Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Telescope tube problem

  • Please log in to reply
9 replies to this topic

#1 Stipe Vladova

Stipe Vladova

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 08 Oct 2010
  • Loc: Split, Croatia

Posted 11 October 2019 - 05:36 AM

Hi to all! My newly made OTA with Sheldon Faworski's ED127 triplet objective has a problem. I centered the lens last night (push pull lens cell screws and radial lens element collimation screws) to elliminate axial decenter coma and lateral colors and all was fine. The scope was on my homemade eq mount and I used 5 star artificial sky lamp. Telescope worked excellent at highest magnification I can get, 310x (barlow 2x and 6mm Gold line ep. The collimation was done with my camera and a laptop in live view mode in APT software for me to see what I am doing. But when I lifted the scope to near zenith to do a real star test, all I saw is a significant coma and lateral colors. Procyon was a rainbow! It was very high at the sky at that time so that was not an atmospheric dispersion! I then returned the scope position back to my artificial star and the image was fine again as it was when I collimated the lens! The collimating screws are tight enough for prevent any movement of the lenses! So I think something is wrong with the tube. It probably sags a lot! The tube is PVC-C used for swimming pool instalations. It is 140mm O.D and 5mm thick wall. Near the focuser I put a finder and a guider next to each other and mounted in their aluminum rings. I have no rings for this telescope so I screwed the Vixen dovetail directly on the tube. Now tell me what to do... Should I afterall need to switch to aluminum tube which was impossible to find here in these dimensions, or is it enough to put the rings on my scope on the longest dovetail I can find so the rings would be at the ends of the tube for stability and to prevent sagging?

 

Thanks

 

S

Attached Thumbnails

  • IMG_20191010_211506-1024x768.jpg
  • IMG_20191010_211816-1024x768.jpg
  • IMG_20191010_212106-768x1024.jpg

Edited by Stipe Vladova, 11 October 2019 - 05:51 AM.


#2 m. allan noah

m. allan noah

    Apollo

  • *****
  • Posts: 1133
  • Joined: 14 Aug 2009
  • Loc: Virginia, USA

Posted 11 October 2019 - 06:05 AM

lock the altitude axis, and push up on that camera with your hand. how does the image change?

 

allan


Edited by m. allan noah, 11 October 2019 - 06:05 AM.

  • cookjaiii likes this

#3 happylimpet

happylimpet

    Soyuz

  • *****
  • Posts: 3989
  • Joined: 29 Sep 2013
  • Loc: Southampton, UK

Posted 11 October 2019 - 06:13 AM

I expect the sag isnt so much in the main OTA itself, but in that impressive array of extension tubes. Assuming you dont need those to focus at infinity, it may be fine in real use?



#4 Stipe Vladova

Stipe Vladova

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 08 Oct 2010
  • Loc: Split, Croatia

Posted 11 October 2019 - 06:58 AM

I tried that. These aluminum extension tubes dont move at all, they are rock solid, there is no plastic parts between them nor any of these tubes are plastic. These extensions I use for planetary imaging since inside is 20mm plossl for eyepiece projection. There is a difference in in the image quality in various OTA positions when I use a diagonal which quality is proven. So let me clear this a bit more; I put the diagonal and 6mm eyepiece to look at Procyon, which is high at the sky in the morning, then look, image is good, then flip the meridian, look again, the lateral colors and coma appear! I repeat, there are no tube rings, just the dovetail screwed directly in the OTA. I will put a laser in the focuser tonight, allign it so it points the center of the lens, and flip meridian to see if it moves. 


Edited by Stipe Vladova, 11 October 2019 - 06:59 AM.


#5 PETER DREW

PETER DREW

    Apollo

  • *****
  • Posts: 1222
  • Joined: 31 May 2017

Posted 11 October 2019 - 07:34 AM

Two possibilities that I can think of. First is that the overall tube length sags, the collimation was performed in the sagged position and then straightens when at high altitude cauasing the collimation to change, or possibly the cell isn't holding the objectives components properly.
  • Kunama likes this

#6 macdonjh

macdonjh

    Skylab

  • *****
  • Posts: 4355
  • Joined: 06 Mar 2006

Posted 11 October 2019 - 07:48 AM

I expect the sag isnt so much in the main OTA itself, but in that impressive array of extension tubes. Assuming you dont need those to focus at infinity, it may be fine in real use?

This was my first guess also.  But you're certain the aluminum extensions are rigid.  Have you checked the joint between the PVC main tube and the aluminum extension tubes?  Perhaps there is a little slop there?

 

Also, I can't see how long your dovetail bar is, but perhaps your main tube, even with the dovetail, isn't as stiff as you want it to be.  I know this only adds weight, but a cheap, quick experiment would be to secure a piece of wood to the OTA as a stiffener and then screw the dovetail bar to the wood.  If that solves your problem you need a different tube for your scope.

 

I also like PETER DREW's idea: you've collimated in a flexed position and when you point toward zenith the tube "unflexes", introducing distortion.  Could you collimate using Procyon and then see what happens when your put your scope back to horizontal to look at your artificial stars?


Edited by macdonjh, 11 October 2019 - 07:48 AM.


#7 Stipe Vladova

Stipe Vladova

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 08 Oct 2010
  • Loc: Split, Croatia

Posted 11 October 2019 - 08:06 AM

When I collimated to Procyon the artificial 5star sky lamp was distorted, comatic and with lateral colors. Then I flipped the mount to see Procyon in a flipped meridian position, the image was distorted, then turned the mount back and Procyon was ok. My Vixen dovetail bar is 8" long. I am affraid that I must ask someone to make aluminum rings on a CNC machine. This is how I mounted the bar. I am affraid that you are right, the tube flexes and unflexes depending on its position... frown.gif

Attached Thumbnails

  • IMG_20191011_150301-800x600.jpg

Edited by Stipe Vladova, 11 October 2019 - 08:14 AM.


#8 dan_h

dan_h

    Mercury-Atlas

  • *****
  • Posts: 2752
  • Joined: 10 Dec 2007

Posted 11 October 2019 - 11:18 AM

I am surprised to hear that a good triplet lens when properly adjusted has any visible coma, either on axis or off axis. Lateral color should also not appear in the off axis images until you are a very long ways off axis. (You wouldn't see these kind of flaws on the edge of a lunar image.) These aberrations should not be easily visible even 1 degree off axis.  If your images are showing these faults you could see where the tube was bending if that is the cause. I would think one of the elements is shifting inside the lens assembly. 

 

Coma is usually a pretty good indication of wedge error in the lens; either a glass element is not optically centered  or airspace is wedged.

 

I've been wrong before. 

 

dan 


Edited by dan_h, 11 October 2019 - 03:56 PM.


#9 Stipe Vladova

Stipe Vladova

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 08 Oct 2010
  • Loc: Split, Croatia

Posted 11 October 2019 - 05:31 PM

The real cause found! Seems that the plossl eyepiece used for projection was the problem. It has wedge and seems that I collimated incorrectly all the time. I machined a 1.25" adapter for my laser pointer and put it in the focuser. It seems that the tube does not sag and the focuser is squared because the laser beam pointed at the center of the lens. Now I must find a better eyepiece for recollimate the lens or maybe put the camera only at the prime focus and try to collimate. 

Attached Thumbnails

  • IMG_20191011_223747-600x800.jpg
  • IMG_20191011_214029-600x800.jpg
  • IMG_20191011_214103-600x800.jpg

  • happylimpet and davidc135 like this

#10 dan_h

dan_h

    Mercury-Atlas

  • *****
  • Posts: 2752
  • Joined: 10 Dec 2007

Posted 11 October 2019 - 06:11 PM

Happy to hear you found the issue and it's not with your lens. I thought that you had seen the problem visually when using a diagonal and your 6mm eyepiece without the projection in place. Glad it has worked out.

 

dan




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics