#1  
Old 15-01-2020, 01:36 AM
srmnm (Will)
Registered User

srmnm is offline
 
Join Date: Aug 2019
Location: Wynnum, Queensland
Posts: 6
RASA Collimation/Focus Issue?

Hello All!!


I purchased a RASA 8" and have used it twice, first time really to get a polar alignment done (which i really struggled with on my NexStar8se on the wedge) and the second time to actually take pictures with ZWO ASI1600mm.

Once polar aligned (second time i used) and in focus I took many light frames of the Horsehead Nebula. Once my session was completed, i performed an auto-stretch in Pixinsight and was surprised upon close examination to find that one side of the frame seemed to be in perfect focus but the other side was not.

There is quite a difference and I'm a little confused as to why this has happened on a new OTA. Could it be a factory defect? OTA needing immediate collimation? (Sounds like this shouldnt happen according to the Technical Instruction Manual) or Camera issues, not sure how?

Any info please have to share would be greatly appreciated?

Please see attached for images showing examples.

Thank you,
Will
Attached Thumbnails
Click for full-size image (1 - Bad Focus.jpg)
200.1 KB50 views
Click for full-size image (2 - Good Focus.jpg)
200.7 KB52 views
Click for full-size image (Main Image.jpg)
167.3 KB53 views
Reply With Quote
  #2  
Old 15-01-2020, 05:59 AM
billdan's Avatar
billdan (Bill)
Registered User

billdan is offline
 
Join Date: Mar 2012
Location: Narangba, SE QLD
Posts: 1,365
Looks like a classic case of "camera tilt".

F2 focal lengths are very unforgiving as the critical focus zone is quite narrow.

There are a lot of threads on Cloudy Nights about this, this is one I came across.

https://www.cloudynights.com/topic/6...gnette-issues/
Reply With Quote
  #3  
Old 15-01-2020, 06:13 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,621
Yes definitely tilt. The faster the scope the more it is an issue.

Teleskop Services sells a few tilt adapters.

Otherwise pack out the offending side with something. Spark plug gap measuring pieces work well.
They come in a set.

Greg.
Reply With Quote
  #4  
Old 15-01-2020, 12:00 PM
The_bluester's Avatar
The_bluester (Paul)
Registered User

The_bluester is offline
 
Join Date: Feb 2011
Location: Kilmore, Australia
Posts: 2,231
First things first would be to try rotating the camera 180 degrees to see if the tilt stays in the same place on the images or moves. If it stays in the same place with respect to the camera framing then it is presumably in the camera or the adapters to fit it to the front of the scope.

The design of the assembly to fit the camera was one reason I decided against a RASA 8 at least for now, it just looks like a source of tilt issues to me and at F2 if they are there, you will see them!
Reply With Quote
  #5  
Old 15-01-2020, 08:41 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,621
Unless you have some weak adapters that are not strong enough then the tilt can be handled either by adding a tilt adapter to the train if there is backfocus or simply packing out the offending corner with some packing material like brass shim or spark plug adapters.

Step 1. Orientation:

You need to know how the corners of the physical camera relate to the images.

I use a torch and a rag to dim it down and take a 10 second image and illuminate one corner only. Then see where the light is in the image.
Its most likely the image is reversed top to bottom and left to right with a compound mirrored scope. Do this step until you know which corner of the camera relates to which corner of the image.

Step2: Correction.

Pack out the corner on the camera that relates to the out of focus stars in the image. To get an idea of how much to pack it out on my F3.8 Honders its about .7mm thick. I placed a packer between the camera and the filter wheel.

Take another shot of the stars and check the focus in all 4 corners. Is it better or worse or no change?

It should be better. If worse then step 1 needs to be done again as you are packing out the wrong corner.

Keep adjusting until you have all 4 corners sharp.

Your image shows badly out of focus stars on one corner so expect a fairly large packer to correct - at a guess around 1mm+ thick.

I do this step with all my scopes and when I do it as above it takes about 10 minutes.

If you find you have large packers on most corners its best to start again as it probably is only 1 corner or at most 1 side.

Greg.
Reply With Quote
  #6  
Old 15-01-2020, 11:30 PM
srmnm (Will)
Registered User

srmnm is offline
 
Join Date: Aug 2019
Location: Wynnum, Queensland
Posts: 6
Thank you all for the input! This guidance really helps.
Will test a little more later in the month and let you know the outcome.

Cheers,
Will
Reply With Quote
  #7  
Old 16-01-2020, 07:49 AM
The_bluester's Avatar
The_bluester (Paul)
Registered User

The_bluester is offline
 
Join Date: Feb 2011
Location: Kilmore, Australia
Posts: 2,231
I should post again, what I wrote sounds a little harsh. I seriously considered a RASA 8 for a long time but decided on the simplicity of a nice short focal length (But much slower) refractor.

Regards the attachment method to the front cell, I would suggest a wipe with a cloth of the mating surfaces any time the adapter is removed as any bit of dirt between the mating faces could introduce tilt that will be visible given the size of the critical focus zone at F2.

I would be interested to hear how stable they are focus wise with temperature changes, the traditional SCT is far less touchy to focus but the cassegrain setup magnifies the focus shift as the tube expands and contracts which I would think the RASA design avoids. So while it might be more difficult to get focus it possibly stays there a lot longer.
Reply With Quote
  #8  
Old 16-01-2020, 01:11 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,621
The trend is definitely towards faster telescopes.
But as a general principle I would think the faster the scope the better the surrounding engineering needs to be.

Greg.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 09:41 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
Advertisement
Astromechanics
Advertisement
EQ8-R
Advertisement
Lunatico Astronomical
Advertisement
Testar
Advertisement
Star Adventurer
Advertisement
Celestron RASA
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement