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Field flattener question

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#1 xanadu30

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Posted 18 December 2020 - 11:52 PM

Hello all, as a VERY beginner in AP, I am interested in a field flattener for my setup.  My question is, are these universal?  Or, do I need a specific one for the setup that I have?  I am using an Orion 120ST EQ refractor and a DSLR in my imaging setup.  Does the field flattener simply fit into the imaging train?  And if so, where exactly?  Thanks so much in advance for any information and advice.



#2 idclimber

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Posted 19 December 2020 - 01:17 AM

The best would be to find the reducer that matches your specific scope. Otherwise searching what others use with the same scope is your best bet. Your scope really is not intended for imaging and Orion does not show a matching reducer on their website. 

 

The flatteners on a refractor are typically installed close to the focuser. They should also specify how far back they suggest placing the camera sensor. This distance is often referred to as backspace. Many of them are 55mm. That spacing makes it pretty easy to hook up a DSLR as you would only need a T-mount for your specific camera to hook things up. 

 

A flattener is not mandatory. You certainly could attempt to image without it. In my quick search here others have proceeded you with this scope. One is a nice narrow band image with a good cooled mono camera without a reducer. You will not get the same results with your DSLR, but that wound not be because of the scope. 


Edited by idclimber, 19 December 2020 - 01:44 AM.

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#3 xanadu30

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Posted 19 December 2020 - 01:49 AM

The best would be to find the reducer that matches your specific scope. Otherwise searching what others use with the same scope is your best bet. Your scope really is not intended for imaging and Orion does not show a matching reducer on their website. 

 

The flatteners on a refractor are typically installed close to the focuser. They should also specify how far back they suggest placing the camera sensor. This distance is often referred to as backspace. Many of them are 55mm. That spacing makes it pretty easy to hook up a DSLR as you would only need a T-mount for your specific camera to hook things up. 

 

A flattener is not mandatory. You certainly could attempt to image without it. In my quick search here others have proceeded you with this scope. One is a nice narrow band image with a good cooled mono camera without a reducer. You will not get the same results with your DSLR, but that wound not be because of the scope. 

Thanks for your input.  When you say "Your scope really is not intended for imaging", can you explain why?



#4 idclimber

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Posted 19 December 2020 - 01:58 AM

Thanks for your input.  When you say "Your scope really is not intended for imaging", can you explain why?

It is a relatively inexpensive doublet. Most (not all) imaging refractors are triplets. This is the number of elements in the scope. It takes more elements to have a fully corrected light path so stars are uniform through the image circle the scope will project onto the camera. The signs of poorly corrected optics are stars that are not round, especially along the edges or in the corners, or colors not lining up correctly. 

 

These corrections are not important for visual astronomy and add cost where it is simply not needed.

 

The other issue is glass, better scopes use better glass to keep the image uniform across the field with less distortion. 


Edited by idclimber, 19 December 2020 - 01:59 AM.


#5 the Elf

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Posted 19 December 2020 - 03:54 AM

Glass refracts the different colors of light by a different amount. The goal is to diffract all colors the same. As you for sure have guessed such glass is expensive. The two best are fluorite and FPL53. The effect can be corrected by using 2 or 3 pieces one canceling out the negative effect of the other. A 2 lens combination is called a doublet, 3 is a triplet. Of course 3 lenses can correct more that two. That is why triplets are preferred. That does not mean a doublet does not work, but you have to use more expensive glass here. The minimum you want for AP is a doublet made of an FPL53 and a lanthanum element. It is better to have a triplet with one FPL53. (FCD100 is about as good as FPL53). If they do not exactly use the terms "FPL53" or "lanthanum" you can be sure it is not in there. "special" "ED" "low dispersive" and such terms are marketing. FPL51 (fifty-one!) is a cheaper glass, not as good as 53 but still acceptable if you don't have high expectations.

I strongly recommend you get yourself a better scope for AP. Please note: there are scopes that already have the flattener inside, so called quadruplets or flat field astrographs. With these things you don't have to worry about where to put a flattener and which distance to use. Just pop the camera in and go. Here is one example:

https://www.teleskop...-Objective.html

If you want excellent images right from the start and an instrument you can use for many years, get one like these.

Here is a low priced that comes with a FPL51/lanthanum doublet. The corrector consists of 2 elements. The price is considerably lower and so is the image quality. You will have blue halos round the brighter stars and small color fringes at the moon.

https://www.teleskop...-Telescope.html

 

If you go for a regular scope and a separate flattener you might also need some spacers to get the distance right. In the end it is not cheaper. Please note, though, that some flat field scopes cannot be used for visual with a diagonal. IMHO you should have two different instruments for imaging and observing. For observing get binos or a cheap but large dobsonian. You can observe while your imaging rig is working for you.


Edited by the Elf, 19 December 2020 - 04:04 AM.

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#6 xanadu30

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Posted 19 December 2020 - 01:42 PM

Glass refracts the different colors of light by a different amount. The goal is to diffract all colors the same. As you for sure have guessed such glass is expensive. The two best are fluorite and FPL53. The effect can be corrected by using 2 or 3 pieces one canceling out the negative effect of the other. A 2 lens combination is called a doublet, 3 is a triplet. Of course 3 lenses can correct more that two. That is why triplets are preferred. That does not mean a doublet does not work, but you have to use more expensive glass here. The minimum you want for AP is a doublet made of an FPL53 and a lanthanum element. It is better to have a triplet with one FPL53. (FCD100 is about as good as FPL53). If they do not exactly use the terms "FPL53" or "lanthanum" you can be sure it is not in there. "special" "ED" "low dispersive" and such terms are marketing. FPL51 (fifty-one!) is a cheaper glass, not as good as 53 but still acceptable if you don't have high expectations.

I strongly recommend you get yourself a better scope for AP. Please note: there are scopes that already have the flattener inside, so called quadruplets or flat field astrographs. With these things you don't have to worry about where to put a flattener and which distance to use. Just pop the camera in and go. Here is one example:

https://www.teleskop...-Objective.html

If you want excellent images right from the start and an instrument you can use for many years, get one like these.

Here is a low priced that comes with a FPL51/lanthanum doublet. The corrector consists of 2 elements. The price is considerably lower and so is the image quality. You will have blue halos round the brighter stars and small color fringes at the moon.

https://www.teleskop...-Telescope.html

 

If you go for a regular scope and a separate flattener you might also need some spacers to get the distance right. In the end it is not cheaper. Please note, though, that some flat field scopes cannot be used for visual with a diagonal. IMHO you should have two different instruments for imaging and observing. For observing get binos or a cheap but large dobsonian. You can observe while your imaging rig is working for you.

Thanks Elf, that is a ton of great information!




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