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drjolo

Reflective slits for ATM spectrograph - where to get from

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This is my first post, so hi to all!

I started to build spectrograph. It will be pretty common solution - with 100mm collimator, 50mm camera lens and blazed reflective grating. However I stuck on the slit :( I would like to use reflective slit, so positioning and guiding will be convenient, however it looks that there is not much to choose. I found only 25um reflective photometric slit from Shelyak for 168EUR. Do you know any other sources? I need 25-30um slit. 

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Thanks Eric! This is interesting one. And probably in the guide camera I could see the index of selected slit in the same time :) 

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I already made an order in OVIO and now waiting for shipping cost estimation.

@Merlin66 - I have seen your site and also studied L200 construction - very nice job. I want to start with classic design and compact instrument to learn some skills in this area. Then I maybe go for Littrow design to reach higher resolution. Do you still manufacture Spectra-L200 kits?

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Thanks!

Unfortunately the Spectra-L200 is no longer in production.

 

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An old thread but perhaps worth a repost: I scavenged some nice front surface mirrors (presumably alu on glass) from an old scanner/printer. There were 5 mirrors!

I tried making reflective slits by cutting the alu lightly with a hobby knife along a straight edge.

Testing with a laser pointer and measuring the diffration pattern indicated that light scoring made a 23um slit, while heavier pressure with a dull tip made about 50um slit.

Looking at the slit under  a magnifier, it looks nice and straight. So will try it in my home built transmission scope (uses old pentax 50mm and 28mm lenses and a piece of 1000lpmm plastic transmission grating but the home made pencil sharpener slit was not even and not reflective)..

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Posted (edited)

How does it look from the back?  (Mirror slits are normally back surfaced so you dont get ghost spectra from internal reflections reflected into  the spectrograph)

Cheers

Robin

Edited by robin_astro

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Yes, I was thinking about that the other day. I guess the internal reflection would be at least 4-6% so no good if one is trying to achieve a decent S/N. I will have a look. I've been concentrating my spare time on finishing the figuring of a big telescope mirror which has been lingering for a couple of years.

Multiple images in the guiding camera are not such a big problem I suppose, especially if they are only faint.

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As Robin has said, the reflection only affects the guide image. Only usually a problem with very bright stars.

 

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Posted (edited)
17 hours ago, hamishbarker said:

Yes, I was thinking about that the other day. I guess the internal reflection would be at least 4-6% so no good if one is trying to achieve a decent S/N. I will have a look. I've been concentrating my spare time on finishing the figuring of a big telescope mirror which has been lingering for a couple of years.

Multiple images in the guiding camera are not such a big problem I suppose, especially if they are only faint.

The loss of light from internal reflection is insignifcant compared to other losses but that is not the problem with the front surface mirror you are proposing to use. It is important that reflective coated surface with the slit engraved in it is on the face towards the spectrograph not towards the telescope (ie mirror slits are back surface, not front surface mirrors) otherwise you risk ghost spectra from the internal reflections.   The back coated slit gives weak ghosts in the guider image but as  Ken pointed out these are not a significant problem.  

Cheers

Robin

Edited by robin_astro
clarity

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