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What refractor do I need to match the image quality of my Canon 300mm lens?

MartinFransson

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MartinFransson

MartinFransson

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Hi all!

I am currently using my Canon EF 300/4L IS lens for imaging, sometimes with a 1.4x converter making it 420mm f/5.6.

I love my Canon but sometimes I need it for other stuff when it´s tangled up in my astro rig.

What kind of refractor and flattener would I need to get similar (or preferably better) image quality? I´m guessing a triplet, but how expensive does it have to be? Suitable suggestions will be appreciated :)

The focal length should be around 350-500mm.

I don´t do far away galaxies that much, I more like the large nebula and molecular clouds complexes.

Today I use a modded Canon 1100D (T3?) and I don´t see myself with a CCD in the near future, but it can´t be ruled out. I know I will go there eventually. Just don´t have the time or budget now. The mount I use is a HEQ5 Pro with an Orion MMAG autoguide kit.

Here is a link to a RAW-file for a light frame taken with my lens and camera. For comparison. Not sure if I had perfect guiding and PA that night but at least it gives an idea of what I use new:

https://www.dropbox.com/s/l9bpxt8d6n8q502/IRIS_LIGHT_600s_800iso_f4_%2B9c_20150311-23h43m51s206ms.CR2?dl=0

Here´s the jpeg (no sharpening added) for those who can´t see Canon RAW files:

https://www.dropbox.com/s/7rhavcmrm4wy1wp/IRIS_LIGHT_600s_800iso_f4_%2B9c_20150311-23h43m51s206ms.JPG?dl=0

It´s a 10 minute sub so if you want me to add a shorter sub I will. There are reddish reflections around the brightest stars. That´s not because of the lens but a misalignment of the Baader replacement filter. It´s been fixed now (or so Canon tells me anyway).

Thanks in advance!

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DaveS

DaveS

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OK, how about one of these

http://www.teleskop-express.de/shop/product_info.php/info/p7085_TS-Imaging-Star71---71mm-f-4-9-Imaging-APO---covers-full-sized-sensors.html

Or one of these?

http://www.teleskop-express.de/shop/product_info.php/info/p7224_TS-80mm-f-4-4-Imaging-Star-APO-Astrograph---aperture-80mm---focal-length-352mm.html

For imaging the aperture is less important than the focal length and focal ratio. For imaging with a DSLR a fast focal ratio is more important than aperture or focal length.

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MartinFransson

MartinFransson

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OK, how about one of these

http://www.teleskop-express.de/shop/product_info.php/info/p7085_TS-Imaging-Star71---71mm-f-4-9-Imaging-APO---covers-full-sized-sensors.html

Or one of these?

http://www.teleskop-express.de/shop/product_info.php/info/p7224_TS-80mm-f-4-4-Imaging-Star-APO-Astrograph---aperture-80mm---focal-length-352mm.html

For imaging the aperture is less important than the focal length and focal ratio. For imaging with a DSLR a fast focal ratio is more important than aperture or focal length.

Yeah, I´ve had my eye on those... would they outperform the Canon lens or would it be more or less same-same?

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ollypenrice

ollypenrice

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From what I've seen from my friend Yves Van den Broek the Star 71 might well be the one. Of course, if you sold a couple of kids and a kidney then you should obviously buy a Baby Q Tak! (I was lucky. I had a lot of kids and a superfluous kidney!)*

:grin: lly

*Should anyone from social services be reading, this is what is known as a joke.

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MartinFransson

MartinFransson

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From what I've seen from my friend Yves Van den Broek the Star 71 might well be the one. Of course, if you sold a couple of kids and a kidney then you should obviously buy a Baby Q Tak! (I was lucky. I had a lot of kids and a superfluous kidney!)*

:grin: lly

*Should anyone from social services be reading, this is what is known as a joke.

I´ve got three kids but I kind of like them so I´ll go the kidney route instead :grin:

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  • 2 months later...
MartinFransson

MartinFransson

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Digging up this thread again to see if there are any other bets than the WO Star 71 in the same price range (or less).

I found this one and the fast focal ratio is something I would like:

http://www.teleskop-express.de/shop/product_info.php/info/p7224_TS-Imaging-Star-80-mm-f-4-4---6-Element-Flatfield-Apo-Teleskop.html

352mm focal length and f/4.4.

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MartinB

MartinB

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I think you've asked a great question Martin. That Canon lens of yours is a fabulous piece of kit and it seems difficult to justify spending mega money to replace it if you are sticking with a DSLR. It will give a good flat field over your APS chip. I guess colour correction won't be quite apo but not at all bad and you have software such as DXO which will be have a tailor made correction for CA of your specific lens.

I don't think the advantage of switching to a telescope is optical really but more around imaging mechanics. A 2 speed crayford focuser will be much easier to use for fine focusing and you have the option of using a filter drawer for LP or narrow band filters (which is a bit less of a faff than clip filters). But you then get into the irritating world of adaptors, spacing and back focus.

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mikeDnight

mikeDnight

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If you love your Canon lens so much, I'd suggest you lean toward Takahashi as they use the finest Canon optics. A sweet little FSQ would be the most perfect companion. They're not cheap though and the price may be a little off putting, but if you see any of Olly's images I'm sure you'll start a Tak fund.

Mike

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MartinFransson

MartinFransson

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Mike, I'd love a Tak and some day I might get one but now is not the time I'm afraid [emoji4] Even spending 1000-1500 Euros on a scope will be stretching my limits at the moment. If money was no object I'd already have a mono CCD and a Tak in the mail.

Sent from my phone using Tapatalk

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DaveS

DaveS

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Looking at those Imaging Star 6-element designs, they look to be a triplet apo with a built in triplet flattener / reducer which means you still have to take note of the BF, whereas the Star 71 variants are 5 element designs with inherent flat fields, so no BF problems beyond getting focus.

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MartinFransson

MartinFransson

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Looking at those Imaging Star 6-element designs, they look to be a triplet apo with a built in triplet flattener / reducer which means you still have to take note of the BF, whereas the Star 71 variants are 5 element designs with inherent flat fields, so no BF problems beyond getting focus.

Excuse a newbie... BF? :)

Sent from my phone using Tapatalk

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DaveS

DaveS

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Yes. All reducer / flatteners have an optimum back focus, usually (But not always) around 55 mm, it doesn't matter if they're built in like the imaging star 80 or added after. The key is if the F/R components move with the focuser. The Star 71 (And Vixen VSD 100) are different in that they are designed from the outset to give a flat field and the flattening components are ahead of the focuser and don't move. All you then have to worry about is bringing the system to focus with whatever hardware you've hung off the end.

Edit: The vixen is a bit pricey though  :eek:

http://www.firstlightoptics.com/vixen/vixen-vsd-100-f38-flat-field-refractor.html

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