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RASA 8 - First Attempt

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#1 starjunkie

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Posted 05 March 2019 - 05:30 PM

Brand new RASA 8, or 203mm for those that go crazy that Americans still use imperial units smirk.gif  Camera is ZWO ASI183MM Pro.  No filter (and I kept the removable optical window installed).

 

Optical path = RASA 8 optical window -> M42 camera adapter -> Latest Starizona filter drawer with female camera-side flange -> ZWO camera.

 

So, my first image (stacked 48 @ 60s each, no calib), shows decently round stars on the left side of the image, but elongated ones on the right.

 

Autosave---M101---Lum-48m---RASA.jpg

 

I ran about 10 of my FITS images through CCD Inspector and I've got quite a bit of tilt, and quite a bit of curvature....and woo-boy, look at that collimation.

 

Curvature.jpg

3D plot.png

 

If I understand the curvature plot correctly, I seem to be too far back with the camera?  As for tilt, I did find a finger smudge on my optical window, but have since cleaned it - but, I don't know how much that would have affected light coming through.  I'm not sure how I can adjust tilt, because the camera just attaches snugly to the corrector plate fittings.

 

I have been very careful not to put pressure on the corrector plate when pushing in the USB cable to the camera, and I haven't gorilla tightened the adapter ring. 

Has anyone seen this level of curvature, tilt and collimation issues?  I'm wondering if the scope got jarred a bit too much in shipping.

 

TIA!

 

-Chris

 

 

 

 

 

 


Edited by starjunkie, 05 March 2019 - 05:31 PM.

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#2 calypsob

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Posted 05 March 2019 - 05:43 PM

Brand new RASA 8, or 203mm for those that go crazy that Americans still use imperial units smirk.gif  Camera is ZWO ASI183MM Pro.  No filter (and I kept the removable optical window installed).

 

Optical path = RASA 8 optical window -> M42 camera adapter -> Latest Starizona filter drawer with female camera-side flange -> ZWO camera.

 

So, my first image (stacked 48 @ 60s each, no calib), shows decently round stars on the left side of the image, but elongated ones on the right.

 

attachicon.gif Autosave---M101---Lum-48m---RASA.jpg

 

I ran about 10 of my FITS images through CCD Inspector and I've got quite a bit of tilt, and quite a bit of curvature....and woo-boy, look at that collimation.

 

attachicon.gif Curvature.jpg

attachicon.gif 3D plot.png

 

If I understand the curvature plot correctly, I seem to be too far back with the camera?  As for tilt, I did find a finger smudge on my optical window, but have since cleaned it - but, I don't know how much that would have affected light coming through.  I'm not sure how I can adjust tilt, because the camera just attaches snugly to the corrector plate fittings.

 

I have been very careful not to put pressure on the corrector plate when pushing in the USB cable to the camera, and I haven't gorilla tightened the adapter ring. 

Has anyone seen this level of curvature, tilt and collimation issues?  I'm wondering if the scope got jarred a bit too much in shipping.

 

TIA!

 

-Chris

Ive been wondering how these rasa scopes are going to do with their factory collimation. Im thinking something like a gerd neuman ctu will be needed to fine tune the tilt. I hope you get the spacing issue resolved. Despite the star shape issues, the data is impressive!


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#3 jtrezzo

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Posted 05 March 2019 - 05:57 PM

It needs to be very precise in the spacing requirements. On my 11" I was off by about 1mm and it wouldn't even come to focus at all. You probably just need to play with the collimation to fix the right side.


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#4 Churmey

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Posted 05 March 2019 - 06:10 PM

It needs to be very precise in the spacing requirements. On my 11" I was off by about 1mm and it wouldn't even come to focus at all. You probably just need to play with the collimation to fix the right side.

That’s odd, and not the case with my R-11. Ive had it as much as +- 5mm when I was initially testing my spacing and was able to come into focus. 


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#5 starjunkie

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Posted 05 March 2019 - 06:36 PM

Ive been wondering how these rasa scopes are going to do with their factory collimation. Im thinking something like a gerd neuman ctu will be needed to fine tune the tilt. I hope you get the spacing issue resolved. Despite the star shape issues, the data is impressive!

Thanks!  And that's from Bortle 8 backyard skies, with no filter ;)  I forgot to turn the gain down on my ZWO, and blew out some other targets.  f/2 is fast!

 

I can't wait to get some narrowband filters and once I figure out the star shape issues, see what this baby can do.



#6 starjunkie

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Posted 05 March 2019 - 06:38 PM

It needs to be very precise in the spacing requirements. On my 11" I was off by about 1mm and it wouldn't even come to focus at all. You probably just need to play with the collimation to fix the right side.

I wasn't sure exactly where to measure from.  I know the backfocus of the camera, and I seem to be right around 25mm...give or take a mm.  I may take off the filter drawer next session and see if that gets me closer (or much worse).  I suppose I'll ask Celestron if I'll need to collimate - as it sure seems like it.



#7 andysea

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Posted 05 March 2019 - 09:10 PM

I have the same scope and I used it with the QHY183c. I have the spacing exactly at 25mm. That is measured from the base of the t-threads on the RASA adapter. I used a 7.5mm baader t-thread extension to reach the 25mm. The back focus of the QHY183c is 17.5mm. I get a very slight elongation at two of the corners but I think that may be caused by slight tilt. The collimation of my scope looks pretty good when I use a defocused star and looks t the diffraction rings, however I want to spend some time studying the collimation a bit more.


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#8 starjunkie

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Posted 05 March 2019 - 09:56 PM

I have the same scope and I used it with the QHY183c. I have the spacing exactly at 25mm. That is measured from the base of the t-threads on the RASA adapter. I used a 7.5mm baader t-thread extension to reach the 25mm. The back focus of the QHY183c is 17.5mm. I get a very slight elongation at two of the corners but I think that may be caused by slight tilt. The collimation of my scope looks pretty good when I use a defocused star and looks t the diffraction rings, however I want to spend some time studying the collimation a bit more.

Thanks a lot for the feedback.  I went back and remeasured everything.  I've got a 6.5mm back focus for the ASI183MM Pro, an 18.5mm filter drawer.  I think I was 1mm off, because Starizona gave me a M48-M42 female flange, but I was using another one which was a bit thicker, because silly me, I had the filter drawer screwed on upside down, and when I tried to pull out the drawer it would get stuck on the RASA adapter threads.  I flipped it over and voila, it works....and I'm at exactly 25mm now.  Amazing what a difference 1mm makes (well, I'll find out for sure when there's a clear night).

 

Thanks for pointing out where to measure from.  I was a bit confused about that.  

 

Curious, have you tried looking at an image in CCD Inspector.  I'm just curious what your values are (collimation, tilt, curvature, etc..).  Thanks again!

 

-Chris



#9 39.1N84.5W

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Posted 06 March 2019 - 08:05 AM

Can the user not collimate these RASA scopes?

#10 starjunkie

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Posted 06 March 2019 - 08:40 AM

Can the user not collimate these RASA scopes?

You sure can.  The question is, do you want to risk fiddling with a hex wrench near the corrector plate, or send it off to Celestron? undecided.gif

 

collimation.jpg



#11 39.1N84.5W

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Posted 06 March 2019 - 09:25 AM

I'd rather not depend on celestron, ups, FedEx, etc to get a spot on collimation. But thank you for the information!
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#12 descott12

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Posted 06 March 2019 - 09:41 AM

 

 

I ran about 10 of my FITS images through CCD Inspector and I've got quite a bit of tilt, and quite a bit of curvature....and woo-boy, look at that collimation.

 

attachicon.gif Curvature.jpg

 

Hi Chris,

I have not used CCD Inspector before. Looks like a very useful too. Would you mind explaining how you interpret the collimation value?  When it says 18 arc-sec, what exactly is that telling you? I am assuming it should be zero?

 

And do you think the high/max FWHM values are coming from the stars on the right were the stars are elongated?

 

Finally, do the tilt values necessarily refer to the camera or is there somewhere else in the optical path that could cause that like the primary or corrector plate?

 

Thanks


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#13 starjunkie

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Posted 06 March 2019 - 09:57 AM

Hi Chris,

I have not used CCD Inspector before. Looks like a very useful too. Would you mind explaining how you interpret the collimation value?  When it says 18 arc-sec, what exactly is that telling you? I am assuming it should be zero?

 

And do you think the high/max FWHM values are coming from the stars on the right were the stars are elongated?

 

Finally, do the tilt values necessarily refer to the camera or is there somewhere else in the optical path that could cause that like the primary or corrector plate?

 

Thanks

Hi, Dave.  I'm no expert on CCD Inspector, but from their help file, here is the explanation of the values:

 

The top left corner contains a number of valuable statistics derived from the measured image:

Min FWHM:  lowest FWHM value in the curvature map
Max FWHM: maximum FWHM value 
Curvature: percent defocus between lowest and highest defocus points on the map
Tilt in X, Y: defocus from left to right, and from top to bottom of the image, expressed in arcseconds or pixels
Total Tilt: is the absolute amount of measured tilt in the image, expressed in %, and its direction shown in degrees, as well as with a pointing arrow
Collimation: the distance between physical and optical centers  in the image, shown in arcseconds or pixels. Assuming a small optical tilt, this is how far the optics are from perfect collimation.

 

 

To answer your questions, yeah, the min/max FWHM would be the diff between the 'good' stars and the elongated ones.  As for tilt, I'm still confused on that one, because unless the corrector plate is off, or perhaps the chip in my camera is tilted, there's not a lot in the path to tilt the image.  Perhaps my collimation might help....not to mention fixing the backfocus distance issue I had.

 

It's a very handy tool.  

HTH!



#14 starjunkie

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Posted 06 March 2019 - 09:59 AM

I'd rather not depend on celestron, ups, FedEx, etc to get a spot on collimation. But thank you for the information!

I hear ya, but since I just bought it, I think they kind of owe me grin.gif



#15 WadeH237

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Posted 06 March 2019 - 12:50 PM

As for tilt, I'm still confused on that one, because unless the corrector plate is off, or perhaps the chip in my camera is tilted, there's not a lot in the path to tilt the image.

It's actually pretty easy to end up with tilt.

 

As you mention, sometimes the camera sensor is off a bit (I've seen it in lower priced cameras).  Focuser sag can do it.  Thumbscrew couplings can do it - especially with only one thumbscrew.  Threaded connections can be off a bit.  Even flexure could do it.

 

The key is that it takes a *very* small amount of tilt to affect an image, especially at faster focal ratios.


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#16 starjunkie

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Posted 06 March 2019 - 12:56 PM

It's actually pretty easy to end up with tilt.

 

As you mention, sometimes the camera sensor is off a bit (I've seen it in lower priced cameras).  Focuser sag can do it.  Thumbscrew couplings can do it - especially with only one thumbscrew.  Threaded connections can be off a bit.  Even flexure could do it.

 

The key is that it takes a *very* small amount of tilt to affect an image, especially at faster focal ratios.

Fortunately, I don't have a focuser or thumb screws in the optical path, as this is a RASA.  But, flexure is a possibility and/or the aforementioned threaded connections and camera sensor....or (hopefully not) the corrector plate.

 

Either way, trial and error is in my future, and hopefully I'll get it figured out.  Thanks!



#17 jtrezzo

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Posted 06 March 2019 - 04:10 PM

That’s odd, and not the case with my R-11. Ive had it as much as +- 5mm when I was initially testing my spacing and was able to come into focus. 

1mm may have been an exaggeration. Probably more like 2mm as I had the spacers that Baader UFC lists for 17.5mm back focus cameras (which calculates out to the incorrect distance of 75.5mm BTW, vs. the spec of 72.8mm). The mirror reached the end point before I could reach focus. It did focus fine if I used less spacer. When it is the right spacing I get a pretty good field across all of full frame. That is certainly interesting that you were able to go 5mm in and out though...no idea why mine would be like that.


Edited by jtrezzo, 06 March 2019 - 04:11 PM.


#18 Jeff B

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Posted 10 March 2019 - 11:09 AM

I'd rather not depend on celestron, ups, FedEx, etc to get a spot on collimation. But thank you for the information!

Me too.

 

Nick, I'm liking the idea of a RASA for us more and more.

 

Jeff


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#19 pyrasanth

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Posted 10 March 2019 - 11:34 AM

Celestron know more about optics than i ever will. The question is why did they produce the RASA 8 with such a limited back focus? Was it design constraints or some other reason. It seems a lot of people will find it difficult to use with their existing equipment. The issue does not affect the other 2 models.



#20 lollywater

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Posted 10 March 2019 - 06:39 PM

I had mirror tilt on my Rasa 8. It was shipped  to Celestron for adjusting, from Australia, and back again in 13 days. Very happy with it now. 

I have a zwo 294 which is 17.5 but only have  10mm and 5 mm extensions so I am at 27.5.

The Starizona filter drawer looks like being the next purchase

cheers

Paul


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#21 starjunkie

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Posted 10 March 2019 - 07:25 PM

I had mirror tilt on my Rasa 8. It was shipped  to Celestron for adjusting, from Australia, and back again in 13 days. Very happy with it now. 

I have a zwo 294 which is 17.5 but only have  10mm and 5 mm extensions so I am at 27.5.

The Starizona filter drawer looks like being the next purchase

cheers

Paul

Thanks for the info, Paul.  Now that I have the correct spacing, I'm going to try once more (when I get a cloud free night) and see the difference.  If the results are similar in CCD Inspector, I'll ship it back.  smile.gif



#22 lollywater

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Posted 10 March 2019 - 08:40 PM

The tilt on mine was more severe than on the image you posted.Stars were pinpoint one side and small donuts on the other. Your image looks pretty good to me.

cheers

Paul



#23 starjunkie

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Posted 10 March 2019 - 10:47 PM

The tilt on mine was more severe than on the image you posted.Stars were pinpoint one side and small donuts on the other. Your image looks pretty good to me.

cheers

Paul

Really?  Wow, I thought mine looked terrible.  lol.gif

 

Either I'm really picky, or I'm expecting too much....or both?  I also took 12@60sec of M45, and this shows donuts a little better.  *Note: also no calibration frames, and no filter....just stacked in DSS and stretched in PS. (This is flip horizontally as well, so elongated stars are on the left now).

 

Autosave.jpg

 

Update:  Well, since I have to shrink the image down to 500KB, it doesn't show up real well in this version.


Edited by starjunkie, 10 March 2019 - 10:50 PM.


#24 lollywater

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Posted 11 March 2019 - 04:00 AM

Chris ,I think the stars at the edges are fine by my standards. Bright stars are more of a problem. 

cheers

Paul



#25 starjunkie

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Posted 18 March 2019 - 11:59 AM

So, I was able to get my spacing corrected, as well as get some clear skies.  As I await the Baader f/2 filters, I am using an Astronomik 12nm Ha filter in the meantime.  I did some integration of IC401 in just Ha (RASA optical window installed -> M48 RASA camera adapter -> Starizona latest filter drawer with Ha filter installed -> ZWOASI183MM Pro).

 

My curvature and 3D plot look much better, as well as the stars throughout the entire field (I did crop just a smidge to account for stacking).  This is 59 @120s stacked, taken at a red site, and calibrated with darks, flats, dark flats (better and more calibration needed).  

 

Curvature.jpg

3D plot.jpg

Autosave.jpg

 

I'll probably answer my own question here, but when I run an LRGB combine in PI (with my S2, Ha, and O3 integrations), the resultant image is somewhat dull, almost gray scale looking.  Would this be due to the filter I'm using, and not actually allowing for the CWL shift, and subsequently not giving me a true 656nm representation?

 

I'm happy with the field, and I don't think I'll need to touch collimation after all.  waytogo.gif


Edited by starjunkie, 18 March 2019 - 12:03 PM.

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