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Cannot focus MallinCam SkyRaider

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#1 D.T.

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Posted 01 April 2016 - 12:46 PM

I recently bought a new MallinCam SkyRaider Deep Sky Mono and I am having a problem which is keeping me from using the device, and I am asking for help on figuring out what I have wrong.

 

The MallinCam is behind an Orion 120mm f5.0.  The problem is that when I try to focus the telescope on a bright star, the best focus I can get is a huge circle of light which I would guess is about 3 or 5 % of the entire sensor field of view.  When I replace the Cam with an eyepiece, I get a perfectly good image, so no problem with the telescope.  I have tried both with a focal reducer, and without, but still the same result.

 

Perhaps there is some setting I could have wrong?  I welcome any suggestions.  I am very eager to use my MallinCam but it is useless to me until I can find some solution to this focusing problem.

 

thanks,

 

Dana

 



#2 Dwight J

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Posted 01 April 2016 - 01:04 PM

You may need more in or out travel in your focuser.  The camera does not focus at a similar position to the eyepiece as the sensor is recessed in the camera body.  If you rack your focuser all the way in and still don't get focus that is a problem.  If you rack out all the way you can add an extension tube, slide the camera out a bit, not too far so it is a risk of falling out, or add a diagonal.  



#3 Don Rudny

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Posted 01 April 2016 - 01:44 PM

With my refractors and a similar camera, I need extensions at the native focal length.  With focal reducers the focus point moves further inward until the focuser is racked fully in.  At that point you either need a camera/focal reducer combo that can slide inward into the focuser or use less focal reduction.



#4 D.T.

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Posted 01 April 2016 - 10:00 PM

I have extension tubes.  I have a 2" long extension tube, and I have a 3.5" long extension tube.  I understand that it doesn't focus at the same point as the eyepieces.  The thing is that I can adjust the focus such that the size of the disk of light, from the star, reaches a minimum.  Turn the focus in and the disk grows.  Turn the focus out, and the disk also grows.  So I'm expecting that this minimum size disk of light is where focus should be.  But the size of the disk is huge, even at that minimal point.  Are you suggesting that if I adjust the focus even further out the size of the light disk may grow for a while, then start to shrink again as I get to the actual focus point?  I wouldn't expect focusing to behave like this.  But I could be wrong.

 

Dana



#5 StarCurious

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Posted 01 April 2016 - 11:58 PM

I have extension tubes.  I have a 2" long extension tube, and I have a 3.5" long extension tube.  I understand that it doesn't focus at the same point as the eyepieces.  The thing is that I can adjust the focus such that the size of the disk of light, from the star, reaches a minimum.  Turn the focus in and the disk grows.  Turn the focus out, and the disk also grows.  So I'm expecting that this minimum size disk of light is where focus should be.  But the size of the disk is huge, even at that minimal point.  Are you suggesting that if I adjust the focus even further out the size of the light disk may grow for a while, then start to shrink again as I get to the actual focus point?  I wouldn't expect focusing to behave like this.  But I could be wrong.

 

Dana

You mentioned "bright star".  Once you get to the minimum disk, point the scope to a not so bright star with focuser locked and if you have just achieved focus with the bright star, the disk should become smaller with the dimmer star.  It is possible that you need a shorter exposure duration for a bright star to make the disk smaller.

 

You may wish to buy a "Bahtinov focus mask" to help you with optimal focusing. 



#6 mclewis1

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Posted 02 April 2016 - 10:33 AM

Dana,

 

Clearly you understand and have a handle on the focus position. You are moving through the focus from inside to outside of focus as you see the "disk" get smaller and smaller and then larger. You are not seeing a clean small dot of an in focus star in your scope because of chromatic aberration (CA). This is the blue fringing around brighter objects that is inherent in achromatic refractors like your Orion 120. On a bright star with a big achromat and a sensitive camera it can make accurate focusing almost impossible.

 

- use a not so bright star for your focusing, this way the CA is much less obvious

- use a yellow or deep yellow color filter to knock down the blue. You'll have to keep the filter on the camera when using it because you won't be able to add the filter, focus, remove the filter ... that will change the focus point. Other filters can be an option too ... they will also offer different levels of reduction in the blue fringing (along with whatever other benefits they may provide).

 

Have a chat with Ken (SkyDragon), he has a lot of good experience with large achromats and fast cameras using yellow filters.



#7 nic35

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Posted 02 April 2016 - 10:37 AM

Dana

 

It sounds like you found the focal point.  assuming your scope is an achromat, you may be suffering IR induced "bloat".  

 

There was a thread going on using achromats for EEA,.  See http://www.cloudynig...ats-for-video/ 

 

It has some links to Ken James' very useful videos.

 

j



#8 nic35

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Posted 02 April 2016 - 10:38 AM

Great minds think alike 



#9 Dragon Man

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Posted 02 April 2016 - 12:05 PM

Dana,

 

Clearly you understand and have a handle on the focus position. You are moving through the focus from inside to outside of focus as you see the "disk" get smaller and smaller and then larger. You are not seeing a clean small dot of an in focus star in your scope because of chromatic aberration (CA). This is the blue fringing around brighter objects that is inherent in achromatic refractors like your Orion 120. On a bright star with a big achromat and a sensitive camera it can make accurate focusing almost impossible.

 

- use a not so bright star for your focusing, this way the CA is much less obvious

- use a yellow or deep yellow color filter to knock down the blue. You'll have to keep the filter on the camera when using it because you won't be able to add the filter, focus, remove the filter ... that will change the focus point. Other filters can be an option too ... they will also offer different levels of reduction in the blue fringing (along with whatever other benefits they may provide).

 

Have a chat with Ken (SkyDragon), he has a lot of good experience with large achromats and fast cameras using yellow filters.

LOL! When did I change my Username ?   :lol:

 

Dana

 

It sounds like you found the focal point.  assuming your scope is an achromat, you may be suffering IR induced "bloat".  

 

There was a thread going on using achromats for EEA,.  See http://www.cloudynig...ats-for-video/ 

 

It has some links to Ken James' very useful videos.

 

j

Dana, it sounds like 3 things to me, and they have all been mentioned above.

Over-exposure, Chromatic Aberration/violet fringing,  and Star bloat.

 

The Orion 120mm f5 is one of the Achromatic Refractors I use (mine is a Saxon, but it's the same scope from the same 'Synta' factory)

 

Don't worry about using extension tubes etc because you have found focus. It is when you say that the bright dot is at its smallest.

 

1. Using a bright star you will need to reduce your exposure time.

2. You will also need to use an IR filter. That will get rid of the Star Bloat in a Refractor.

3. Also, because the scope is an Achromat you will also need as Mark has suggested, and that is a Yellow filter. You can use either a #8 or #12 Wratten Yellow filter. They are only about $12.

What the Yellow filter does is gets rid of the terrible Chromatic Aberration in Achromats, and Violet Fringing.

See my examples HERE

 

This will show you the necessity of an IR filter on stars: HERE

 

Another tip would be to try use the Focal Reducer. Not only does it give a wider FOV but also smaller sharper stars.

By doing all the suggestions I have given you shouldn't have any problems because the 120mm f5 Achromat works great for Live Viewing.



#10 D.T.

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Posted 02 April 2016 - 01:20 PM

Questions:

1) If I use both the Yellow 12 filter and the Infrared blocking filter how many stops of light do I lose?

2) Where do I get these filters?  And for $12 or something in that ballpark?

3) I will need to get filters which I can put on my Mallincam?  I want to get the right size and threading.

4) Assuming that I get the right size and threading I would guess that I would stack them with the focal reducer I got from Rock Mallin, UV/IR, Yellow 12, focal reducer threaded stacked to each other right?  If that is so, then in what order?

 

thanks,

 

Dana



#11 mclewis1

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Posted 02 April 2016 - 03:38 PM

Dana,

 

Look for inexpensive 1.25" color filters. These are threaded to attach to eyepieces, Barlows, etc. and will indeed thread onto the front of the Mallincam 1.25" focal reducers. You generally want to attach the filters onto the front of the focal reducer. If you are using spacers with the focal reducer a filter can often be used in place of a 5mm 1.25" spacers. Watch out for C mount threaded spacers however (C threads are what is on the front of the camera and they are smaller in diameter than 1.25" filter threads), if you have C mount spacers for your focal reducer then the filters will have to go in front of the reducer.

 

If you are not using a focal reducer then the 1.25" nosepiece that's included with your camera is also threaded for 1.25" filters and they can simply be stacked onto the front of that.



#12 D.T.

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Posted 02 April 2016 - 07:54 PM

Many thanks to all.  I have a direction.  Hopefully this will fix my problem.



#13 Dragon Man

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Posted 03 April 2016 - 08:09 AM

Questions:

1) If I use both the Yellow 12 filter and the Infrared blocking filter how many stops of light do I lose?

2) Where do I get these filters?  And for $12 or something in that ballpark?

3) I will need to get filters which I can put on my Mallincam?  I want to get the right size and threading.

4) Assuming that I get the right size and threading I would guess that I would stack them with the focal reducer I got from Rock Mallin, UV/IR, Yellow 12, focal reducer threaded stacked to each other right?  If that is so, then in what order?

 

thanks,

 

Dana

Dana,

 

1. I doubt you will lose any stops of light because the cameras we use are so sensitive that a tiny loss wouldn't even be noticeable.

When I use a #8 yellow I see no noticeable light loss.

When I use a #12 I actually get an increase in Contrast which makes the objects actually appear brighter.

Of course they aren't really brighter. The view is cleaned up and the contrast is boosted giving a brighter looking object and a darker looking background. A more 'pleasant' contrasty result.

 

2. The filters should be available at any Astronomy shop (online or walk-in), and Yes, the Yellow filters range from about $12 to about $19

IR filters are a bit more and can range from $29 to $79

 

3. 1.25" filters all have the same thread and will screw onto the front of all the 1.25" C mount nosepieces and 1.25" focal reducers we use, no matter what brand of camera.

 

4. If using a Focal Reducer: Camera > FR > IR filter > Yellow 

 

If NOT using a Focal reducer: Camera > Cmount nosepiece > IR filter > Yellow.

 

:waytogo:



#14 D.T.

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Posted 05 May 2016 - 10:35 PM

For the record, the suggested solution of Yellow 12 filter and Infrared blocking filter seems to work pretty well.  So, this is the solution for the posted problem.

 

Thanks All


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#15 jimthompson

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Posted 06 May 2016 - 12:16 PM

Dana,

 

1. I doubt you will lose any stops of light because the cameras we use are so sensitive that a tiny loss wouldn't even be noticeable.

When I use a #8 yellow I see no noticeable light loss.

When I use a #12 I actually get an increase in Contrast which makes the objects actually appear brighter.

Of course they aren't really brighter. The view is cleaned up and the contrast is boosted giving a brighter looking object and a darker looking background. A more 'pleasant' contrasty result....

Hi Dana,

 

An important caveat to Ken's recommendations is that he is observing from a dark site.  If you are as well then his recommendations should work for you.  If you are observing from an urban or suburban location, you may want to consider a different approach.

 

To back up a step, I strongly recommend you get yourself a focus mask to use with your setup.  Using a focus mask there is no question whether or not you are at the best focus.  I have used the link below numerous times to custom print my own masks, and they work great.

 

http://astrojargon.n...CookieSupport=1

 

The next thing you should know is that there are two reasons for star bloat:

 

1.  Overexposed sensor pixels, and

2.  Chromatic aberration.

 

The sensors used in astronomy cameras are very sensitive, so sensitive that the light from stars tends to over expose them.  Picture each pixel on the sensor as a bucket collecting water.  At some point the bucket overflows and the extra water spills into neighbouring buckets.  The result is that the light from a single star, because it is so bright, spills over and fills multiple pixels making the star look larger.  The way to combat this is to somehow reduce the brightness of stars but not the object you want to view.  A UV/IR cut filter is a good way to do this, and should work well on all types of objects when under dark skies. Under light polluted skies a UV/IR cut filter will help emission nebulae, but will hurt the view of galaxies so use only if your bloat problem is bad.  Another way to reduce the brightness of stars is to use a Light Pollution filter.  An LP filter is designed to do just what we need, reduce visibility of things like stars and leave visibility of nebulae mostly unaffected.  Under light polluted skies LP filters will also help the contrast on galaxies; they will appear darker and you will need more exposure time, but the contrast will be better.  Under dark skies you are better off not using an LP filter on galaxies.  On emission nebulae an LP filter helps regardless of your LP level.

 

The second cause of star bloat, chromatic aberration, can be dealt with using the same kinds of filters mentioned above.  LP filters and UV/IR cut filters reduce the range of wavelengths you are trying to focus at the same time, making it easier to get a good focus.  This is also why Ken's suggestion of the yellow filter helps, you are reducing the range of wavelengths you are trying to focus.  The extreme end of this is using narrowband filters such as Halpha or O-III, which will provide the sharpest focus using an achromat.

 

After you have confirmed using a focus mask that you are indeed well focused, you will probably need to try each of the filter types mentioned here or by Ken to see what works best for your combination of conditions, telescope, camera, and object observed.

 

Best Regards,

 

Jim T.




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