Hi Richard...I saw your October post in a forum where you said you guided with your BORG 76ED at 3.7 asp (or so) when you are imaging at 0.6asp.
I have tried this several times as I too own a BORG 76ED and I have posted several questions regarding this, in particular why I am unable to get satisfactory images on my RCOS at 0.65 asp when guiding through the BORG.

Responses generally stated that differential flexure will prevent me from getting round stars on the RCOS when guiding this way.

I have often thought why SBIG would produce a remote guiding head for their cameras, when it seems that you cannot use a short FL refractor to guide with when imaging with a much longer focal length on a different scope.

Clearly, the need to have this work when narrow band imaging would be a real benefit.

Do you have some advice for me please on how I can get the two to co-operate?

many thanks

Martin

Martin,

I have been successful guiding with my external Borg 76ED guidescope.

Here are a few examples:

http://www.ewellobservatory.com/gallery/viewer_zoom.cfm?ID=55

http://www.ewellobservatory.com/gallery/viewer_zoom.cfm?ID=54

http://www.ewellobservatory.com/gallery/viewer_zoom.cfm?ID=57

When imaging at very high resolution I do prefer to self-guide as I am able to get very low FWHM (1.7"-2.0"). With an external guider I find that I can image fine with round stars, its just the FWHM values are not as good (2.5"-3.0"). BTW - This was with my old AP900...I am sure with my ME, the FWHM values would be smaller now, I just have not used my external guide scope with this setup.

The main reasons to self-guide (or use an off-axis guider) is to avoid flexure and to increase the accuarcy of guiding.

The more your guiding image scale differs from your image scale, the less accurate the guiding. In my case, I image at .60" and guide at 1.19". When imaging at > 1.5", I guide at 6.0" with the external scope.

There is no reason why an external guide scope would not work on your RC. Your FWHM may be a bit larger, but as long as you have minimized flexure as much as possible, this should be an achieve able goal.

Post some images (or sub-exposures) taken with your external guide scope and I will take a look at them and try to tell you what I see is going wrong.

rb

Hi Richard...and thanks for the advice. I will get those images you have requested, but in a couple of days time, as I have just remounted the BORG atop of my RCOS and attached the ST10XME to it.

Just to clear up some confusion for me, what do you actually mean by self-guide? Is this using the internal guide chip of an SBIG camera?
Where you say you image at 0.60" and guide at 1.19" I assume you mean that you bin your guide chip 2x2?

thanks

Martin

Yes, Self-Guide means internal guider in SBIG camera.

Yes, I bin the guide chip 2x2.

rb

Hi Richard
Finally got an image for you to assess.
Its a 10 minute image taken through the BORG 76ED/ST10XME and guided by the RCOS.

Its a full FITS image, 6mb in size, and I have only dark subtracted and debloomed it.

Here is the link

http://www.martinpughastrophotography.id.au/images/BORG...ded%20by%20RCOS1.fit

It does look better than my previous attempts..but I would be grateful for your thoughts.

tks

Martin

Sorry Richard..forgot to mention that I will get an image guided by the BORG but imaged through the RCOS and post it....but some weather has moved in.

tks
Martin

Martin,

First observation is that you are suffering from coma in all four corners of the image.

Also, the deblooming is interferring with measurements as it is distorting a majority of the stars in the image.

Can you send me a copy of the same image without de-blooming.

BTW - What software are you using to debloom with?

rb

Hi Richard.
Its amazing isnt it? This is the new BORG 0.85x focal reducer, designed to give a flat field for 35mm format chips and DSLRs..and this is only an ST10XME. I have the backfocus distance specified for the focal reducer at 5mm too long..and HUTECH say that is too much. I am not even getting anywhere near the specified focal reduction, the focal length of the BORG reduces down from 500mm to 473mm instead of around 425mm...thats just 95%.
Actually I think most of the problem is flexure. Even though the draw tube on the BORG is all the way in, and I have used screw fit adapters all the way, the camera is still too heavy...what can you expect for a $800 scope?

Anyway, I ran CCDI over it and the curvature and tilt are very significant, not to mention the miscollimation figures it throws up. Yes, I am happy this scope is very well collimated, and I have assessed that visually. You cant collimate this scope yourself anyway.

I have uploaded the new file without the blooming, and I use Maxim V4.11 to debloom but I use the automatic settings....is there any software out there that does it better?

cheers

Martin

Martin,

Where is the new file?

rb

Sorry Richard...

http://www.martinpughastrophotography.id.au/images/BORG...ded%20by%20RCOS1.fit

cheers for helping

Martin

Martin, sorry to jump in to the middle of this thread..

The only thing I see that might be slightly wrong with the last image you posted is the curvature is a bit large (16.8%): the collimation and tilt look very much within the normal ranges.

A curvature of about 17% is not bad at all, but if the focal reducer promises a flat field, this is not exactly flat.

17% curvature is pretty good, to be sure. My TEC140 refractor produces about 20% without a field flattener on an ST10 chip. With a FF though, I get closer to 10% curvature. The best I've seen is the Tak FSQ refractor that produces about 5% curvature.

Remember that the further the chip is from the reducer, the larger the reduction factor. If you are not getting the specified reduction, try moving the chip further away from the reducer.

Flexure is always a problem when guiding with a guide scope. You may want to consider a way to mount the camera on the Borg focuser so that it has some additional support (such as a dovetail or a rail that the camera itself can be attached to). In the past, I've built a guide scope by replacing the adjustable focuser with a fixed position one: the gude camera was placed and adjusted to be in the proper position, and then everything was tightened down so no shifting or flexing could occur.

Regards,

-Paul

Martin:

First Issue:

If you are not getting the desired focal reduction, then you need to move the chip farther back from the focal reducer. This will also flatten the field and reduce coma.

Questions:

1. How far is the Hutech FR supposed to be from the CCD focal plane?
2. How far back is the CCD focal plane now and how did you arrive to that distance?
3. Does the ST10XME have a color wheel or not?

Second Issue:

Your stars are pretty darn round in the center of the field (we are ignoring coma in the corners). They are showing an aspect of less than 15% on average. That means that Max FWHM is 15% larger than MIN FWHM on a particular star. This is considered round. I have been able to accomplish 6-9% when self guiding, but this is a really good sub-exposure.

So there propably is some flexure going on.

I have a borg 76ED with autoguider, and I would make the following suggestions:

1. The ST10XME is a pretty darn heavy camera to be placing on the back of the Borg. I used to have a ST7XME without filter wheel and I found that to be pretty heavy, so I moved to a ST402 and now its as rigid as can be.

2. Second, I moved the focuser in the front of the Borg rather than the back. Please see this picture:



- Notice that the focuser no longer can cause flexure based on the wieght of the camera

- Notice that I have a direct connect to the back of the borg via a t-thread adapter that attaches directly to the camera.

- Also notice that I have solid rings holding the scope in place rather than 3 point holders. I dont know what you are using, but those three point tube ring holders do not provide the rigidy that you are requiring.

The focus issue is the biggest one to solve. Call Ted at hutech and he will get you setup with the parts you need to accomplish what I have done. Show him my picture if need be...he consulted me on this design.

I can imagine if you are guiding with your Borg for your RCOS at very low image scales, that the flexure problem will be very pronounced.

Over all, this image looks good, but it so undersampled that it would be hard to evaluate how much flexure you have if I dont see an image from the RCOS guided with the Borg.

rb

Hello Paul and Richard and thanks for the detailed help there.

Let me address some of the suggestions you have made, although I am amazed at how you have set up there....Didnt even know you could do that with the BORG!

1. I take it you need special adapters in order to move the focusser to the front like you have done?
2. How far is the Hutech FR supposed to be from the CCD focal plane?
According to HUTECH, 55mm.
2. How far back is the CCD focal plane now and how did you arrive to that distance?
First of all, you have to add an extension tube BEFORE the reducer in order to get to focus. I use a 35mm extension tube for this..20mm is too short. So my optical train is: extension tube, focal reducer, then part numbers 7920/7522 (these mate the reducer to the ST10XME), then the ST10 which has a CFW8 and the Muscle Plate So the camera back focus distance alone is 47mm, leaving 8mm. Part 7920 is 8mm, and Part 7522 is 4.5mm...so altogether, I am around 5mm too long. The only way to get it exact is to have a custom part made up. But Preciseparts say that 5mm is well within tolerance of any focal reducer.

So all of my connections are screw-fit and I also use the tube rings you show in your picture...but I can tell you that I can slightly lift the camera once everything is attached, so it is not a good fit and flexure is going on. With the focusser all the way in, this is somewhat reduced. I will however, investigate what Paul suggests by supporting the camera some how.

I have had a very long debate with Ted over this, because with the STL11000 I did get an adapter made up so the back focus distance is perfect. Guess what....yes, I still get flexure of course, but the amount of reduction is absolutely identical with both cameras and you know, I belive the coma with the ST10 is worse than it is with the STL. Ted stated that SBIG were wrong in specifying their back focus distances of the STL11000 with the filters fitted. However, the entire conversation was nonsense because we are talking 0.03mm (1.5mm v 1.47mm backfocus of the STL with 3mm filters fitted), and I dont think that alters the price of fish whatsoever.

BORG do supply spacers with the reducer, but they are designed for the 100ED and 125ED and you have to disassemble the reducer to insert these. Take a look at this URL.
http://www.sciencecenter.net/hutech/borg/parts/html/7887.htm

I dont quite understand the graphs on this page....on the X axis...what do you think D represents....distance from what? It seems that at 27mm, everything is optimal, but it does not specify in what context D is made.

Regarding your observation that if the reduction is not enough, then the CCD needs to go further back. How does this apply here, I am already 5mm beyond the optimal focus distance. I suppose there is no harm in trying, but I dont quite understand it.

You also say that the image is under-sampled...well I guess we cannot avoid that when widefield imaging correct?

I will get the other image you asked for as soon as I can.

thanks a lot...I would really like to get this one sorted out...I love how you have assembled the BORG and are definately interested in doing that. Do you have the part numbers required to achieve this please?

Martin

Martin,

My guess is that D in the Hutech diagram is the diameter. At D=27mm, the graph says that the image on a chip with a diagonal of 27mm will be flat all the way out to the corners.

Focus position does not change the amount of reduction for a refractor: the distance between the reducer and the chip does. To get more reduction, the chip should be further away from the reducer. Once the appropriate distance is established between the reducer and the camera, you can then find the best focus position by any of the standard focusing methods, while keeping the reducer-to-chip distance constant.

Regards,

-Paul

quote:

Originally posted by Martin Pugh:
BORG do supply spacers with the reducer, but they are designed for the 100ED and 125ED and you have to disassemble the reducer to insert these. Take a look at this URL.
http://www.sciencecenter.net/hutech/borg/parts/html/7887.htm

I dont quite understand the graphs on this page....on the X axis...what do you think D represents....distance from what? It seems that at 27mm, everything is optimal, but it does not specify in what context D is made.

Regarding your observation that if the reduction is not enough, then the CCD needs to go further back. How does this apply here, I am already 5mm beyond the optimal focus distance. I suppose there is no harm in trying, but I dont quite understand it.

Hi Paul and thanks for that. As I am already beyond the optimal distance for the focal reducer i.e backfocus distance is 55mm and I am at 60mm, then I am not sure going further back will improve things....but its worth a try....just got to figure out how to do it as I dont think I have any other adapters or extension tubes.

thanks
Martin