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Unable to focus camera with flip mirror

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#1 Psionmark

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Posted 11 April 2021 - 06:30 AM

Hi all.

I purchased a flip mirror as it seems a good idea being able to switch easily between the eyepiece and the camera. However, I’m unable to focus the camera when it’s in the flip mirror.

I’ve experimented during the day and it seems the camera (SV305) is too far out to be able to focus.

I’m still a newbie in this game so please excuse beginner gaffs but do I need a focal reducer to be able to achieve focus? Or some other gizmo?

#2 StarBurger

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Posted 11 April 2021 - 07:09 AM

I had exactly this problem with my first flip mirror. It pushed the camera too far back to reach focus. 

I did fix it by carefully taking measurements to see that it needed much lower profile adapters both front and back of the flip. Luckily I had some on hand.

Even after, the focus tube only had about 5 mm remaining before bottoming inwards to play with.

Your flip and focus tube are probably not the ones I have (Astromania flip on an SW ED 80) so I can't say precisely what you will need.

A FR may work but I would be reluctant to limit your options in this way.

 

Actually, thinking about it, you may need a focal extender rather than reducer, an even worse combination.


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#3 Visit-the-Moon

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Posted 11 April 2021 - 07:41 AM

Psionmark, I use a flip mirror for high power planetary and lunar imaging at f/16 and f/20, it is very useful in this case. For wide field EAA I'm not sure it is so useful. As you are looking at a screen your night vision for observing through an eyepiece, assuming you are looking at nebulae or galaxies, is ruined. I tend to either view through a 16" Newtonian with no lights, red spot finders and computers around so that my eyes are dark adapted, or I do EAA and use a big bright screen. For EAA finding and pointing I use SkySense to find most objects. If well set up the object of interest is usually close to the centre of the field (which is 1.2° in my case). 

 

If you are using an SCT, or a refractor with a field flattener/reducer, there will be an optimum back-focus, that is the distance between the rear thread on the telescope and the imaging chip. You can look that up in a search. Get that right then set up the flip mirror - you might need to use an extension tube. You need to be able to separately focus the eyepiece. If the flip mirror doesn't incorporate a focuser for the eyepiece you can add for example ZWO's 1.25" helical focuser. 


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#4 Psionmark

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Posted 11 April 2021 - 08:07 AM

Psionmark, I use a flip mirror for high power planetary and lunar imaging at f/16 and f/20, it is very useful in this case. For wide field EAA I'm not sure it is so useful. As you are looking at a screen your night vision for observing through an eyepiece, assuming you are looking at nebulae or galaxies, is ruined. I tend to either view through a 16" Newtonian with no lights, red spot finders and computers around so that my eyes are dark adapted, or I do EAA and use a big bright screen. For EAA finding and pointing I use SkySense to find most objects. If well set up the object of interest is usually close to the centre of the field (which is 1.2° in my case). 

 

If you are using an SCT, or a refractor with a field flattener/reducer, there will be an optimum back-focus, that is the distance between the rear thread on the telescope and the imaging chip. You can look that up in a search. Get that right then set up the flip mirror - you might need to use an extension tube. You need to be able to separately focus the eyepiece. If the flip mirror doesn't incorporate a focuser for the eyepiece you can add for example ZWO's 1.25" helical focuser. 

Very good point re the night vision - I'd not considered that. My main use case really was to use the eyepiece to get the scope aligned and then switch over to the camera. It's not the biggest hassle in the world to remove the eyepiece and insert the camera though. Just trying to reduce the number of steps for each session to the minimum.



#5 c2m2t

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Posted 11 April 2021 - 08:09 AM

Hi Psionmark!

I can understand the thoughts of cutting the tube of your OTA, but that is the simplest and most effective way to mitigate the problem. A similar issue presents itself with bino-viewers. I am in the midst of a double stars imaging project that began about 11 years ago and will likely take another 10 years to complete...I just hope I don't run out of gas. As a result, I decided to shorten the tube of my SW 100 ProED scope since it is permanently teamed up with my 2" Meade flip mirror.  I have had a bit of experience with shortening tubes when I began using bino-viewers. It seems that most refractors will not come to focus with binoviewers unless you place a barlow in the light path. Adding a barlow increases unwanted magnification. I started out with some inexpensive achromats and I very quickly realized the benefits of shortening the OTA. I have two dedicated scopes to use with a bino-viewer, a 4" and 5" and both are F9. The solution to getting  back the OTA length is to add a suitable length extension tube...the most popular lengths are 1 and 2 inch.

 

The only word of caution, make sure you have bought into the idea and that the tube shortening with not hinder future use of the telescope. The second recommendation is that you have an experienced machine shop perform the work...they are simply better equipped to do accurate work....unless of course...like a few CN'ers, you too have your own fully equipped machine shop. In the case of my SW 100 ProED, I had to shorten my tube by about 1" which kept the focus tube on the focuser extended by about 3/4" to bring the setup to focus.

 

The other alternative is to have custom fittings made that will reduce the length of the flip-mirror assembly....or one can find a focuser body that is shorter than what you already have. A final suggestion is to see if you can track down an old OTA tube that has the same thread as your lens cell or is such that it can be adapted to fit your lens cell and focuser and then simply have a second  shorter tube that is dedicated to flip-mirror use.

 

Good luck!!

 

Cheers, Chris.


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#6 alphatripleplus

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Posted 11 April 2021 - 08:12 AM

I’ve experimented during the day and it seems the camera (SV305) is too far out to be able to focus.

I’m still a newbie in this game so please excuse beginner gaffs but do I need a focal reducer to be able to achieve focus? Or some other gizmo?

If the camera is too far out to achieve focus, that implies that the focal plane is closer to the objective. Adding a focal reducer will not help as it will pull the focal plane even further in. You don't mention what scope you have, or what you have the flip mirror attached to. If you are using extension tubes between the flip mirror and the focuser, you may have to remove them to get focus with the flip mirror attached.


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#7 StarBurger

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Posted 11 April 2021 - 09:42 AM

One further problem I found with my flip was that the mirror was not set accurately to 45 degrees. What was centered in the EP was not centered in the camera.

Helpfully there was a set screw built in to adjust the mirror stop point.

My EP branch has a helical so that the mirror focus is bang on the same as the camera. 



#8 Psionmark

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Posted 12 April 2021 - 04:54 AM

Thanks everyone for all the feedback and great ideas! Lots of food for thought - I'll let you know how I get on smile.gif


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#9 nic35

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Posted 12 April 2021 - 12:30 PM

P-mark

 

If you only want to use the flip mirror to center objects, I'd suggest trying plate solving instead..  Pretty easy if you use Shaprcap and ASTAP.   With this combo, Sharpcap will take an image of where you are actually pointing, and recenter it to the object you wanted to go to.  I'm assuming you have connected your mount, and camera to your computer and can control it through ASCOM.  This way you can dispense with the flip mirror gizmo.

 

If not -   

 

One way to analyze your situation is the scotch tape trick i describe in post 10 of https://www.cloudyni...-10-cant-focus/

 

Obviously, you put the tape on the tube where you are putting the camera. 

 

If you know how much backfocus the camera "consumes"  then you can figure out if you have enough focuser travel to accommodate the camera. I couldn't find an on-line spec for the 305.  I suspect it is on the 10 to 17.5 mm range.

 

The focuser will need to move inwards by the amount of camera backfocus to put the image on the cameras sensor.

 

You should mention what kind of telescope you are using.  Scopes with fixed position focal planes (refractors, newtonians) can run into problems.  Scope with focal planes that move - most notably SCT's - do not suffer this problem.

 

 

Hope this helps.

 

john


Edited by nic35, 12 April 2021 - 12:36 PM.

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#10 Psionmark

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Posted 13 April 2021 - 05:26 AM

Thanks again everyone. I was trying this with my tiny reflector. I have switched over to the 90mm refractor and can now successfully focus both the camera and the eyepiece at the same time.

 

I shall see how I get on with this, but I do prefer my little reflector so I shall try out some of the tips from all of these terrific replies.



#11 alphatripleplus

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Posted 13 April 2021 - 08:03 AM

Good that you have it working on one scope, and let us know how it works out.




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