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CFF 250mm 10" RC Arrived

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#26 buckeyestargazer

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Posted 13 May 2017 - 09:20 AM

I'd like to get the collected wisdom of the group...

Last night it was supposed to be clear with pretty good seeing conditions...but the forecast was wrong as usual.  It started out pretty clear before some puffy intermittent clouds started rolling in.  

 

Nevertheless I was able to take some star images, did a very careful polar align with Pempro, and also did some PEC training before packing it in.

 

I'd like to hear what people think of the below star images.  The images are:

1.  A single 5min LUM image of a mag 2 star near the meridian

2.  The same bright star as above but defocused and heavily stretched to show another nearby star

3.  The same bright star defocused, no stretching.

 

 

To the practiced people here, what do these images show?  In particular, I'm curious why all of the stars in the 5min image appear to be eggs in one direction.  (For info, guiding was quite good with an RMS error of around 0.35 in both RA and DEC.)  Is that coma, and does the bright star reflection, which goes in the same direction, indicate that the primary needs to be adjusted slightly? 

 

I should also note that I took some 5s and 30s images and the stars in those images also look like eggs in the same direction.  

Attached Thumbnails

  • Star_NoStretch_Crop.jpg
  • CFF_F8_Star_Stretch_Crop.jpg
  • CFF_F8_QHY_300s.jpg

Edited by buckeyestargazer, 13 May 2017 - 01:31 PM.


#27 buckeyestargazer

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Posted 13 May 2017 - 09:25 AM

 

 

 

All of their RCs come as F/8 as standard, the cass's are F/15-20.

Interesting.  The only 250mm cassegrain I could find on their site was the classical cassegrain.  I can't find the R/Cs...

 

Here you go:  http://cfftelescopes...tor-rc250mm-f8/

 

 

 

 

 

 

 

Nice - hopefully you'll get some time to set it up!

Collimation isn't too bad.  One trick, at least for me, if you choose to do rough collimation (laser and/or Tak scope, etc), temporarily unscrew the primary baffle and this will greatly lesson the confusion of interpreting the concentric rings (or what should be concentric rings! laugh.gif)

 

For final fine tuning collimation, via a star test, remove any coma via small primary adjustments of the 3 graduated-marked knobs on the back.  Generally I've not needed to adjust mine more than maybe 1/20 to 1/5 of a turn.  Astignatism is removed by very small adjustments to the secondary, which I've only done one-time.  For me at least, I need very good seeing to do adequate star testing and critical to only defocus slightly on the airy disc and its rings (rather than a great amount of defocus which will result in testing more mechanical and not optical alignment).

 

My scope does not have the optional secondary heater.  I only have the 3 rear fans.  They do help to cool or keep the primary near an ambient temperature, but of course if you experience a rapid temp drops they'll need to make the primary catch up after maybe 5-10 min if temp's steady.  But the biggest plus to the fans, in my experience, is the boundary air sweep they do of the primary.  Air flows around the primary sides, across the primary face and exits out the front of the scope.  In general I've found this to steady planetary imaging and/or star test patterns.

 

cheers, DJ

 

Thanks so much for the advice.  

 

I already used the laser collimator indoors and got the scope aligned as well as I can or know how at this point.  Even with removing the baffle is was incredibly hard to see the concentric rings on the primary and thankfully the secondary didn't appear to need very much adjustment.  

 

Unfortunately last night was not clear after all.  I was able to set up and do a few things, but there were thin clouds the whole time and there's no way I could do any sort of star test.  Tonight looks clear...

 

sorry, my bad, as I meant removing the baffle when using a Tak scope (somehow I always forget to say that).  If you're not using a Tak scope, then maybe no need to remove the baffle in this case.  Either way, your rings look nice and concentric to me!  Good job!  I would say you're good for a star test now when clouds go away and seeing behaves.  cheers, DJ

 

Actually it was extremely important to remove the scope baffle, in order to see the laser rings on the primary from the secondary.  I think it would have been impossible with the scope baffle in place.


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#28 Alterf

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Posted 14 May 2017 - 12:34 AM

Hi, Joel.

 

For guilding, I've had eggs even when the guiding looks good if I have a cable dragging or some other such variable that affects the whole system.  You've probably checked all that, so . . .

 

You are getting quite a lot of diffraction spiking around the bright star.  It's not a very nice star shape.  I'm curious what causes that.  My CFF Scope has a retaining ring on the primary mirror; there are no clips on any mirrors to leave a mark on the image.  The stars in images from my 290 are almost dead-on round when the scope is collimated, at least in the optical center (the coma-free are is smaller for my scope than yours).  The spikiness in your image also seems to be a similar pattern in both the focused bright star and the unfocused star.

 

Also, you are getting an off-focus back image behind your bright star.  That's not something I've every seen.  Does it suggest an internal reflection?  It is interesting to me that the reflection seems to be perfectly round and collimated.  Is that a hint that the mirrors are working and that something else in the system is causing the diffraction effects?

 

These sorts of things would make me look for causes in the camera setup.  A filter can cause reflections like that.  A stressed lens or filter could cause diffraction, too.  I notice that there is a little bit of a square pattern to the diffraction around the brighter star. 

 

It's a large and complex system, you have.  Will be curioius what you find.

 

Best,

 

Val



#29 Richard Whalen

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Posted 14 May 2017 - 12:32 PM

Looks to me like your imager is not square to the optical path? Perhaps focuser not square?



#30 buckeyestargazer

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Posted 14 May 2017 - 02:44 PM

Hi, Joel.

 

For guilding, I've had eggs even when the guiding looks good if I have a cable dragging or some other such variable that affects the whole system.  You've probably checked all that, so . . .

 

You are getting quite a lot of diffraction spiking around the bright star.  It's not a very nice star shape.  I'm curious what causes that.  My CFF Scope has a retaining ring on the primary mirror; there are no clips on any mirrors to leave a mark on the image.  The stars in images from my 290 are almost dead-on round when the scope is collimated, at least in the optical center (the coma-free are is smaller for my scope than yours).  The spikiness in your image also seems to be a similar pattern in both the focused bright star and the unfocused star.

 

Also, you are getting an off-focus back image behind your bright star.  That's not something I've every seen.  Does it suggest an internal reflection?  It is interesting to me that the reflection seems to be perfectly round and collimated.  Is that a hint that the mirrors are working and that something else in the system is causing the diffraction effects?

 

These sorts of things would make me look for causes in the camera setup.  A filter can cause reflections like that.  A stressed lens or filter could cause diffraction, too.  I notice that there is a little bit of a square pattern to the diffraction around the brighter star. 

 

It's a large and complex system, you have.  Will be curioius what you find.

 

Best,

 

Val

Val, I appreciate your thoughts about this stuff as I know you have good experience.

 

First I want to say that I am 100% certain the issues I'm seeing are user inexperience and not a property of the scope. 

 

Right now I'm not concerned with the off focus back image/reflection around the bright star.  I'm reasonably sure that's a reflection off a glass surface in the optical train and I'll deal with that later.

 

For now I'm focused on nailing down collimation.  Last night was nice and steady and I was able to improve the coma by adjusting the primary mirror a little.  It is much better now but still isn't perfect and I am intentionally taking my time to familiarize myself with the process and do it right.  After making some adjustments last night I wanted to move on to some other things while the seeing was so steady.  So I left collimation incomplete.  Hopefully tonight I'll have the same steady, clear skies and I can further refine the collimation.  

 

I have no idea what might be causing the spikes around the bright star but again I'll deal with that in due time.

Attached Thumbnails

  • frame_and_focus_5.jpg


#31 farn

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Posted 29 May 2017 - 03:31 PM

Hi

 

Is there any progress about your collimation issues. I am an owner of a CFF 132mm APO and I am very happy with it.

I also would like to buy the CFF 250mm RC. I have no experience with RC telescopes, no experience with the collimation of an RC.  

So I am happy, that I found this thread to find out if I could handle such a scope.



#32 buckeyestargazer

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Posted 29 May 2017 - 03:40 PM

Unfortunately the weather hasn't cooperated and I haven't spent much time with the scope and collimation.  However I have made some progress.  One thing I can say is that using a Tak collimation scope makes collimation a lot easier.  It gets collimation quite close and after that small adjustments to the primary are all that is needed.  I had some clear sky last night and will again tonight and I'm close...hoping I can nail it down soon.  


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#33 Guest_djhanson_*

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Posted 29 May 2017 - 04:58 PM

Joel, that's good to hear!  I do like the Tak scope myself...the only thing if they could maybe figure a way to reduce the price of the 2" adapter - dang near more than the scope itself! :D  DJ



#34 buckeyestargazer

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Posted 29 May 2017 - 05:56 PM

Joel, that's good to hear!  I do like the Tak scope myself...the only thing if they could maybe figure a way to reduce the price of the 2" adapter - dang near more than the scope itself! laugh.gif  DJ

When I found out that I needed to buy an adapter with it...I was not happy.  In my opinion that borders on robbery.

 

Thankfully I found a good used collimation scope that came with an adapter for Astro-Physics threads.  I was already intending to use an AP reducer so I needed to have an adapter made for that anyway.  

 

Regarding collimation...something just isn't quite right yet and to be honest I'm not really sure where to go from here.  If anyone wants to take a look, here's a link to download 5 images I took last night while collimating and after.  Some of the stars still have a weird flare on them.  I'm not convinced that the primary mirror retention nuts are tight (or loose) enough.  I'm pretty sure they were too tight because I was getting triangular stars and when I loosened it the triangular shape mostly went away.  But loosening the nuts further didn't completely eliminate the aberration I'm seeing.  

 

Anyway if someone could take a look and tell me what they think I would appreciate it.  



#35 dpastern

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Posted 30 May 2017 - 08:28 AM

Not sure if this will help Joel, I have zero personal experience with collimating a RC, and it's been many years since I've collimated a Newtonian...

 

https://www.youtube....5N5bfB9A&t=133s

 

and 

 

https://deepspacepla...rccollimate.php

 

I know that both refer to the GSO RC and that that has some unique issues with the design and thus collimation but perhaps either reference will assist you further!

 

if in doubt, ring the manufacturer and talk to someone there.  You've paid an awful lot of money for a scope, make them work for their $!


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#36 farn

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Posted 04 June 2017 - 04:07 AM

Hi Joel

 

May I ask you if you bought the Supremax or the fusec silica main mirror version.



#37 buckeyestargazer

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Posted 04 June 2017 - 01:51 PM

Hi Joel

 

May I ask you if you bought the Supremax or the fusec silica main mirror version.

I ordered the Supremax.



#38 buckeyestargazer

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Posted 08 June 2017 - 12:40 PM

I have had some wonderfully clear and steady nights to collimate the CFF scope.  I am not too proud to admit that since I've never done this before it was a steep learning curve and I struggled with collimation.  But I have learned a lot and now (after several attempts) I can probably collimate precisely in 1-2hrs.

 

For collimation I used one of two cameras I own, a QHY163M, because the download times are way faster than my other camera (Moravian G3-16200).  I used the DSI collmation procedure which worked very well.

 

I got the scope aligned as well as I think I possibly could with the QHY camera.  But the G3-16200 camera is the main camera I will use with the CFF RC, so I put that camera on and was disappointed that collimation seemed to be quite a bit off again after changing cameras.  Is that typical?  I'm guessing so but it means I'll have to collimate again with the camera in place and in the orientation I'll use for imaging.

 

The attached images showing collimation were what I ended up with last night from the QHY camera.  The round reflection is from my LUM filter (an Astrodon).  But I figured that Vega is super bright so I shouldn't be surprised at the reflection.

Attached Thumbnails

  • Vega_Final.jpg
  • Collimation6.png


#39 akulapanam

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Posted 08 June 2017 - 02:37 PM

Your collimation shouldn't change with cameras but your tilt can and the fov is so much bigger that great collimation on the small chip may look worse in the bigger chip.

#40 buckeyestargazer

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Posted 08 June 2017 - 03:14 PM

Regarding tilt, that is very true and I suspect that the tilt in my G3-16200 camera itself is further off than the QHY camera.  The large round reflection is from the LUM filter (I've tested for other sources and it comes down to the filter).  When I switched cameras the reflection was much more noticeably de-centered.  

 

So I guess I'll start over...again!  lol.gif



#41 Suavi

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Posted 12 June 2017 - 06:53 PM

The scope looks great Joel and the last star test looks pretty good too waytogo.gif

 

Not sure if Moravian cameras have capacity for tilt adjustement, so a tilt adaptor might be needed to get the CCD orthogonal to the optical axis. Gerd Neumann makes them, and I am confident that Catalin could manufacture one for you if needed.

 

If there is a tilt within the camera's body, then adjusting mirrors to compensate for CCD being not precisely positioned within the camera's body may not necessarily lead to perfect stars.

 

But once you get the scope fine-tuned, it should perform beautifully. It may take a bit of time to learn to precisely and confidently collimate the scope.

 

Looking forward to the next update.

Suavi



#42 akulapanam

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Posted 12 June 2017 - 09:31 PM

If the Moravian does have tilt adjustment in the camera getting the camera CCD centered is going to be A LOT easier than the scope based on my RH200 experience.  SXCCD has a good doc 

 

https://www.sxccd.co...ligning_CCD.pdf .  I have never changed tilt using this method but I did check alignment using it.



#43 buckeyestargazer

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Posted 13 June 2017 - 08:31 AM

Unfortunately I don't believe that the tilt of the Moravian camera can be adjusted.  I do have a question in to Moravian to know for sure.  Still, it seems like having a tilt adjuster would be a good thing in the long run, especially on a precise instrument like this.  



#44 Jared

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Posted 13 June 2017 - 10:19 AM

If the tilt is not adjustable in the Moravian, you'll need to use shims instead. Aluminum foil works well for this sort of thing. You would put the shim(s) between the camera body and the mounting plate. The challenge, of course, is knowing how thick a shim to use (thicknesses of foil) and which corner or corners. I don't know of any solution aside from trial and error and CCDInspector. If you think it's a camera tilt issue, not a collimation issue (and your evidence with the smaller chip suggests that) then don't mess with the mirrors--address this through the camera.

#45 Suavi

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Posted 13 June 2017 - 03:06 PM

Shims are a cheap and sure way to fix tilt in a camera, but finding the right adjustment can be quite fiddly. This tilt adjusting unit should thread directly into G3 camera: http://www.gerdneuma...h-xl-m68x1.html


Edited by Suavi, 13 June 2017 - 03:07 PM.


#46 buckeyestargazer

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Posted 13 June 2017 - 04:03 PM

I'll have to think about this for a bit.  I think if I need to adjust the tilt in the camera I'd rather have a tilt adjuster than shims.  But the other thing I'm not sure about is which camera has tilt, or both?  lol.gif



#47 akulapanam

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Posted 13 June 2017 - 09:26 PM

I'll have to think about this for a bit.  I think if I need to adjust the tilt in the camera I'd rather have a tilt adjuster than shims.  But the other thing I'm not sure about is which camera has tilt, or both?  lol.gif

If I were betting it would be the ASI.  I would also verify that its not your focuser that is tilting based on the weight.  The best way to do that is rotate the camera and see if the tilt moves.


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#48 Suavi

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Posted 14 June 2017 - 01:09 AM

I looks like a 3.5" FTF, so it really shouldn't flex under lightweight ASI nor G3, unless the drawtube itself is already tilted/not precisely aligned. 



#49 buckeyestargazer

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Posted 14 June 2017 - 08:14 AM

The focuser is a FTF3215, a "true" 3 inch focuser.  This is a relatively new focuser for Starlight.  

 

You both bring up another issue that I have not mentioned in this thread and that has to do with the focuser.  In the typical way of collimating a scope a laser is first inserted into the focuser and the focuser flange is adjusted so that the laser dot is centered on the secondary mirror.  But in my case I found that when rotating the focuser the laser dot does not remain stationary but rather traces a small circle.  I created a small video looking at the laser dot as I rotate the focuser.  I am not sure if this is normal, so I contacted Starlight and hope to hear back from them soon.

 

So what I did was instead of centering the laser dot precisely on the secondary, I centered the circle that the laser dot traces.  

 

Also, I've gone back to some old images taken with both cameras and measured them in CCDInspector, and the Moravian camera clearly shows more (dramatic!) tilt than the QHY camera does.  



#50 KJL

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Posted 14 June 2017 - 11:27 AM

You are getting quite a lot of diffraction spiking around the bright star.  It's not a very nice star shape.  I'm curious what causes that.  My CFF Scope has a retaining ring on the primary mirror; there are no clips on any mirrors to leave a mark on the image.  The stars in images from my 290 are almost dead-on round when the scope is collimated, at least in the optical center (the coma-free are is smaller for my scope than yours).  The spikiness in your image also seems to be a similar pattern in both the focused bright star and the unfocused star.

The many non-symmetric spikes (even in the Vega shot from the 8th) is odd: if this happened in a newtonian I'd actually first conclude that the mirrors themselves needed to be checked before I wasted time on other parts of the scope. Am I crazy to think this?

 

Out of curiosity, where does one even send mirrors on exotic scopes like these to be checked? The same as I might for a dobsonian, e.g. Mike Lockwood, Carl Zambuto, etc? Send the entire scope to William Rohr? This has been one of my genuine hangups about buying exotic scopes from smaller manufacturers, so I am genuinely interested in the process of verifying the quality optics.




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