainter (and can even disappear, needing an increase in exposure to see the overspill ) as the star comes to focus and more of the light goes through the slit. When in focus and centred on the slit the star image will be split by the slit and look like a "burger"

Edited July 3, 2022 by robin_astro

[robin_astro]

Another affect you can see with mirror slit guiders like the ALPY if the star is over exposed in the guide camera  is a faint out of focus ghost image of the star next to the main image  due to internal reflections in the mirror (Unlike usual astronomical mirrors is is silvered on the back, deliberately to prevent this reflected light  entering the spectrograph) This is not a problem as it is very faint when the star is correctly exposed and does not prevent guiding on the overspill of the main star image on the slit.

[Graham Beamson]

Hi Robin,

Yes, it is with the star away from the slit. On the slit the star almost disappears and does indeed look like a burger. Away from the slit the behaviour of the star as I go through 'optimum' focus is so strange that there must be something wrong. Moving the focuser one way the star becomes a large 'blob' and moving it the other it very quickly gets bigger and so faint that it disappears. At 'optimum' focus there are moving streamers coming out of the star image (some sort of seeing variation ?) and a long exposure gives a triangular star image that looks like a bit like a comet. Unfortunately I didn't record any images to show what was going on.

Even with this strange focus I managed to get some reasonable-looking spectra of Vega, Deneb and Altair.

I've just tried readjusting the guide camera focus and the Alpy rotation relative to the guider, as described in the manual. Maybe I've got a better focus of the guide camera on the slit, so I'll try again the next time I have some stars.

By the way, I've got a lot of dust on the reflective slit that won't come off with a blow duster. As the coating is on the back is it safe to gently wash the front surface with a lens cleaning cloth and lens cleaning solvent ?

Here are my Altair, Deneb and Vega spectra. I'll probably put them on the RSpec forum as well.

Regards, Graham.

[Graham Beamson]

Ah, I've just read your first post - I missed it to begin with. The ASI178 does indeed have 12.5 mm backfocus and playing around with it I discovered I got a better focus with a 5 mm spacer - so by trail and error maybe I've got it about right !

Something else I could do is to try my 120mm f7 refractor - that might give less coma than the 80mm f6. While I was looking at Vega I thought I would try to find M57 but with a 30 second exposure on the guide camera the stars were all so 'mangled' that I gave up on it.

[SteveBz]

6 hours ago, Graham Beamson said:

Hi,

I purchased an Alpy 600 just recently and have been setting it up, focussing the various parts etc. I have the base module and guide module, but not the calibration module. It has been rather slow because of the short nights and cloud. I was out last night and was able to put a star on the entrance slit (as seen in the output of the guide camera - a ZWO ASI178) and obtain reasonable-looking spectra of Vega, Deneb and Altair. 

However, I’m concerned about two things:

1. My spectra are not centered on the sensor of the spectroscopy camera - they seem to be off-centre by about 1 mm in the non-dispersive direction. The spectroscopy camera is a Lodestar X2.

2. My focus, as seen by the guide camera, seems quite bad and strangely behaved. When I adjust the telescope focus (an 80 mm, f6 refractor) one way I can see a star get bigger as I go away from focus, but going the other way the star gets fainter and disappears. Also, at the best focus I can get the shape of the star is very strange - there is lots of flaring so that it looks a bit like a comet.

I must be doing something wrong - can anyone advise me ?

Thanks, Graham.

Hi Graham,

I've eventually decided to go the LowSpec 3.0 route, which is a bit of a nightmare because of learning 3D printing etc.  However, lots of friends are rallying around.

However I also have a LodeStar x2 and a 178mm (QHY rather than ZWO, but same sensor).  I note that the pixels are very different sizes.  The X2 is about 8.x um, whereas the 178 is 2.4 um - a factor of 3 different.  I'm using the 178 as my spectrum camera and a ZWO asi290 mm (2.9 um) for the guide camera.  Then I'm doing imaging with another large format camera (SX H16) and guiding that with the X2.

Everyone says that guiding is less sensitive to sampling than image quality, so you might improve the resolution of your spectra if you switched round X2 for guiding and 178 for spectra.  But I'm not even off the ramps yet!!! What does @robin_astro think?

Steve.

[robin_astro]

4 hours ago, Graham Beamson said:

By the way, I've got a lot of dust on the reflective slit that won't come off with a blow duster. As the coating is on the back is it safe to gently wash the front surface with a lens cleaning cloth and lens cleaning solvent ?

Yes you can clean it no problem, the same way you would clean a lens or eyepiece etc

[robin_astro]

4 hours ago, SteveBz said:

Everyone says that guiding is less sensitive to sampling than image quality, so you might improve the resolution of your spectra if you switched round X2 for guiding and 178 for spectra.  But I'm not even off the ramps yet!!! What does @robin_astro think?

 

I don't know the relative sensitivity/noise levels of these two cameras but pixel size-wise either way round should be ok with the the ALPY and the standard 23um slit. (To avoid undersampling of the spectrum you need to have at least 2 pixels per slit width). If you were to move to a narrower eg 10 or 15um slit though the Lodestar X2 used as a main camera would give an undersampled spectrum so should be avoided. 

Cheers

Robin

[robin_astro]

5 hours ago, Graham Beamson said:

Here are my Altair, Deneb and Vega spectra

The FWHM looks about right. The resolution of the ALPY600 is ~12A and the lines in giant stars like Deneb are narrow enough to estimate the resolution but the Balmer lines of main sequence stars like Altair/Vega are intrinsically wider than this so don't represent the true resolution of the spectrograph. Here is H beta in Vega for example

[robin_astro]

9 minutes ago, robin_astro said:

Here is H beta in Vega for example

Contrast that with H Beta in supergiant star Rigel for example. Although similar temperature to Vega and more massive, it is much larger  so the surface gravity (which reduces as the square of the radius) is much lower so the lines (which are produced in the photosphere at the surface of the star) are less pressure broadened)

[Graham Beamson]

Thank you Robin, Steve,

Some interesting linewidth effects there.

Next week I should have an Atik314 (pixels = 6.5 x 6.5 microns) that I'll use as my spectroscopy camera and I'll use either the Lodestar or the ASI178 as the guide camera. 

I've got field flatteners for my f6 and f7 refractors  - would using one before the Alpy be of any benefit ?

Regards, Graham.