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Hi everyone, after months of study and evaluation I finally took courage and tackled a complete upgrade of my equipment, following this forum's advices. I kept my trusty modified Canon 90D and first of all I changed my mount, switching to the EQ6-R Pro (actually a notable leap forward compared to the GTI). I then finally decided to move on a refractor, purchasing a TS-Optics AP 115/800 ED Photoline. Using a 0.79x flattener and after an update of the guidescope (now a 240mm) everything is fine and works perfectly: focusing, guiding, field, no aberrations. Great! On the other hand, if I use the 115/800 without the 0.79x flattener I notice a lot of aberration moving away from the center of the image, with the stars progressively getting longer in a noticeable way (never seen such an effect with my old Sigma 600mm, not even multiplied 2x). Okay, so far not bad: flattener, a few micro adjustments on backfocus with a couple of very thin rings and everything is ok. Now I'm carrying out some tests with a Barlow 3x suitable for photographic use: without a flattener (in my head it makes no sense to put a reducer in front of a Barlow, or not?) the result is the one here attached. To center the focus perfectly I used an adequate extension (8cm!), but the farer I'm from the image center, the longer the stars get and I have a lot of aberration. Is this a backfocus issue and/or do I need to use a flattener (1.0x) with the Barlow? And if it's a backfocus problem, considering that I use a Canon 90D reflex, how do I calculate the correct one? Am I completely lost and as usual doing something gross wrong? Thank you!  |
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| Lens focal extenders do not work very well on telescope (in fact one may question whether they work very well at all). You need a proper extender, which also flattens the field. Question is why would you want to do that (and 3x at that!). But, whatever is the reason, the extender needs to be telecentric not a normal barlow lens. Best of all, it would need to be designed for your specific refractor of which there are none. |
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andrea tasselli: Actually the extender is telecentric and it is supposed (or at least, the seller says it is intended...) to be used in astrophotography even with refractors. Honestly I expected some distortion, but not such a kind of it. This is why I'm wondering if there is some precaution I could adopt, for example coupling it with a 1.0x flattener (The reason to use the Barlow, for example, is just getting closer to small planetary nebulae or galaxies, until I'll save some money for a more powerful telescope :-) Just dealing with the gear available. |
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| You're better off with a planetary camera with high pixel count and small pixels to achieve your aims (to the degree afforded by the optics). A camera based on the IMX664 chip, for example, would afford a sampling of 0.75"/px which is above what you could realistically consider to be the very upper limit of the pixel scale on such a small scope, accounting for efficiency and noise. Even with your 90D the sampling will still be rather high compared to the size of the scope so I would look no further. |
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| Oh, right! I was not considering the point from the sensor perspective and btw I've just a similar camera in my pipeline I was not taking in account. Thank you very much! |
