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| Hello, For a few years, I have been shooting astrophotos with a Nikon D610. I bought at the beginning of 2020 a NIKKOR 70-300mm f/4.5-5.6E ED VR lens. Then I decided to upgrade my setup. I equipped myself more seriously (AZEQ6 mount, QHYCCD 294M cameras, QHYFW3 filter wheel, guiding, Stellarmate, etc.). I decided to tame my new setup before changing my optics... Also to spread the costs over two or three years. So, I use my NIKKOR 70-300m lens with the QHYCCD294M and the filter wheel. When I designed my optical chain, I discovered the principle of the Flange focal distance (FFD). After a lot of trial and error, I discovered that each focal length had a different FFD. So I finally decided on two sharp focal lengths, at 600mm and 200mm. Nevertheless, at each of these two focal lengths, the focuser remains without effect... I don't understand why. Would someone have an idea of explanation? Thank you for your collaboration. CVO |
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Hi @Voirol Christian , I understand that when you adjust the focus using the focus ring of the lense, you never reach critical focus on your sensor, correct ? I understand from what you wrote that you use (i) a Nikon to M42 or M48 adapter, and also put a filter wheel between the lense and the QHY camera, is that correct ? If yes, then the problem is simple : all lenses made for DSLR have a specific and fixed flange focal distance by type of mount among a brand (46,5mm for Nikon F, 44mm for Canon etc.), which means that you can't reach focus at infinity if the distance between the sensor and the flange of the lense is superior to that. Usually, when using lenses on astro-camera, you just have the space to put an adapter that can contain a filter, nothing more. If I do the maths : the QHY 294 Pro (if that's yours) has its sensor 17,5mm behind its flange. But an adapter Nikon to M42 already takes around 21mm, so we are already at 38,5mm, which would leave only 8mm (to get to max 46,5mm) to put a filter wheel. They usually are 20mm thick, so it can't fit in your optical train... Usually you would put a 8mm tube between the adapter and the camera, to get exactly to 46,5mm. This adapter, for example, allows you to put a small filter inside (https://www.astroshop.de/fr/adaptateurs-pour-objectifs-photographiques/geoptik-adaptateur-t2-pour-nikon-digital/p,18706) but with DSLR lenses you will have to leave the filter wheel in the drawer... Hope this helps |
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Dear Jérémie, Thank you for your answer. In fact, I did all this adjustment work. By playing with the FFD, I could see that I could get a sharp image at any focal length, as long as I adjusted the FFD. For example, at the 100mm focal length, I needed an FFD of 70mm to get a sharp image. So, I finally opted for the famous 46.5mm FFD required on Nikon. And currently, I am able to find a very good focus at 600mm and 200mm (in fact, 300mm and 100mm in FF) with an HFR of about 1 to 2. But I can only focus by changing the focal length. The focus wheel has no effect on the image. That's my question. My hypothesis is that the Nikon-M42 adapter prevents the aperture from being adjusted. It's wide open all the time. So, it's mechanically that I have to adjust the focus of the image. Which I do, but I was wondering if there is another cleaner option. Because here, the focuser is only used to know the HFR of my stars. But I adjust with the focal length. In details: 9mm for the QHYCCD 294M - 8mm for the adapter to the filterwheel - 20.5mm for the filterwheel - 10mm for the Nikon lens-Filterwheel adapter -->Total : 9+8+20.5+10=47.5 mm - But, I use filters in the path… I add 0.67mm to the standard 46.5mm --> 47.2mm for the FFD. Thank you for your collaboration. CVO |
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| In details: 9mm for the QHYCCD 294M - 8mm for the adapter to the filterwheel - 20.5mm for the filterwheel - 10mm for the Nikon lens-Filterwheel adapter -->Total : 9+8+20.5+10=47.5 mm - But, I use filters in the path… I add 0.67mm to the standard 46.5mm --> 47.2mm for the FFD. |
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Hi Christian, you need a FFD of 46.5 mm for the Nikon lens, because the tube of this lens extends by f-46.5mm to the flange. All of your mechanics sum up to 47.5 mm (with filter it's even ~48.2 mm). Maybe you can still reach focus locally, but your image plane will be curved and thus not be in focus everywhere. With increasing distance to the image center, your stars should also be more and more elongated in tangential direction (overcorrection). So you should somehow reduce the length of the image train by 1.7 mm. Make it 2 mm and leave some space for fine tuning. Maybe you can find a shorter adapter. CS Hoschie |
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| Dear all, Thank you for your comments and suggestions. I will explore possible alternatives. I have all possible adapters... I ordered them all from QHYCCD when I designed the optical train. But I'm afraid I'll have to change them again... As they say in computer science "if it works, don't touch it". Clear sky. |
