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Back Focus on TOA-130

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#1 mmalik

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Posted 31 July 2012 - 04:10 PM

I have been taking pictures using diagonal on 130mm APOs (TMB/TOA); this time I tried adding backfocus extensions to TOA-130 and result is attached. This is an image of M51 taken last night (July 30, 2012) which was roughly in northwesterly direction, well above the horizon, and camera/sensor orientation was pretty what you see in the image (top of the image is top of the OTA/camera).

Notice oval stars in upper right right/left corners, while bottom half doesn't show this behavior or not as pronounced. Is this flexure in the imaging train? Or could it be loose focuser? Regardless, my take is why wouldn't one get flexure after adding this much backfocus, even if it wasn’t a loose focuser. I doubt this is field curvature since that should have been all around and equal. While we are on the subject I would like to know how much of field curvature would/should there be in an APO? I presume it will be much less compared to SCTs?

In short, I am looking for your analysis of this image and seeing if my conclusion is correct (i.e., it being a flexure in the long awkward imaging train, primarily, and/or possible lose focuser, secondarily). Note: I’ll do more tests after tightening things (primarily the focuser, since I had the extensions/camera well tightened for the image attached).

Image specs are as follows:

•17x3 min (guided)
•50 min integration
•Canon 60Da/TOA-130/Paramount ME
•ISO 400
•Full frame (no cropping)

Attached Files



#2 blueman

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Posted 05 August 2012 - 12:17 PM

I do see radiating elongation on both the upper right and left, that looks a lot like field curvature. Flexure is usually all in on direction, where curvature radiates from the center.
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#3 Bert

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Posted 05 August 2012 - 06:11 PM

All image trains have flexure due to gravity. Some have more than others for a given load. The other consideration is the critical focus zone. In general for fast systems any flexure present will show up far more than for slow systems.
That being said there are two components to flexure. The amount you have when adjusting focus and the dynamic change due to mount orientation while collecting data. The dynamic change can be partly overcome by an OAG rather than a guide scope. It is this myth of guide scope flexure that comes from this dynamic flexure due to orientation.
Your image shows elongation of stars due to non orthogonality of your sensor to the optic axis. It is not field curvature. It is sag in your image train.
I have been setting up an F3 system with a focal length of 600mm and a sensor that is 36.8x36.8 mm. This very fast system was very sensitive to flexure. The best solution was to stabilise the image train completely by restraining the camera with an adjustable frame.
See image below.

Here is an animated gif of two crops from two 16 minute exposures.

http://d1355990.i49....2_07/tightT.gif

The difference is a firm tighten of the adjustments that hold the camera.

Without the frame there would be about ten pixels of movement per exposure.

This is 10x16 minutes of 3nm NII of NGC6334 and NGC 6357. 10MB

http://d1355990.i49....34&6357_ITS.jpg

This image is the proof that the image train stabilizer works!
Bert

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#4 hiro

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Posted 05 August 2012 - 11:11 PM

Hi mmalik,

The elongated stars look to be due to focal plane curvature. TOA-130 is a very nice refractor OTA, but the focal plane is not flat without field flattener. If your OTA is TOA-130F, you can get incredibly flat focal plane with 67 flattener and adequate spacer as below.
http://www.cloudynig...91/Main/3698884

If your OTA is TOA-130S, you can get near-perfectly flat focal plane with 35 flattener enough for APSc sensor.

Watch the adequate system chart in the link below, and consult Art Ciampi in Takahashi America.
http://www.takahashiamerica.com/

#5 gmartin02

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Posted 06 August 2012 - 11:46 AM

Hi mmalik,

I see that you are now using a TOA-130 on a Paramount ME for imaging.

Did you just purchase this setup?

In previous posts, you said you are also imaging with a TMB 254mm on an Astro Physics 3600GTO in an observatory that is owned by an astronomical society that you belong to.

Which astronomical society do you belong to?

Perhaps you can get help from other members in your astronomical society - If they have a TMB 254/3600GTO, I am guessing that members in the astronomical society are very experienced with imaging setups and could help you with your setup in person.

Greg

#6 alpal

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Posted 07 August 2012 - 08:26 AM

All image trains have flexure due to gravity. Some have more than others for a given load. The other consideration is the critical focus zone. In general for fast systems any flexure present will show up far more than for slow systems.
That being said there are two components to flexure. The amount you have when adjusting focus and the dynamic change due to mount orientation while collecting data. The dynamic change can be partly overcome by an OAG rather than a guide scope. It is this myth of guide scope flexure that comes from this dynamic flexure due to orientation.
Your image shows elongation of stars due to non orthogonality of your sensor to the optic axis. It is not field curvature. It is sag in your image train.
I have been setting up an F3 system with a focal length of 600mm and a sensor that is 36.8x36.8 mm. This very fast system was very sensitive to flexure. The best solution was to stabilise the image train completely by restraining the camera with an adjustable frame.
See image below.

Here is an animated gif of two crops from two 16 minute exposures.

http://d1355990.i49....2_07/tightT.gif

The difference is a firm tighten of the adjustments that hold the camera.

Without the frame there would be about ten pixels of movement per exposure.

This is 10x16 minutes of 3nm NII of NGC6334 and NGC 6357. 10MB

http://d1355990.i49....34&6357_ITS.jpg

This image is the proof that the image train stabilizer works!
Bert


That's a great result Bert.
You really have sorted that system out.
I look forward to your future photos.

#7 mmalik

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Posted 08 August 2012 - 06:16 AM

Your image shows elongation of stars due to non orthogonality of your sensor to the optic axis. It is not field curvature. It is sag in your image train.


Thanks everyone for the feedback.

And thanks Bert; appreciate the pic. I am pretty sure it is sag as you said. Besides the sag of the imaging train, I am bit disappointed with the quality and firmness of the Tak focuser on TOA-130 which I am quite sure is contributing to the sag. Wonder why wouldn't Tak start putting Feather Touch focuser on their scopes by default?

#8 Bert

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Posted 08 August 2012 - 05:21 PM

A good test to prove what is happening is to take some data with the telescope on the other side of the meridian at about the same elevation and see if the problem moves to the other side of the sensor. If it does not, then Hiro's suggestion could be far more correct.

It could also be a combination of both flexure and field curvature.

Hope this helps rather than confuse.

Bert

#9 mmalik

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Posted 08 August 2012 - 06:31 PM

Bert/Hiro, you both are partly correct; field curvature is part of the equation. But I when I just look at the long back focus and imaging train on a Tak, it is an ugly sight. If one looked hard enough, you could even visually see the sag. Point I am making is that scopes of Tak caliber don't need to have this problem but they do. Even if one made the imaging train stiff and tight, the Tak focuser is quite flimsy in itself.






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