Forum der Vereinigung der Sternfreunde

Grüße an alle.

Ich interessiere mich für stellar und Sonnenspektroskopie und dachte, dass ich werde ein neues Thema "Spektroheliographie/Spektroskopie" im Sonne-Forum beginnen. Ich bin ein Englischer Sprecher, aber Deutsch kann ich gut verstehen. Wahrscheinlich, die meisten meiner Beiträge werden in Englisch sein. Sie sind willkommen in Deutsch antworten.

Ich habe beobachtet die Sonne mit einem Spektroheliograph ich im Jahr 2013 gebaut. Fotos in dieser Links zeigen das Instrument:
http://www.pbase.com/p_zetner/image/151819182
http://www.pbase.com/p_zetner/image/151819181
http://www.pbase.com/p_zetner/image/152202602/original

Hier sind vier Sonnen Bilder im Lichte der vier Fraunhoferlinien; Ca II H3 (3968.5 A), H beta (4861.3 A), He D3 (5875.6 A) und H alpha (6562.8 A). Diese geben vier Ansichten der Chromosphäre.



Jetzt kommt Englisch!

Because the spectroheliograph (SHG) is a grating instrument, it is easily tuneable to any wavelength of choice. In the present configuration, I achieve a resolution of ~ 0.2 A. The images below show a series of spectroheliograms taken by "stepping" the SHG, in small wavelength increments, through the broad Ca II (H) and H alpha absorption lines. Physically, as we tune from the absorption line centre outward to the line wings, we observe the disk as it appears at different depths in the solar atmosphere. The farther from line centre, the deeper into the atmosphere we look. At line centre of strong absorption lines, we observe pure upper chromosphere and, as we move into the line wings, the photosphere comes into view. By tuning through strong lines with the SHG we are accomplishing solar "tomography".




There is further complication to this simple picture! Some solar features (especially filaments and prominences) have large line-of-sight velocity components with respect to the Earth. This contributes a Doppler shift to the absorption or emission lines associated with these features. I have measured some of these velocities in earlier observations and will discuss them further in future posts.
Here, for example, is a study of prominences observed in Ca II K light. The prominence shape changes strongly from the red side to blue side of line centre because of the different velocity components in the prominence plasma.

http://www.pbase.com/p_zetner/image/160940150

Ich hoffe, das für Sie von Interesse ist, und ich hoffe, Ihr Feedback zu hören.

Grüße.
Peter.

Peter, das ist sehr spannend, Vor allem, dass Du Radialgeschwindigkeiten sehen kannst und viel andere Physik auf der Sonne. Hast Du Lust, darüber einen Artikel zu schreiben, Dein Instrument zu zeigen und die Möglichkeiten (also genauso wie Du es hier machst)? Es gibt die Zeitschrift SONNE der deutschen Sonnenbeobachter. Hier kannst Du Dir eine Ausgabe anschauen (alle Hefte können als pdf heruntergeladen werden): http://vds-sonne.de/Archiv/Sonne/so137.pdf
Im Frühjahr 2017 (wahrscheinlich April) erscheint SONNE 140. Wir feiern damit 40 Jahre SONNE. Dort würde Dein Artikel wunderbar hineinpassen. Du kannst ihn gerne in Englisch schreiben, wenn Du möchtest. Wenn Du Deutsch schreiben möchtest, kann ich die kleinen Fehler korrigieren.

Ich würde mich riesig freuen und grüße Dich herzlich
Heinz

Hallo Peter,

ich hätte zwei Fragen zu deinem Aufbau:
1. Welche Auflösung R erreichst du?
2. Wie lange benötigst du für eine komplette (räumliche) Aufnahme der Sonne?

Schöne Grüße,

Gerrit

Hallo Heinz.

Vielen Dank. Das interessiert mich sehr so ein artikel zu schreiben, aber, fur mich, etwas so technisches wird viel schneller in Englisch gehen. Vielleicht kannst du dann übersetzen? "Sonne" scheint eine sehr interessante und gut geschriebene Zeitschrift zu sein.


Hallo Gerrit.

Hier habe ich ein Hg-Spektrum (gelbes Dublett) hochgeladen. Mit einer Linienbreite von 0,25 A erreiche ich R ~ 23000. (Für Optik-Konfiguration: 500mmf Kollimatorlinse; 25 um x 18mm Schlitz; 25mm x 50mm, 2400 l / mm Gitter und 400mmf Kameraobjektiv).

Im Betrieb wird das SHG fixiert und die Sonne durch das "field of view" driftet. Ein vollständiges Bild der Sonnenscheibe erfordert die Sonnenscheibe seinen eigenen Durchmesser zu reisen. Das dauert ~ 128 Sekunden.


Schöne Grüße.
Peter.

Lieber Peter,
ein Artikel für SONNE von Dir wäre wunderbar. Englisch verstehen alle. Das muss nicht übersetzt werden. Darf ich Dich einladen? Bitte fange an zu schreiben. Ich freue mich auf Deinen Artikel.
Alles Gute
Heinz

Hi Peter, that is also highly interesting for the spectroscopy section (http://forum.vdsastro.de/viewforum.php?f=38)! Maybe you can introduce your work in our forum, too, and write an article for our section journal SPEKTRUM (http://spektroskopie.fg-vds.de/index_e.htm => Our Journal - would be best in English). Or we re-use the article written for SONNE. In any case, we would highly appreciate your contribution.
Cheers, Thomas

Hi Peter and Thomas, double-publication of the article in SPEKTRUM and SONNE ist fine with the Sonne-group. Excellent idea to spread the message in both journals, for a broader readership.
Best wishes
Heinz

Thanks a lot, Heinz! The spectroscopy section lacks observers of the sun. Only Dieter Goretzki does respective measurements (you can find it in some issues of our journal). Maybe such an exchange motivates some spectroscopists to go solar.
Cheers, Thomas

Hi Thomas and Heinz.
Thomas, thank you for the invitation. I will give some thought to writing the article in two parts with some overlap but the second part more focused on spectroscopy. Maybe this would be a good way to address the interests of the readership in both forums.
Kind Regards.
Peter.

... and it would support the visibility and connection between both sections. Very well, Peter!
Cheers, Thomas

Peter, your plans go the right way. The collaborative aspects, I love them. Thank you very much, indeed.

Hallo zusammen,

nachdem ich aus der Schweiz wieder zurück nach Norddeutschland gezogen bin, habe ich nun allerdings schlechtere Bedingungen wegen erhöhter Nachthimmelshelligkeit durch heftige Beleuchtung und naher Bebauung. Meine Schwerpunkte sind die Veränderlichenbeobachtung und die Spektroskopie. Wegen der kurzen Sommernächte wende ich mich aber mehr der Sonne zu, die ich vom Frühjahr bis Herbst ganz gut beobachten kann; danach ist zwangsweise Winterpause in dem Thema.
Letzten Sommer habe ich erste Versuche mit der Spektroheliografie unternommen. Die Ergebnisse sind aber noch heftig suboptimal. Als Aufnahmegerät benutze ich einen 3Zoll-Refraktor mit dem LHIRES (2400 L/mm, Spalt 15 µm) und der QHY5-II. Die empfangenen Daten werden mit der französischen Software IRIS bearbeitet. Diese kann die aufgenommene avi-Datei in eine Serie von fits-Dateien umwandeln. Da ist aber je nach Auflösung der Kamera die Anzahl der erstellten fits-Dateien begrenzt. Hier wäre demnach eine mächtigere Software angebracht. Mit welcher Software arbeitest du, Peter? Die Befehle zum Zusammenfügen der fits-Dateien zu einem neuen Gesamtbild arbeiten aber gut.
Ich hänge einmal zwei Beispiele an, die nach der Durchlaufmethode entstanden sind (Sonne wandert bei abgeschalteter Nachführung durch das Bildfeld).
Das H-alpha-Bild wurde mit höherer Auflösung gemacht und anschließend in der x-Achse mit dem Faktor 2 gestaucht. Nur der Rot-Kanal wurde ausgewertet. Das Segment ist aber noch nicht gleichförmig rund. Die kleinen Randausfransungen könnten vom schlechten Seeing kommen. Die Software beendete die Konvertierung nach 577 Bildstreifen.
Das Ca-H-Bild wurde mit der Auflösung 640 x 480 Pixel der Kamera aufgenommen und anschließend in der x-Achse mit dem Faktor 2 gestaucht. Der Sonnenrand ist aber auch noch nicht gleichförmig rund. Hier hat die Software die gesamte avi-Datei konvertiert.

Wie bekomme ich den Sonnenrand gleichförmig rund? Wie bekomme ich ein gleichmäßiges (glattes) Bild?

Schöne Grüße, Jörg!

Leider funktioniert das Hochladen der jpg-Dateien (50 kb) nicht. Ich bekomme zwar das kleine Fenster, in dem eifrige Aktivität angezeigt wird plus Kommentar: "Die Datei wird derzeit hochgeladen". Aber auch nach drei Minuten sieht das noch so aus. Gleichzeitig wechselt Hintergrund auf die Anmeldeseite. Ich komme ebenfalls auf die Anmeldeseite, wenn ich den Beitrag Absenden will.
Ich habe mich also nicht abgemeldet, sondern gleich erneut angemeldet, was auch geklappt hat. Mit dem Rückwärtspfeil des Browsers kamm ich wieder auf die Texterstellungsseite. Der Text war aber weg. Zum Glück hatte ich ihn kopiert. Nun funktioniert auch das Hinzufügen der Dateien. Merkwürdig!!!!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Welcome to the world of the spectroheliograph!!
In my recent book " Imaging Sunlight using a digital spectroheliograph" I review the currently available processing software.
The favourite at the moment seems to be Wah!s SLiM (Spectral line merge)
This is available for download over on the solar chat forum library
https://solarchatforum.com/viewtopic.php?f=7&t=21608

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Thank you very much for the link. I tried the software but it turned out that it was unable to process files with more than 2 GB, neither with Vista nor with Win7. See the attached message. Choosing "Weiter" doesn't make any sense, the program is halted anyway.

@Peter
Which software do you use for merging and possible geometrical corrections?

Schöne Grüße,
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Jorg,
The 2Gb limit can be overcome by first cropping your AVI video to a region only 100 pixel wide or so at your target wavelength by using Virtual Dub. This is explained in "Imaging Sunlight", p 155.
SLiM does a good job tilt/ curvature correction.

the latest version of BASS Project
https://uk.groups.yahoo.com/neo/groups/astrobodger/info
has a SHG processing option which will handle larger files.... See p.161

Final corrections to the aspect ratio to improve the shape of the solar disk can be done in PaintShopPro (recommended!) or PS.
Hope this helps.

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Wah! has also prepared a write up on his SHG processing....
This is unfortunately too large to upload!!
If you send me an email address I'll forward a copy (1.2Mb)
Ken

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Jorg,
Did the Wah! document help you?

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Hello Ken,

thank you for the document. Yes, it was really useful especially with VirtualDub. I think the chapter about Wah!'s own program needs an update, because the items dealing with geometrical correction have been revised and updated and got new labels.
But VirtualDub is really great. After the treatment of a 9 GB avi-file (1280 x 960 pix ---> 100 x 960 pix) IRIS was able to handle the file and did a good merging job. Thus I could apply the geometrical correction commands of IRIS (Ver. 5.59).
In the end all my results point out that I have to take special care of the image acquisition.

Schöne Grüße,
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Hi there,

when using SLiM by Wah! a strange error occurs. If I use 'Left/Right Skew' or 'Up/Down Skew' with an avi-file cropped by VirtualDub and try to use 'Remove Curve' too the spectrum gets skewed in itself (see attached screenshot). Any suggestions?

Greetings Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Jorg,
Hmmmmm......
I usually work with the absorption line horizontal....

Going back to Wah! recent comments on the latest upgrade:
""
If you use the mono 8bit or 16bit SER file format to capture the video, it should be able to support without problem (I hope)
I was too lazy to implement the code for 16bit SER vertical mode, please make the spectral lines horizontally while using 16bit SER
""
Could it be this issue????

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Well, let's have a look!

Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Hi Jorg.

My apologies for not responding earlier. Congratulations on your first efforts in spectroheliography ... very promising! I see you have been making progress with software recommendations of Ken Harrison. Here is a brief description of the workflow I use to convert the avi into a solar image.

The video file is loaded into the VirtualDub video editing software in step 1.

In step 2, each frame of the video is cropped to isolate the single spectral line of interest. In most instruments, depending largely on the speed of the optics, the recorded spectral "lines" have some curvature associated with them ("smile") and may be rotated somewhat in the video frame. Step 2, therefore, generally involves video frame image processing in addition to the cropping. Fortunately, VirtualDub has a number of video filters available which can be used to remove smile and rotation from each video frame. I have found the "General Quadrilateral Transform", "Barrel Distortion" and "Rotate2" filters to be especially useful. Once the spectral lines have been straightened by filtering, the cropping can be applied. Most VDub video filters have a cropping feature. I simply use the "Brightness Contrast" filter, without altering brightness or contrast, and introduce the cropping here.

Once each frame has been suitably cropped, the cropped frames need to be assembled (concatenated) to produce the solar disk image. In my processing workflow, step 3 involves exporting the cropped frames, as a sequence of separate images, to a folder using the VirtualDub "Export" command. Assembly of these individual images into a single image is carried out with ImageJ. The commands "Import > Image Sequence" followed by "Image > Stacks > Make Montage" achieve the (nearly!) desired result.

The image resulting from concatenation of the cropped video frames in step 3 is, invariably, elongated by an amount dependent on the camera frame rate and the chosen pixel width of the crop. The elongation can be easily corrected (assuming you have captured the entire solar disk) by adjusting the aspect ratio in any image processing software (eg ImageJ or Adobe Photoshop).

Step 4 involves this aspect ratio correction along with other image enhancements like contrast and sharpness adjustments. At this point, it is also desirable to remove the transversalium lines associated with dust and irregularities on the entrance slit. This can be painstaking and difficult, depending on the severity of the problem, but can be accomplished by various filtering actions. I use a technique somewhat similar to that described by Rousselle (http://www.astrosurf.com/spectrohelio/t ... shg-en.php).

I hope this is useful and wish you continued success with your imaging!
Peter.

(==>) Ken
Horizontal spectral lines not tested yet.

(==>) Peter
Thank you for your explanation. As I couldn't find two of the filters in VD I tried to get them from the net. I found a download link for "VirtualDub Filter Pack 2014 v1.3" but it didn't work because the file could not be found. Is there any chance to get this package?

Hope I can take new videos in March when the sun is somewhat higher in the sky and will look over the nearby trees.

Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Jorg,
The filter pack by default installs to the Program Files directory...under windows 10 you need to change this to the Vdub folder which is located under program files (x86)
Unfortunately it doesn't include the filters mentioned by Peter!!

These can be found here :
http://www.infognition.com/VirtualDubFi ... fects.html
(scroll down the page)

Also the free transform and divide functions used by Wah! are only available in CS5 and newer versions of Photoshop.
Ken

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Hi Jorg and Ken.

Here are links (active as of today; 22Feb2017) to VDub filters I have found valuable to my SHG image processing.

The filter pack available from http://emiliano.deepabyss.org/ is extremely useful to me. I always use "Grid" (for visual alignment / straightening of spectral lines) in combination with "Barrel Distortion" (to remove spectral line curvature -- "smile"). Lately, I have been using the "Canvas Size" filter as well. I have had to be careful that my anti-virus software (Norton) doesn't quarantine the filters that I extract from the zipped folder of filters. I've tried to place extracted filter files in the VDub plugin folder only to find that they have disappeared! There doesn't appear to be any danger of virus but, because the community of users is small, Norton perceives these files as a potential threat. Norton allows me to restore a quarantined file.

The Barrel Distortion filter is available to videographers to alleviate either barrel or pincushion distortion introduced by the video camera lens. Photoshop offers a similar "lens correction" filter but for single images only. SHG imaging requires correction at the video level, across all frames of the video. In practice, there are two adjustable parameters (alpha and beta) in the filter which I adjust while viewing a preview video frame with the grid overlay in place. The correction is somewhat subjective in this sense ... no mathematical "best fit" is involved. My latest SHG work has made use of in-camera cropping to isolate a region of interest (ROI) and boost the camera frame rate. Over a limited ROI, the Barrel Distortion filter may not work well because of the nature of the interpolation curve (unknown to me) used to effect the geometrical transformation. For curved spectral lines in such smaller regions of interest, I find the Canvas Size filter to be useful in combination with Barrel Distortion. I can introduce a larger canvas around the small ROI in such a way that the "smile" effect in the ROI more closely approximates a portion of the barrel or pincushion distortion across the full canvas. With some experimentation (ROI placement and relative size within the canvas), line curvature can be satisfactorily corrected by the filter and cropping is then used to get rid of the excess canvas.

In my earlier attempts to straighten spectral lines, I made use of the "General Quadrilateral Transform Filter".
A description of its action is here: http://software.acobase.com/video/quadrilateral.html
However, the filter file downloadable at this site doesn't appear to work. It crashes VDub! (I haven't tried this download lately.)
A corrected version of the file is available here: http://forum.doom9.org/showthread.php?t=165912
(downloadable at post #20 in the forum thread: forum.doom9.org/attachment.php?attachmentid=13011&d=1349682681)
Despite the somewhat worrisome name of the forum (!) the downloadable filter file seems to be virus free.

In truth, I don't use the Quadrilateral filter too often. I've found that I can straighten curved spectral lines effectively and consistently with the Barrel Distortion filter.

Hopefully, this is helpful to you.

Cheers.
Peter.

Hi Peter and Ken,

I found Peter's explanations really helpful and tried to make myself familiar with this distortion filter. Well, I'll see ......
At last I tried Wha!'s program in the horizontal mode, as Ken had suggested, and it worked fine. The result isn't that good because of bad aquisition conditions.

Cheers,
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Jorg,
That looks very promising....
Onwards and Upwards.

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Hi Jörg,

as I already said to Peter, maybe you can write an article about your experience for our section journal SPEKTRUM (http://spektroskopie.fg-vds.de/index_e.htm => Our Journal - would be best in English). Here is the descriptive artikel about SPEKTRUM (german and english versions) to get an idea about it.
German: http://stsci.de/pdf/Artikel_Spektrum.pdf
English: http://stsci.de/pdf/SpektrumEng.pdf

Cheers, Thomas

I also have a spreadsheet for the digital spectroheliograph design and performance - SimSpec SHG Feedback/ comments welcome.

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Ken,
thanks for the spreadsheet. As I use a Littrow I can't fill in Camera-Distance to grating. Is it OK when I put the value of Camera-Focal length in this field too?
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Jorg,
The Camera Distance in a Littrow is the distance between the Collimating lens and the grating. If in doubt, stick with 30 to 40mm.
Hope that helps.

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Hi Ken and Peter.
Finally got the chance to take some avi-files of the sun. The results are below.
Before the acquisition I took care that the sun moves perpendicular to the slit. I managed this by letting the sun move parallel to the slit in declination. A sunspot was really helpful in this case. Afterwards I aligned the spectral lines in an upright position by turning the camera (snapshot.jpg; the dark horizontal line is a sunspot). I found that these provisions decrease the distortions of the raw picture.
1. After all geometrical corrections with the software fitswork the sun disc istn't perfectly round, especially in the upper left part. I saw that this appears in some of Peter's pictures too. I don't no why.
2. Are the tiny sawteeth (seeing.jpg; magnified snippet) a product of bad seeing? I've attached two frames of the avi-file (rot192.jpg, rot195.jpg) where you can see that one is really fuzzy at the lower part.

Cheers.
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Jorg,
based on my experiences with the long focus Littrow SHG, the "edge effects" can be caused by a few issues:
1. General seeing and wind vibration causing the image to shift slightly during the exposure. Not much you can do about it....the answer I think is faster image acquisition, hence my ongoing trials with the fast frame (500fps) ASI 17MM and a wobble mirror.
2. Slight drifting in dec during the exposure. This can cause a "diagonal" shift of the image which then needs to be corrected. The impact is seen more at the edges.
3. Collimation and alignment of the solar image to the entrance slit. One edge of the solar disk may be better focused on the slit gap than the other. Should be verified and checked.
4. Long slit aberrations - ??? Imaging at a distance from the optical axis.....
5. The spectrum should be horizontal to the CCD chip (no obvious tilt) and the Littrow slant corrected during processing. This should give a cleaner top/ bottom edge to the individual extracted images.
Hope this helps.

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Peter, your article has been published in SONNE 140. You can download the issue as a pdf from this site (click on cover of SONNE 140 in the upper left margin of the page): http://www.vds-sonne.de/

Thank you very much for part 1. We look forward to part 2 of your article.

Best wishes
Heinz

Hi Peter,

I really enjoyed reading your article. Great stuff!

Cheers,
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Ken's post (30.März.2017)

Ken,
I think a (my) Littrow isn't first choice for SHG. The image is not of uniform sharpness along the slit. Can be best seen, when focusing on a star at different positions.

Cheers,
Jörg!

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Wer hat's gemacht? .... Die Schweizer ...

Thank you Heinz and Jorg!

Heinz, you have done a fine job with Sonne. Nr. 140 is a very nice issue.

I have recently "re-configured" my spectroheliograph.
Here are two solar images as well as a picture of the instrument.

Alles Gute.
Peter.

Hi Peter,

what did you change in your SHG setup?
By the way, I took a SHG-image at the same day. As you can see, my slit broke at the beginning of the session, because I didn't limit the entrance aperture of my 3"refractor to 50 mm and used it without any filter.
Can anyone give me a hint, why the attached image isn't as sharp in the upper third as in the lower third?

Best regards,
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

Peter explains he has changed to a 4" SCT telescope here:
https://solarchatforum.com/viewtopic.php?f=8&t=22752

I use a 100mm refractor with no ERF on my SHG - no issues.....

Don't know about the image softness - seeing or focus issue??

_________________
" Astronomical Spectroscopy for Amateurs" - Springer

Hi Jörg.
The major change introduced into the instrument was replacement of the 1000mmf f/10 Astro-Rubinar telephoto lens by the Celestron 4SE Maksutov-Cassegrain telescope. I attached a motorized focuser which makes imaging much, much simpler than with the Astro-Rubinar! Also, I decided not to use a front-mounted uv-ir cut filter as I used with the Astro-Rubinar (cemented achromatic lenses near the focus). I also replaced a 400mmf camera lens (200mm Pentax Takumar with Komura 2x Tele-extender) with Takumar 300mmf telephoto lens. This reduces dispersion and image scale a little but, overall, there is less glass in the optical path and better uv transmission.

I'm not sure what to say about your focus problem. Is it possible that the slit does not lie perfectly in the telescope focal plane? A slight tilt out of the focal plane would give this effect, no?

Alles gute.
Peter.

Hi Peter and Ken,

thank you for your explanations and hints. No ERF on 100 mm objectives ..... are you using metal slits? If not, it can't be the heat alone, that broke my slit. The four slits of my spec are evaporated onto a glass plate. See attached photo with the damaged plate. When I looked at it from the side (next picture) I saw that the top plate that keeps the slit plate in place is slightly bent. This may put pressure onto the left and right side of the glass plate. Together with the heat of the sun it might have broken the plate????
So I turned the plate 90° clockwise put flat washers left and right and will try it without ERF or something like that on my 3" and 4" refractor.

Best regards,
Jörg!

_________________
Wer hat's gemacht? .... Die Schweizer ...

@Peter and Ken and all the other amateurs who use SHG setups without ERF

Which slit constructions do you use and what are the slits made of?
The construction of my spec-slit can be clearly seen in my post (25.06.2017). The reflective surface is chromium, chemichally deposited on the glass.

Here is the answer of the LhiresIII-manufacturer to the broken slit affair:
"By the way, if the slit is broken, it is not totally flat anymore, and it can explain (at least partially) why the spectrum is not well focused all accross the slit, no ? Can you tell me if you already had the same issue before the slit is broken ?
The bending of the top plate is normal : it is made in a spring material, and its function is to press the slit holder (the slit holder is flat, and the part behind also). There is no reason that this effort - which is normal - breaks the slit (except if there is a small piece (like dust) below the slit holder. But if this is the case, the slit would have been broken for a long time.
Also, the glue we use to stick the glass on its holder is made for optics, and can tolerate some differential dilatation (it is a little bit flexible).
Yes, the chromium is a metal, then it can heat a lot. But the glass can resist to a very high temperature... Anyway, I think that all elements we have in hand say that the slit has been broken by the Sun exposition - no ?"

Best regards,
Jörg!

Jorg,
I have documented many SHG designs in my recent book "Imaging Sunlight - using an SHG"
Most slit assemblies are based on metal slits - from copper to stainless steel. None are based on glass reflective slits.
Having said that, I'm also aware of newer SHG using reflective glass slit plates (I have one on my 100mm Littrow SHG)
The "limitation" seems to be the ability to vary the gap - much easier with metal slits.
Hope this helps.
Ken

_________________
" Astronomical Spectroscopy for Amateurs" - Springer