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g__day
18-04-2014, 11:46 AM
Not sure where to post this - as it cover both equipment (on/off axis guiders) software and processes - how to maintain optimal focus over a long duration imaging run.

The article / process / software works with MaximDL and other software and comes ForeSight Innovation (the very clever folks that pioneer On Axis Guiding).

http://www.innovationsforesight.com/SharpLock.htm

Rather than leverage expensive temperature compensating focusers Sharplock works by tracking the guidestars shape and pattern to work if focus needs adjustment and by how much and in which direction.

Clever stuff - has anyone tried this?

I note ONAG and Sharplock are written up in the latest issue of Astronomy Technology Today - https://www.astronomytechnologytoday.com/?l=/viewissue.asp&id=Mar14

Will have to see if I can find this somewhere (or subscribe online)!

gaston
29-04-2014, 08:36 AM
Hi Matthew,

Here is a link to the ONAG and SharpLock real time auto-focus ATT article:

http://innovationsforesight.com/Onag%20and%20Sharplock.pdf

Clear Skies!

coldlegs
29-04-2014, 10:44 AM
Mathew
Haven't tried it but it seems to be a great idea although
it relies on a reasonable cross shape for the guide star
and I rarely get even close to this mythical shape they
keep showing. Usually I get some kind of "t" shape. The
biggest pain in the neck with ONAGs is the manual guide
focuser which you have to crudely push/pull until you get
some kind of focus. The focuser is so crude you can tip/tilt
it and get better or worse star shapes. I tried gluing a
3.3mm oval shaped piece of glass in the end of the focuser
tube and although there were indications of better (normal)
star shapes I couldn't get the thickness right so gave up
for the time being. There are rumors that they are making
an optional mini crayford focuser to ease focusing problems
but I suspect that is an optimist daydream as is a correcting
glass insert to give normal star shapes!

Cheers
Stephen

PRejto
29-04-2014, 02:39 PM
Stephen!

I also use the ONAG and I can understand your frustration with focusing as I experienced exactly the same. I came up with two solutions. One is crude but does work, the other more elegant but perhaps subject to some flexure. The first you can sort of see here:

http://www.pbase.com/prejto/image/153978120

What you cannot see is blocked in the photo by the ST-i guide camera. It's a long screw that threads through that aluminium plate and presses against the body of the ONAG. I fixed a bit of plastic to the ONAG where the screw hits to avoid scratching the housing. The bubber band provide pressure for the screw to act against. Once focus is achieved you tighten down the ONAG set screw for focus. Admittedly focus works better moving out rather than "in." There just isn't enough spring pressure to pull inwards. This allowed me to get focus because I could just turn the screw a small amount vs trying to slide the focus tube in/out which in my situation was never satisfactory. The advantage of this crude focuser I dreamed up in desperation(!) is that it's firm once set and it doesn't add any back focus to the imaging camera (except for the thickness of the aluminium plate.

However, if you don't have a problem setting up the camera a little further back from the ONAG (and still have enough room to move forward on your main focuser) the Hutech helical focuser does the job. It's not absolutely rock steady but I have not had a problem with flexure.

http://www.pbase.com/prejto/image/152290400

I intend to try Sharplock soon and I will certainly post my results.I'm just waiting for a motorized focuser solution for my TEC180 which I should have in a matter of weeks.

Re star shapes I was able to get the X shape using both focusing methods. I do think the ONAG needs a bit of light to work most effectively. What I'm saying is that there is a discernible difference between my TEC140 and TEC180. The TEC180 gives better shapes, is easier to focus, and guides with shorter exposures. The 40 mm makes a lot of difference though that difference might well be because it is a refractor and these instruments are not so well corrected in IR (compared to reflectors as I understand).

Peter

coldlegs
29-04-2014, 03:26 PM
Peter
Normally I've got a baader varilock on the guider side as there is a filter plus MPCC plus spacer plus QHY9M camera on the other so there's room for the varilock. When I swap the camera for my QHY10C there is a slight difference and I have to go through re-focusing etc. Ill get the spacing exactly the same when my small spacers arrive. So maybe no more re-focusing... until I change something as usual.
Cheers
Stephen

PRejto
29-04-2014, 05:54 PM
Stephen,

Thanks for pointing out the Baader Varilock. I wasn't aware of that product. But, I assume that it doesn't allow for easy focus but does "hold" focus firmly once you find it. I assume from your first post that this isn't an answer to the actual focusing issue. Let me know if I've got that wrong.
Peter

TR
29-04-2014, 05:57 PM
Hello Peter,

I can see your ST-i guide camera in your photo. How hard is it to locate a guide star using the ST-i? Or, is there always a guide star in the field? If you were to perform a meridian flip, how likely is it that there would be a star in the field of view of the ST-i autoguider, or is the field to narrow and requires the human touch?

I am interested, but I have a feeling the FOV would be the same size as my guide chip on the STL, and I have to hunt for a suitable guide star. Can you share your experiences?

Terry

PRejto
29-04-2014, 06:19 PM
Hi Terry,

The nice feature about the ONAG is that you can find any star seen by the entire FOV of the telescope! Now, you won't see the whole field with the ST-i due to the CCD size, but the mechanical stage allows you to change where the ST-i is looking. Now, to specifically answer your question(s).

1. If the ST-i is exactly centered in the FOV (by centering the ST-i on the stage) doing a meridian flip should yield the same guide star but 180 degrees rotated in the FOV.

2. I often see stars! When you are using the full aperture as a guiding scope a lot of faint stars come into play. This is mitigated by the lower energy to some extent as only the IR signal is available to use.

3. I have never failed to find a star by just moving the guide camera on the stage of the ONAG. Also, I have never needed to rotate the imaging camera to find a guide star. You can frame your target exactly as you want and very very likely there will be a guide star!

4. Do I wish I had a guide camera with a large FOV. Yes. I don't love needed to sometimes hunt around with the stage given that my computer is indoors and I need to see whats going on with images when I'm outside. So, it's not a perfect solution for those trying remote imaging. I've been told that a motorized stage is in the works (but this was a year ago and there still is nothing like that).

Any more questions?

Peter

coldlegs
29-04-2014, 06:27 PM
Peter
The baader varilock makes the fine focus a little easier although
the guider and cables can get a little twisted up. It's not an
answer to the crude focuser. It's a shame as they created a great
device and then added a primitive guide focuser as an afterthought.

Cheers
Stephen

PRejto
29-04-2014, 08:18 PM
Stephan,

I can understand your frustration and I share your feelings about this. But only partially. I've had discussions with Gaston about the "focuser." I think he would be open to any viable suggestion for improvement. You might think a helical focuser is the answer. It certainly tics a lot of the boxes, but, the drawback is the extra back focus it will add to the system. Some scopes/cameras can deal with this while others cannot. The difficulty is in finding something that works easily and doesn't appreciably add back focus. My idea behind my crude rubber band tensioned device is close, I think. A proper design and construction of something like this might be an answer. On the other hand some people don't find the focuser impossible to use as it is. This perhaps depends on the F value of the system and critical focus zone and total amount of energy of the guide star used for focusing with "brighter" obviously easier to deal with. At the end of the day, however, I agree with you that this is the weakest link in an otherwise superbly engineered piece of equipment.

Peter

TR
29-04-2014, 08:24 PM
Hi Peter,
Thank you for your comments. I have been looking at this for a while. I run everything automated, currently using a guide scope. Stars are always present, even after a meridian flip. But, I will be increasing my focal length and have been looking for a reliable way to locate guide stars before and after a meridian flip. That's why I was asking all the questions. I have a gut feeling that I would have to manually locate a guide star as my guide camera has to small a chip. Are you using the ONAG in an automated fashion. Like you, I am away from the scope. Makes some things difficult at times.

Terry

PRejto
30-04-2014, 05:58 AM
Hi Terry,

I certainly have the potential to go fully automated with my gear but find there always seem to be adjustments here and there that need to be taken care of manually. And the ONAG in its present configuration without a motorized stage is just one of those things. Now, if it were possible to stick a larger CCD on the guider port I think that objection would become a non-issue. That's an expensive solution but not impossible if one had an older camera around that was gathering dust. Consider, though, that without the ONAG you need to invest in a rotator and that the OAG will have an even smaller FOV and more difficuty finding a star. I think a motorized stage or a large FOV guide camera would make the ONAG an ideal solution.

There are other considerations that need to figure in such as the back focus needed in order to use the ONAG. My refractors have enough available as do most SCTs. But, I don't think it's a viable solution if you want to use a fast system as available BF really shrinks. Then you need to figure out how to use a focal reducer in conjunction. After much research and head scratching I could not find any solution that allowed the FR to sit in front of the ONAG. The total optical length was just a bit too long no matter what brand I looked at. So, the FR would need to go after the ONAG. This introduces even further BF. It isn't a problem for me and I have done this with an Astrophysics barlow and have a FR if I want to try in the future. There also is a FR available for just the guide camera if you want both cameras to see the same field. These devices (balows, FRs) require that the guide camera also move back from the ONAG, but that is easy to do with the supplied tubes.

Just to complete this discussion, light pollution filters pose an additional issue to consider because they must go after the ONAG otherwise the IR frequencies are blocked from entering the ONAG. I had more head scratching moments figuring out camera mounting + LP filter on the ONAG. Mostly this was due to the smaller ONAG terminating in t-threads and I needed to use a 2" Hutech filter with threads only on one end of the filter. It has forced me to use a clamping system on the camera nosepiece which is not ideal. Recently I am incorporating the LP filter into the filter wheel in place of the "L' filter. There is always something to figure out...

Peter

coldlegs
30-04-2014, 09:56 AM
Peter
If I were to redesign the ONAG I would turn the guider
focuser into a miniature crayford focuser. The tube is already
long enough to allow for at least 10mm travel and there's plenty
of room inside and in the thickness of the X/Y platform to
allow room for it. Once the general spacing was correct the fine
tuning could be easily achieved. Also would glue a glass corrector
into the focuser tube for better star shapes. It's nice to dream.

Cheers
Stephen

PRejto
30-04-2014, 02:13 PM
This just might be the focusing solution I have been searching for! It's quite brilliant and rock steady with appreciably little back focus:

https://m.youtube.com/watch?v=YX-nyWe0qp0

Peter

gaston
01-05-2014, 10:23 AM
The fundamental idea behind the ONAG was to provide a much wide FOV without any flexure, and also eventually a real time auto-focus capability (SharpLock).

To reach those goals the guider focuser as been designed first to avoid any flexure using a full body compression ring, not just a thin brass ring. The result is even full extended the guider focuser assembly is as rigid as one solid piece of aluminum. But it come at a price.
I have worked and still working to improve the product and find a better solution for the guider focuser. Beside flexure, which is very tricky to avoid (trust me), few microns could ruin your life, there are other requirements, such as back focus and the capability to remove the guider focuser drawtube for using directly 1.25" nosepiece or any camera body of same size (this may be useful on some configurations). this is important if you have a ST-4 for instance (I have customers still using a ST-4).

There are applications where the ONAG is used for NIR imaging, with guiding in visible, this is why the XT version have an integrated corrector inside the guider drawtube. However to be able to use SharpLock the drawtube needs to be rotated to create some astigmatism.

Experimental designs with Crayford focuser style did not go well, there is simply not enough room to have a dependable rigid solution, but I do not give up yet.
I which this one would have worked though. But I do not want to compromise on rigidity and back focus.

However I may have another solution which could improve the guider fine focus, in design phase.

From personal and customer experiences I would suggest the following approaches for fine focusing:

When close to best focus with the guider camera you could tight a bit the focuser more, enough to keep it from sliding, but not too much. Then twist back and forth the guide camera while pushing or pulling it, this helps to move it by just a little.

Another option is to use the T2 thread (M42) and locking ring, screwing or unscrewing the guider camera. We have added extra length to the thread for this. This would act like an helicoidal focuser. The guider camera body will roated some, but auto-guiding algorithms, as well as SharpLock, work with any guider camera orientation. T2 is an M42 x 0.75mm thread system which means a full rotation (360 degress) is 750 microns, likely you would need less than 360 degrees.

I am totally aware that I could make an even better product, I am certainly committed to this goal and I am opened to any suggestion.
It is just that some time good ideas could be more difficult than expected in the reality. He toke me more than 5 years to come to the current design on the market and I had to overcome many problems. At glance an ONAG seems an easy device to make, but in practice this is a totally different story (trust me on this too).

Finally I would like to thank you all for the feedback and suggestions made in this forum, I do value them. I am an astronomer first and my goal it to come with new innovative solutions to make our hobby even more fun to do and enjoy! :)