The Lowspec spectrometer

[Paul Gerlach]

Remember that the guide lens is at a distance of about 71.6 mm from the slit. So 71.6 / 8.5 = 8.4
Thus it can accept a f/8 light cone. Or do I make an error here?

[Merlin66]

Paul,

That's correct.....

If you use the instrument at <f8.5 you'll get severe vignetting at the edges of the guide FOV. May not be a show stopper, depends on the guide camera and the telescope focal length.

 

[Paul Gerlach]

Well, it my opinion that the LOWSPEC should not be used with a telescope that is faster than f/8. I've seen the focussing issues at the outer edges of the full spectrum at low resolution (300 l/mm grating) and at 'high' resolution (1800 l/mm) it make no sense to use a telescope faster than f/10. The required width of the grating should be 26.8 mm at f/10 according to SIMSPEC which means that a 25 x 25 mm grating will not capture all the light from the collimator, That's why I included a 30 x 30 mm alternative grating.

[Bajastro]

Yes, It will be vignette, but Low Spec is optimized for f/10 telescopes. I use f/4 Newtonian telescope and cone of light is cutted on the collimating lens holder. I'm planning to buy a Barlow lens 2x to get the telescope's longer focal length. Maybe in the future I will have SCT, which it should be better for slit spectroscopy.

[Thalestris24]

5 minutes ago, Merlin66 said:

Louise,

Think of the reflective slit plate as a on- axis guider or a star diagonal. All it does is reflect the FOV at the slit position.

The transfer lens - depending on the spacing between the slit plate and the final guide image then acts as a reducer - giving a slightly larger FOV in the camera. As far as I can see the reduction is x0.72.

The actual guide FOV will depend on the area of the slit plate visible to the guider, the reduction factor and the guide camera chip size.

In my case the slit plate has a front cover, with an aperture of 8 x 6 mm, centred on the slit gap. This effectively limits the actual area of the sky visible ( the camera FOV is much larger, but obviously blocked by the cover plate!)

In my C11 @ f10 I get 9.8 x 7.4 arc min coverage.

I can easily put a target star close to centre in the 60/220 eFinder and find the same field in the spectrograph guider. If you use PHD2 you can set up the virtual slit and nudge the target into/onto the slit gap.

Much easier than you think.

 

Hi Ken

I was looking at it from the point of view of being able to plate solve a view and so get the star on or near the slit. The side-by-side pylon pics show the actual comparison. It won't be that easy in practice unless my e-finder is accurately lined up with the Lowspec guide image - even quite a small deviation will make it difficult, if not impossible, to find the target star. It also won't be that easy to maintain the alignment.

Louise

[drjolo]

53 minutes ago, Paul Gerlach said:

Great work @drjolo ! Unfortunately I still don't have my own setup (for a couple of years now 🙄) so I can't test these things myself.
By that 'small holding ring'you mean that little protruding edge (see picture red circle)?
The inner diameter of the guide lens holder is 12.75 mm and the lens itself has a diameter of 12.7 mm. This tolerance is a bit of guess work. I can set my printer at 12.7mm and it will fit but every 3D printer is different. The lens has to be pushed in until it hits this small protruding edge. The flange on the holder should help in placing the lens flat against the edge of the opening in the main body. Is the holder a tight fit in your case or is there still some space for the lens to move around? If this little edge turns out to be to small for some printers, maybe I should enlarge it a bit. If I double the edge the opening will be 12 mm which means it could accept a f/6 light cone.

The lens fit perfectly into the holder inner diameter, so this dimension is okay. But (at least in my case) this protruding edge was too thin, and was not able to stop lens in the stable position. Lens fits tight to the 12.75mm hole, but it tilts and wobbles at this thin edge. I made this edge a little bit larger, so the aperture is now 11mm, printed it with 0.12mm layer height and it seems to be much more stable now. 

[Thalestris24]

8 minutes ago, Paul Gerlach said:

Well, it my opinion that the LOWSPEC should not be used with a telescope that is faster than f/8. I've seen the focussing issues at the outer edges of the full spectrum at low resolution (300 l/mm grating) and at 'high' resolution (1800 l/mm) it make no sense to use a telescope faster than f/10. The required width of the grating should be 26.8 mm at f/10 according to SIMSPEC which means that a 25 x 25 mm grating will not capture all the light from the collimator, That's why I included a 30 x 30 mm alternative grating.

Mine is only F7...  

Louise

[Bajastro]

@Thalestris24 You and me have a loss of light 😆
You can improve parallelism your telescopes during the day with using very far terrestrial objects (over 0.5 km), eventually Moon. It isn't difficult.

[Merlin66]

Louise,

I think you are worrying unnecessarily.....

You'll find it easy to centre the star in the guide slit plate - trust me!

Your LowSpec will work at f7 and give acceptable results - My Spectra-L200 was designed for a maximum of f7 but I have used it many times with scopes down to f5 and still obtained useable results.....

I think you'll be pleasantly surprised when you get your first stellar spectra! Just remember to start with an A type star......

 

[Paul Gerlach]

Louise,

Ken is absolutely right! Don't let the fact that your telescope is not a 'correct fit' for the LOWSPEC put you of.

[Paul Gerlach]

11 minutes ago, drjolo said:

The lens fit perfectly into the holder inner diameter, so this dimension is okay. But (at least in my case) this protruding edge was too thin, and was not able to stop lens in the stable position. Lens fits tight to the 12.75mm hole, but it tilts and wobbles at this thin edge. I made this edge a little bit larger, so the aperture is now 11mm, printed it with 0.12mm layer height and it seems to be much more stable now. 

Okay, well my nozzle is 0.35 mm and my Felix Pro 3 printer is maybe a bit more precise in the x-y direction.  Good point. I will leave the inner diameter at 17.5 mm but will extend that edge more so that all printer can actually print it.

[Merlin66]

Marius,

Re using a barlow...

Caution, as you know the magnification varies with the back focus distance, when adding the spectrograph the slit plate will be probably further back than normal (with an eyepiece) and the magnification higher.

Also consider how to give the instrument some support - to maintain the alignment on the optical axis.

I occasionally use a x2 Powermate (with my Genesis f5) and use the T thread adaptor to improve rigidity.

On the C11 I added a support bar to hold and align my Spectra-L200.

 

[Thalestris24]

5 minutes ago, Merlin66 said:

Louise,

I think you are worrying unnecessarily.....

You'll find it easy to centre the star in the guide slit plate - trust me!

Your LowSpec will work at f7 and give acceptable results - My Spectra-L200 was designed for a maximum of f7 but I have used it many times with scopes down to f5 and still obtained useable results.....

I think you'll be pleasantly surprised when you get your first stellar spectra! Just remember to start with an A type star......

 

Hi Ken

It may not be difficult if the target star is visible in the guide camera. If it isn't, it will be impossible!. It took quite a lot of effort to get the e-finder and imaging scope aligned well enough to get the Coma Berenices stars in the two fields of view. If I happen to lose that alignment after dark then that will be end of session...

Louise

[Merlin66]

The target star just needs to be visible in the guide FOV, not necessarily spot on the middle.....you do have a lot of lee way......

Your guider field of view will be much larger than mine!!

When you try it for the first time I'm sure you know what I mean.

 

Edited January 19, 2020 by Merlin66

[Bajastro]

@Merlin66 I'm a bit afraid of that. My focuser isn't high technology, it is cheap China product.

It slightly moves when I change a position of my telescope, but the guiding works well.

Edited January 19, 2020 by Bajastro

[Merlin66]

Marius,

Hmmm

It unfortunately will be worse when/ if you add a Barlow. You may be able to rig up a support from the end of the dovetail????

[Thalestris24]

4 minutes ago, Merlin66 said:

The target star just needs to be visible in the guide FOV, not necessarily spot on the middle.....you do have a lot of lee way......

When you try it for the first time I'm sure you know what I mean.

 

Well, it also has to be identifiable, so ideally needs to be seen (by me) as part of a pattern. The images I took of the Coma Berenices cluster above are quite thin on the ground in terms of stars with a small fov. I calculated the e-finder fov to be 26.4 x 21.2' and the guide cam was 20.5' x 15.37. Quite a small area of sky (for someone used to imaging dso's). I guess the side-by-side pylon pics show the reducing effect of the Lowspec guide lens.

Louise

[Merlin66]

OK.....

What about adding a x0.5 reducer to the front of your eFinder camera?

This would increase the finder FOV to approx 54 x 42 arc min - larger than the full moon.......

 

[Thalestris24]

1 minute ago, Merlin66 said:

OK.....

What about adding a x0.5 reducer to the front of your eFinder camera?

This would increase the finder FOV to approx 54 x 42 arc min - larger than the full moon.......

 

Oh, the finder has a much bigger fov at full resolution (1.06 deg x 0.7 deg) but I did the final comparison/alignment at a similar roi to what's seen by the guide cam since both then show a very similar fov. Also, you have to remember my scope is in the living room which means I can't go anywhere near it when I'm normally guiding/imaging. If I just stand up it messes up the guiding!

Louise

[Thalestris24]

27 minutes ago, Merlin66 said:

Marius,

Hmmm

It unfortunately will be worse when/ if you add a Barlow. You may be able to rig up a support from the end of the dovetail????

😮 

[Merlin66]

Louise,

based on your numbers  on the eFinder I shows the relative guider/ finder FOV......

 

 

Edited January 19, 2020 by Merlin66

[Thalestris24]

11 minutes ago, Thalestris24 said:

😮 

Yep, yours is bigger than mine, Ken! Ha ha

That's the window I image through... It's not very big...

Louise

 

[Jagho]

1 hour ago, Paul Gerlach said:

Well, it my opinion that the LOWSPEC should not be used with a telescope that is faster than f/8. I've seen the focussing issues at the outer edges of the full spectrum at low resolution (300 l/mm grating) and at 'high' resolution (1800 l/mm) it make no sense to use a telescope faster than f/10. The required width of the grating should be 26.8 mm at f/10 according to SIMSPEC which means that a 25 x 25 mm grating will not capture all the light from the collimator, That's why I included a 30 x 30 mm alternative grating.

Paul, what is the "Collimator/Camera -Total angle" ?  -> 56° ?

[Thalestris24]

1 hour ago, Merlin66 said:

Louise,

based on your numbers  on the eFinder I shows the relative guider/ finder FOV......

 

 

See my post above, Ken. It's not the e-finder fov that is itself the problem, it's really my guide scope fov that's difficult. Maybe I should look at changing it but anything with bigger pixels and the same qe will be expensive, I think.

Cheers

Louise

[Merlin66]

Louise.

The eFinder will get you to the target area..... the guider will give you a view of the slit position. I don't think you'll have any difficulty working between them

Start with a bright star target....once you see what's happening -you'll be OK.

 

Edited January 20, 2020 by Merlin66