Infrared guiding

[cfranks]

Has anyone had any experience guiding using infrared light? The Sharplock system uses it and Innovations Foresight are quite enthusiastic about it. ;)
I can't afford to buy such a system but will try and make one if it seems useful. There is a diy doc around that uses an Edmunds Cold Mirror in a converted Vixen Flip Mirror to make the beam splitter.

[bojan]

I just had a look at Edmund Optics.
They have mirror that may be exactly what you need, and it doesn't cost arm and leg:
http://www.edmundoptics.com/optics/optical-mirrors/hot-cold-mirrors/high-performance-hot-mirrors/64457/
Other sizes:
http://www.edmundoptics.com/optics/optical-mirrors/hot-cold-mirrors/high-performance-hot-mirrors/3150

The suface flatness may be the issue..
Surface Flatness 4 - 6λ

I am tempted to try this myself.
BW CCD camera (Vesta) should be OK for IR below 750nm

[PRejto]

Hi Charles,

I've been using the ONAG for a few years now. I've actually got two - one on each of my TECs. As I write this I'm imaging NGC1532 using FocusLock on both scopes. I have not focused once the entire evening and the temperature has changed from 21.7 c to 20.7. Only 1 degree but you can see how many times the focus was automatically adjusted to stay well withing the CFZ. I started out with 3 x 480 sec and then 4 x 600 sec subs. My FWHM values from CCDInspector are: 1.84, 1.85 2.06, 1.87, 1.94, 2.28 even though I started out at ca 75 degrees and am now at 63 degrees altitude. Pretty cool!

Peter

[cfranks]

Thanks Bojan, those are Hot mirrors and the one I want to use is a Cold mirror. http://www.edmundoptics.com.sg/optics/optical-mirrors/hot-cold-mirrors/high-performance-cold-mirrors/64449/ I hadn't noticed the flatness though, 6 lambda doesn't look too good.

Peter, I'm impressed with your results. I have always had problems with focussing, using @Focus2, with 3 different focusers and would certainly go your route if I was a bit more that an OAP. I haven't got enough back-focus on my FLT132 but should have enough on my RC10C. I looked at the information from Innovation Foresight and wonder if I could go part way with DIY and then purchase the FocusLock software to control it all. I've got the Vixen Flip Mirror on the way and will see what that looks like before buying too much more.

[PRejto]

Hi Charles,

The two biggest issues you will face making your own device will be back focus and rigidity, and I believe the exact mirror placement is quite critical too. I think the ONAGs mirror is laser aligned. The BF gets tricky to say the least if you also need to fit a reducer or flattener. Putting those optics before the mirror seems best in my view but then BF can be a game stopper. If you need to place the camera very close to the body of the ONAG like device there can be clearance issues. If you place after the mirror you might get more vignetting but you also then effect the guide camera focus (which also needs to be extended. When the imaging camera is close then so must also be the guide camera chip. The latter can be tricky. It's not insurmountable for sure but you need to calculate carefully in advance.

Another issue of high importance is how you intend to focus the guide camera? To use FocusLock the guide camera needs to be focused critically...not by hand which is not nearly exact enough. The IF helical focuser is good (and expensive) but so is Garry Jarrett's KISS focuser (much less expensive). Gary's focuser is very rigid and allows the guide camera to get very close (much closer than the IF helical model). Gary will probably work with you to make an adapter to mount his focuser to whatever device you end up modifying. I've used Gary's focuser with the ST-i and Lodestar, and it should operate perfectly with any camera having similar size barrel (1.25").

This is a recent setup though I have added a TAK flattener in front of the ONAG-XT. This forced me to swap guide cameras as I now need to get extremely close to the back of the XT model. Gary's focuser saved the day!

http://www.pbase.com/prejto/image/160513305

Let me know if I can assist you making this up!

Peter

[billdan]

I am also looking at making a DIY ONAG using a 50mm square cold mirror from Edmund.
6 lambda wavelength for the surface accuracy does not sound good, however the on-axis light cone at that point is only about 10mm ( if the CCD chip was 50mm away at F5). Being so close to the CCD may mean the 6 lambda is irrelevant in practical terms. (wishful thinking here)

Biggest issue with these is the increase in back focus required which probably means for a Newt that the only coma corrector suitable is the RCC1 with 90mm back focus.

As Peter said another mirror to collimate and adjust.

For $90 odd dollars I think its worth a gamble.

Regards
Bill

[PRejto]

Hi Bill,

Check your calculations carefully! My ONAG -XT BF is 69mm and I'm certain IF did everything possible to reduce BF to the absolute minimum. 90 mm means that you must consider 2 adapters (one for each end of your "ONAG") + camera BF with filter wheel. Even the ATK ONE requires 27mm BF with inbuilt FW. On my TEC180 the best location for the TAK TOA-67 was ca 112mm. from the CCD. My camera is as flush as possible on the ONAG and I had to custom design a special fitting (Precise Parts couldn't do it!) to mount the TOA-67 (saved me ca 4 mm). Bottom line, I'm not sure how you will get an ONAG like device with smaller BF than around 68-70mm unless you use a smaller mirror. You might get away with that if your imaging camera CCD is small.

This photo shows my current setup using the TAK flattener. Notice how close the imaging camera is to the ONAG body and how close the TOA-67 is. It just fit when I shaved off 1.5mm from the Moravian t-thread adapter.

http://www.pbase.com/prejto/image/162166083

The other question is on your newt scope do you actually have enough in focus to mount ca 100 mm of gear? You might need to make too many modifications to your secondary. The ONAG seems to work best on the CAT type scopes.

Peter

[billdan]

Yes I see what you mean about how close everything has to be.

Well my QHY12 CCD is ~ 30mm diag so a smaller cold mirror is not on.

I would probably have to move the primary mirror closer to the secondary by at least an inch. Don't really want to buy a larger secondary though.

I will have a re-think and learn how to fall in love with an OAG.

Regards
Bill

[gaston]

For information see the attached PSFs and related Strehl's ratios on axis and 0.3 degree off axis (@f/10) for a 6 lambdas smooth symmetrical (best case) mirror flatness error set at 45 degrees:

f/10 - on axis - back working distance (BWD) = 110mm
f/10 - 0.3 degree of axis - back working distance = 110mm
f/5 - on axis - back working distance = 50mm (which may be challenging to reach)

If the mirror flatness is not a simple smooth deviation across its clear aperture, or does not exhibit the same error in x/y directions (likely) the induced optical aberrations could be worse (such as strong field dependent astigmatism and curvature).


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[bojan]

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[billdan]

Hi Gaston,

Thanks for those PSF's, if those spikes are induced errors, I don't think I would bother with the Edmund product.

May have a look and see if there are other cold mirrors with better specs.

Regards

Bill

P.S that link doesn't work for me either.

[codemonkey]

You don't necessarily need a cold mirror to guide in NIR. I've been doing it for a little while with an OAG. Honestly can't say how easy it will be to find a star with this approach, as it'll depend on many factors (focal length, sensor size, response curve of camera, camera sensitivity, filter bandpass etc) but so far I've managed with mine on a couple of targets.

[Eden]

Is the emphasis here guiding in NIR or using Sharplock without an ONAG?

There are some gains to be had from guiding in NIR with the aid of a bandpass filter (as Lee is doing with the OAG), I've done it using an Orion 70mm guider/finder and an Astronomik ProPlanet 807. Most filters (ProPlanet included) will still transmit varying amounts of light below the NIR, though.

Going down that path means no SharpLock and leaves you at the mercy of incidental flexure, etc. I wouldn't recommend DIY with a cold/hot mirror, a few folks have tried it with off-the-shelf parts and ran into issues with optical aberration.

An OAG + bandpass filter would give you some NIR guiding but finding the right star in the limited FoV is challenging.

[gaston]

The spikes are the result of some astigmatism error induced by the mirror curved surface. There is also some spherical and field curvature aberrations involved here.
The aberrations and resulting PDF are function of the BWD (the larger the worse) and therefore field dependent.
Also faster scopes (smaller f/#) would exhibit larger aberrations. The problem becomes very challenging with large dichroic mirrors and off axis targets.

The link does not work for me too...

[billdan]

Thanks for the further update Gaston, I'm now wondering if the Edmund Specs of 6 lambda is a misprint.

I had a look at Thor Labs Cold Mirror specs and they claim one tenth lambda surface error.

Unfortunately their biggest size is 25mm at this spec in fused silica. Their Soda-Lime mirrors are larger but no mention of surface quality.

Sorry for hijacking your thread Charles but if you intend to still go ahead with building one, have a look at the Thor specs before committing to the Edmund product.

http://www.thorlabs.hk/newgrouppage9.cfm?objectgroup_id=61 08

Regards

Bill

[gaston]

The Edmund Optics's specification is correct and provided in peak to valley (PTV) unit. Those mirrors are often used to manage heat in light sources, not for imaging applications, for which the surface flatness is not much an issue.

The Thorlabs cold mirrors are small since it is much easier to manage flatness with diameters at or below one inch. My guess, for the price, the flatness is a 10th of a wave RMS, the PTV error is more in the range of a half wave or more, seldom enough.
With one inch diameter, when set at 45 degree, the useful working surface becomes an ellipsoid with a minor axis below 18mm, not much left for most imaging cameras.
Also they do not have an AR coating on the back which may create significant out of focus ghost images.
Larger multi-coated dichroic mirrors with a good enough flatness are much more expensive, they are usually custom designs.
Soda lime float glass is a common, inexpensive, substrate used in windows, bottles and jars. I would expect a PTV flatness in the 5 to 10 waves, or more depending of the size. For such glass flatness is usually not provided since it is understood it will be large.

[cfranks]

No worry about hijacking, Bill, I'm really enjoying the knowledge that is passed here. I ordered a couple of IR pass filters a couple of weeks ago, to see what I could do with them on my Lodestar or ST-I guiders. I did look at Thorlabs at that time and noted the better surface accuracy of their mirrors but, being 79yo, promptly forgot about them! :question:
As mentioned here though, using the guide camera with an IR filter will make it more difficult to find a suitable star but I will test that when they arrive. My cameras are QSI wsg and their prism is fairly small.
I've got plenty of time, and most of the skills to precisely engineer this project but will take note of what is said here. I'm not too concerned yet, about BF for the guider as one can easily guide with slightly out-of-focus stars and, being a DIY, I can hack into a homemade ONAG if neccessary. I couldn't ever do that to a commercial one like the SharpLock system! :)