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Trying to go in the right direction before buying the better gear.

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#1 Tropobob

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Posted 16 August 2022 - 06:18 PM

I have began to dabble in astrophotography, after being a visual observer for 50+ years.  Its quite a learning curve, which has not been helped by my history of preferring simple, non-goto altz mounts. 

 

I use a Canon M50 mirrorless camera at the prime focus of an Orion 80mm F6 Triplett on an old EQ5 mount.  I am finding distortion at the edges of my images and have tried to minimize this by using square (1:1) images for keepsake images.  However, even so, I have progressed to the stage where I can see that this is ruining my images. I think that I need a field flattener.  

 

However, I am only learning with the 80mm and hope to progress to using one of my larger scopes for astrophotography, so I do not want to buy things that I will not use in a longer timeframe. 

 

I am pondering my options and this is where I need some help.  

 

Will it help if I move from prime-focus to using a low powered EP with a large field?

 

Or perhaps I am better of trying one of my larger scopes and working upon my learning curve with that? If so, if seeking a sharper field, is it better to have a longer F ratio, or just a longer actual focal length?

 

To be more specific, I have a Tak 100 F7.4 fluorite, a Vixen ED115mm F7.7 and a Vixen 140SS F5.7 Achromatic. The Vixen 140mm is often referred to as having a flat field, which is good, but it is the heaviest, which is bad for both the mount and me.  However, as my skills improve, I aim to buy a more suitable mount.   I also have a 100mm ED F11, but suspect that will be beyond my current ability to track effectively. 

 

So will any of the above scopes improve the image quality of my edges, without having to buy a flatteners?   

 

 



#2 Phishin_phool

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Posted 16 August 2022 - 08:29 PM

I have began to dabble in astrophotography, after being a visual observer for 50+ years.  Its quite a learning curve, which has not been helped by my history of preferring simple, non-goto altz mounts. 

 

I use a Canon M50 mirrorless camera at the prime focus of an Orion 80mm F6 Triplett on an old EQ5 mount.  I am finding distortion at the edges of my images and have tried to minimize this by using square (1:1) images for keepsake images.  However, even so, I have progressed to the stage where I can see that this is ruining my images. I think that I need a field flattener.  

 

However, I am only learning with the 80mm and hope to progress to using one of my larger scopes for astrophotography, so I do not want to buy things that I will not use in a longer timeframe. 

 

I am pondering my options and this is where I need some help.  

 

Will it help if I move from prime-focus to using a low powered EP with a large field?NO you do not want to do that eyepiece focus is hot garbage compared to prime focus.

 

Or perhaps I am better of trying one of my larger scopes and working upon my learning curve with that? If so, if seeking a sharper field, is it better to have a longer F ratio, or just a longer actual focal length? Focal length and f-stop will matter more for your framing and time that needs spent on a target.Sharpness will depend on the glass composition and doublet vs triplet and how apochromatic the optics system is and can not easily be discerned by focal length or speed.

 

To be more specific, I have a Tak 100 F7.4 fluorite, a Vixen ED115mm F7.7 and a Vixen 140SS F5.7 Achromatic. The Vixen 140mm is often referred to as having a flat field, which is good, but it is the heaviest, which is bad for both the mount and me.  However, as my skills improve, I aim to buy a more suitable mount.   I also have a 100mm ED F11, but suspect that will be beyond my current ability to track effectively. 

So will any of the above scopes improve the image quality of my edges, without having to buy a flatteners?   I would certainly try with the TAK 100 if you have all necessary connections. Also it is going to leave you at a shorter fl than the others which is helpful  I had considered that scope on occasion when seen in classifieds If not satisfied I would look at what you like and don't like about shooting with one of the longer fl.

See above, basically I would get your technique down with the 80 your using. If convinced you're ready to move up try each scope - TAK first and see if you like the framing and time required for imaging. Then try the others and get the flattener (I would               recommend a .80 FR/FF. 


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#3 photoracer18

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Posted 16 August 2022 - 08:34 PM

To be honest if you can't get decent images with the 80mm your problems will multiply when you try to use scope with a longer focal length. Nobody does eyepiece projection or afocal imaging these days. That is what others and I did when I was in high school in the 1960's with film astrophotography.  One problem maybe the size of the focuser on your 80mm. You generally need a 2.5" focuser to do APS-C imaging because a 2" focuser draw tube generally vignettes an APS-C sensor. This is when using either a focal reducer or a flattener.  The Tak 100 is going to be your best scope but you really need either a flattener or a focal reducer/flattener to make this all work. Telescopes don't normally have flat fields but a few scopes do.


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#4 the Elf

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Posted 17 August 2022 - 03:15 AM

There are few so called flat field astrographs, also called quadruplet or quintuplet astrographs depending an the number of elements. These have a flattener built in. You will soon find that a regular scope plus a flattener is about as expensive as an astrograph of same size and quality but the flat field device is very easy to use. Camera in, focus, done. I'm happily imaging with an AT65EDQ clone. The scope is discontinued but you might get a used copy or a similar new scope, e.g. this:

https://www.teleskop...3-Objektiv.html

You can probably find a clone with a different brand name. They all come from the same Chinese factory. These often don't work for visual, the back focus is too short for a diagonal. Straight view is possible but not enjoyable. For photography these are ideal. A fast scope gives you better results because signal matters. Otoh too fast a scope is hard to focus and needs to be refocused often. Somewhere between f/5 and f/7 is the sweet spot, easy to use but still enough signal. A short focal length is easy to handle and there are large objects in the sky. If you want to enjoy the journey start with a short focal length. Chromatic aberrations are far more important in photography than in visual so you want to go for an FPL53 or FPL55 triplet. If you are on a budget an FPL53/lanthan (equals FCD100/lanthan) doublet is the minimum for somewhat decent images. If it is just called "triplet" with no information which glass is in there, don't expect high quality.

Short focal length designs are often Petzval designs, these come with a flat field as well. The RedCat being the most prominent but not necessarily the best example of this type. A short focal length allows for unguided imaging provided your polar alignment is on spot. When you go for medium or long focal length you will need guiding. For very long (1000mm+) you need off axis guiding. Not a technique recommended for beginners. Start on the short side and move up slowly.

I can recommend the SW EQ6-R mount. I'm using it with my 8'' RC at 1100mm focal length at 8kg payload with no problem at all. When your payload exceeds 10kg you need a stronger mount. I cannot over emphasize how important a big sturdy and precise mount is. If the scope looks small on the mount you are doing the right thing. Keep the big scopes you have for later use with a flattener when you have some experience with a shorter one. If you are frustration proof and have stamina you can jump in at the deep end (the scopes you have) but be prepared not to see any decent image during the first year. Only you know which sort of character you are - instant success or long term challenge seeker.


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