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Problems focusing 8" F4 Quattro with ASA 0.73x reducer

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#1 CarlightExpress

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Posted 12 July 2019 - 05:30 PM

Hi All

 

I purchased an ASA 0.73x reducer for my F4 Quattro, I have the right backfocus from the reducer to the sensor, however according to documentation I need to have 90mm backfocus for the telescope from the 2" connector of the focuser, so I am presuming I need to add say 80mm extension tube to the focuser...would that be right?

 

If what I am asking is correct, then I do not know what the hell is going wrong, focus seems to be far worse than it is with no extension tube and focuser at 0, I just cannot seem to work out where the focus point should be.  Does anyone have any experience with this reducer?

 

TIA

Simon


Edited by CarlightExpress, 12 July 2019 - 05:37 PM.


#2 Jerome Ni

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Posted 12 July 2019 - 05:45 PM

Most likely not. While I have not used an ASA 0.73x, the 90mm in your documentation probably means the distance from the IMAGE PLANE to the 2" connection, WITHOUT the reducer.


Edited by Jerome Ni, 12 July 2019 - 07:38 PM.


#3 CarlightExpress

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Posted 12 July 2019 - 05:49 PM

Very strange indeed, if the 90mm is from the 2" Connector on the focuser to the sensor then my focuser travels out at least 45mm, so with the 65mm back focus from the reducer, +25mm of travel back on the focuser I should hit focus right......well not in this case, the star goes more towards focus the more I focus inwards, doesn't seem to be enough travel on the focuser to go any further "Inwards", this doesn't tie up with everything I have read online about the 90mm back focus from the 2" Focus Connector for the telescope



#4 Starman1

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Posted 12 July 2019 - 07:02 PM

Focal reducers typically reduce the distance to the focal plane and require additional in travel of the focuser to achieve focus.

#5 Eddgie

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Posted 12 July 2019 - 10:10 PM

Focal reducers typically reduce the distance to the focal plane and require additional in travel of the focuser to achieve focus.

Yes, this is exactly the case. 

 

The spacing on the ASA corrector is something like 55mm, and as with any other reducer, you have to rack the focuser in more than it would require without the reducer.

 

If the focal plane of the scope is not at least 85mm above the top of the fully racked in focuser, there will not be enough inward  travel on the focuser to reach focus and have the necessary spacing between the corrector and the sensor.

 

If the scope is short of the necessary inward focus, the primary mirror will have to be brought closer to the secondary, but this is only a partial solution.  As the mirror is moved closer to the secondary, field illumination falls off so it would be best to have a slightly larger secondary with slightly more offset if it were to work as well as possible. 

 

And last but not least, one would have to check to make sure that the fully racked in focuser tube does not extend into the light path.

 

I have a 150mm scope that was designed specifically for the ASA corrector.  I has to have a 40% secondary to make sure the light cone is big enough to fully illuminate an APS-C size sensor when used at f/2.8.  When the scope is in focus, the low profile focuser has only about 8mm of travel left. The focal plane is 90mm above the top of the racked in focuser. The fully racked in focuser tube does not extend into the light path.

 

Boren Simon on Minitower.jpg

 

So, you really need to have a focal plane that sits 90mm above the top of the fully racked in focuser, and most f/4 imaging Newts don't have this much. 

 

ASA reducer focal plane requirement.jpg

 

If the scope is only 10mm or 15mm short, it might be possible to get it to reach focus by raising the primary closer to secondary, but many imaging Newts only have about 60mm spacing from the top of the focuser to the focal plane.  Again, it would also be important to make sure the focuser tube was not sticking too far into the light path when racked all the way in.  Good starting point is to measure the distance from the top of the focuser to the focal plan without the reducer.  


Edited by Eddgie, 12 July 2019 - 10:52 PM.


#6 Eddgie

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Posted 12 July 2019 - 10:18 PM

And as an aside, I saw that someone in the US is selling this reducer, but as can be deduced by my above post, it will not be plug and play in the vast majority of imaging Newts without some modification. 

 

The ASA corrector is a great reducer/corrector, and working at f/2.8 is amazing, but the best result will be achieved with a scope that is properly set up to use it to full advantage.  Most standard f/4 imaging Newts would require pretty extensive modification to work to the full potential of the corrector.

So, the problem is you are going the wrong way.. You need to get the corrector closer to the secondary to reach focus and you will run out of inward travel before that happens. Minimum modification would be to raise the primary mirror maybe 30mm to 40mm closer to the secondary and optimal mod would be to enlarge the secondary and put in the necessary secondary offset for the new spacing.

 

I suspect that you will wind up returning the ASA, and if the vendor is smart, they will edit their page to explain that it will not work with most standard f/4 imaging Newts without modifying the scope.  


Edited by Eddgie, 12 July 2019 - 10:42 PM.


#7 Eddgie

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Posted 12 July 2019 - 11:23 PM

And the camera has to be mounted about 55mm above the top of the corrector. I forget the exact spec but it is something like this.



#8 CarlightExpress

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Posted 13 July 2019 - 02:51 AM

Thanks guys, that makes a lot more sense now, return the reducer I will :(



#9 CarlightExpress

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Posted 13 July 2019 - 03:38 AM

And the camera has to be mounted about 55mm above the top of the corrector. I forget the exact spec but it is something like this.

It's 65mm




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