inFINNity Deck

Indeed he was asking about visual observing, I was confused by his line "...give me better lunar and planetary images". The larger the aperture the sooner seeing is going to play a negative role in the stability of the image, so less is more when it comes to aperture and seeing as Chris already stated. Nicolàs

That indeed is a key point: the Celestron SCTs do not hold collimation very well, and may not 'survive' a meridian flip due to excessive mirror-flop. I currently have two C11s here, a XLT and a XLT EdgeHD, and both show collimation goes astray when going through the meridian (see the link in my previous line, it is in Dutch but should translate properly when opened in Chrome). I had the EdgeHD serviced by Celestron in the US and apart from a big hole in my pocket it did not have any effect. Celestron did not want to specify the mirror-flop, so I could not check if it was damaged by the return shipment, and apparently this is not something Celestron has control over. Ever since I got it back I first slew to a nearby star, collimate the SCT, and then do the imaging. It then gives reasonable results. I also see nice planetary images being taken with Maksutovs, perhaps that is a good alternative. Nicolàs

Hi Tico, That the Moon gives better result simply is due to its massive size, all the planets are much smaller: the largest planet (Jupiter) is only about 2.5% the apparent diameter of the Sun and Moon (depending on its distance to Earth), the smaller planets only a fraction of that. The advantage of a reflector over a refractor are the additional aperture and focal length for the same price. More aperture means more detail, more focal length means more magnification. Downside of the refractor is that the aperture comes at a price... 😞 Below images or Saturn were shot on one night using a SkyWatcher Esprit 150ED refractor (150mm aperture f/7, but thanks to 2x PowerMate f/14) and a Celestron C11 EdgeHD reflector (280mm aperture f/10, no Barlow). Personally I find the refractor giving a better image with more details and less artefacts. But then I have to say that the C11 was undersampling at f/10, so I should have added a Barlow and then reduce the size after processing. Nicolàs

Hi Ken, like this: From from focuser down to the camera I use: - 2" TeleVue 2x PowerMate - ZWO EFW mini filter wheel with LRGB filters - a 1.25" clamp that allows me to rotate the ADC (although I could do the same by turning the filter wheel in the PowerMate or by turning the PowerMate in the focuser, but in this way I find it easier) - a 1.25" ZWO ADC - another 1.25" clamp that allows me to rotate the camera in respect to the ADC (although I could do that in the T2 to 2" adapter as well) - a T2 to 2" adapter that allows me to swap the ASI174MM for the ASI290MC without the need to refocus - the ASI290MC or ASI174MM camera The T2 to 2" adapter is very useful for adjusting the ADC when you want to image in mono as even within the bandwidth of the monochromatic filters dispersion will occur. So when I image in mono I first attach the ASI290MC, adjust the ADC using ASICAP (please note that the FireCapture ADC tool is not functioning properly), and then swap the ASI290MC for the ASI174MM. Using this adapter causes the two cameras to have the same focus position. Before I used a T2 to 1.25 adapter that screws into the camera, but when doing so both cameras have different focus points and that affects the setting of the ADC again. The combination of the 1.25" clamp and T2 to 2" adapter creates more distance between ADC and camera, which makes it easier to adjust the ADC due to the longer path. Prior to imaging I slew the C11 to a close by star and collimate it. Nicolàs

Hi Vladimir, thanks for your elaborate and interesting answer. I have to say that I image using FireCapture and it seems to handle histograms very well. As soon as I go over 100% the image clearly shows overexposed areas, so it seems to do its calculations in respect to the bit-depth of the image, but I could be wrong there. Mars I never tried at f-ratios higher than f/20, but I will try next time at f20 and f/40, see how that relates to each other and to the theory. Nicolàs

Best is of course to keep an eye on the histogram, keeping it around 80%-90% filled. As long as it does not significantly increases exposure times it does not harm to oversample. Better to over- than to undersample as the latter does reduces detail. Regarding noise due to higher gain: even if the signal gets noisy by it, then the stacking will take care of it, provided we have enough data: Seeing was not particular brilliant that evening, but I think the idea is clear... 😉 Nicolàs

Hi Ken, I can confirm Vladimir's advice to increase by a factor 2 the focal ration when imaging in colour. You may be interested in how we arrived at these factors, which you can find in an article that I wrote on determining the f-number. In it I dedicated a section on the colour camera where I too arrive at a f-number twice the f-number for a monochrome camera. For a monochrome camera the f-number should be at least 3 times the pixel size in microns, for a colour camera this is 6 times. The article is in Dutch, but if you open it in Chrome it should translate reasonably well. Strictly spoken the image of a colour camera should be reduced in size by 50% as half of the pixels that are produced by a colour camera are the result of interpolation. I do planetary imaging myself using a C11 EdgeHD and ZWO ASI174MM (pixel size 5.86 micron) or ZWO ASI290MC (pixel size 2.9 micron). For both I image at f/20 (2 x PowerMate) and that seems to work well: ASI174: 5.86 x 3 = 17.58 ASI290MC: 2.9 x 3 x 2 = 17.40 C11 with 2 x Barlow = f/20 Here is Mars by C11/2x PowerMate/ASI174MM in LRGB: After the processing I have enlarged the image by 200% using bicubic interpolation and added another sharpening. You could go to f/30 or even f/40, but that will only enlarge the image (like the Mars image shown here), hardly add more detail (although some gain may be achieved from stacking at that f-number). Nicolàs

You can shoot at a high rate and afterwards reduce the output using PIPP. Among a plethora of other useful things it allows you to skip frames, to compress them and to run the output video at another frame-rate than the original (both slower and faster). I use it a lot, mainly to reduce the filesize of my time-lapses, a few of which are on my website. Nicolàs

I remember very well that I was very pleased with the views of Jupiter and Saturn with my first (loaned) telescope: a Celestron Nexstar 5SE with some simple eyepieces. Nicolàs

Hi Semitte, welcome to the forum. Jupiter and Saturn are quite low and due to this their colours get smeared out like sunlight in a rainbow. This can be counteracted by using an ADC (Atmospheric Dispersion Corrector) and should result in better colours, although it remains difficult to see that visually, much depending on seeing. HTH, Nicolàs

Last night I received the confirmation that the adjustment of the RC8 was successful. The owner sent me below image of Vega. The spider's diffraction spikes are razor-sharp. The photo was taken without flattener, which gives the opportunity to see whether the collimation is well balanced. Coma is visible in the corners and they all point towards the centre of the image, which is a good sign. Prior to this final adjustment, the focal length was determined by astrometry to be 1646mm, while it was designed to be 1624mm. Now I had read that adjusting the secondary mirror would have a tenfold effect on the focal length. So we moved the secondary mirror outward by approximately 2.25mm (three turns with the M5 bolts). Using a caliper I checked this displacement and came to a real difference of 2.6mm. Astrometry on the image of Vega taught us that the new focal length has become 1617mm, a difference of 29mm, which brings the factor by which the focal length changes to approximately 11 times the inter-mirror distance (increasing the distance lowers the OTA's focal length). Nicolàs

Hi Kookoo, yes, that could be the case. The reflection of the camera lens is very sensitive to camera and mirror misalignment. That was exactly why I started using that reflection in the process, it magnifies those misalignments significantly. Nicolàs

Hi Kookoo, aligning the camera with the scope is indeed far from easy when using a standard tripod, it takes me as will quite some effort. If I look at your image I can see the camera s properly aiming at the OTA, but the dot in the visual back is not properly centred. The refection of the camera lens is even worse, that has a clear offset to the upper left, I have marked that with a yellow dashed circle in a crop of your image: The spider-arms at the lower left and upper right also clearly show an offset sideways. The other two show an offset in longitudinal direction. Today I had another go at the RC8 I saw a while ago, trying to get the inter-mirror distance slightly better and checking the focuser-tilt. A screen-dump was taken with a light source (artificial star) in the centre of the focuser with the MoonLite focuser fully out. As you can see the lens, light source and secondary attachment are all centred (although the secondary attachment could be slightly better). Last week I built a new alt/azi table for the alignment. The previous one was of wood and a bit wobbly. The new one is of aluminium bar (30x40mm) and aluminium strip (200x20mm) and makes aligning a fair bit easier: Something similar is required for the camera, so that will be the next project. The box with the two knobs in front is an artificial star made by the scope's owner after my design. Nicolàs

The evil shed vs astro set-up... 🙂

I usually have an additional ZWO ASI290MC with fish-eye lens running from a laptop during imaging, just to check what the weather conditions were during the night, this is one of the many recorded sessions: The equipment is listed in my signature, imaging was done with the SkyWatcher Esprit 150ED. The recording was my second attempt to perform an anti-meridian flip using SGP and two events (north is above in the video). SGP cannot automatically do this, so the second event is fired in time to avoid the mount stopping in the middle of the night. Nicolàs

Hi Vulisha, I am not familiar with this type of Newton. I should have mentioned that for testing the donut, it should be mirrored in two perpendicular directions: So although the first image looked right along your initial mirror-line, in the direction perpendicular to that it is still far off. The second image is near to perfect in that direction, just as the first image in the initial mirrored direction. This on itself is very strange and can not be easily explained other than by flexure in the optical system. Could it be that the secondary holder can rotate/flex along its length? Nicolàs

What kind of telescope is it? Nicolàs

Hi Vulisha, if you flip the right half of both images you can tell that both are quite off, although the first is slightly better. The scope could do with collimation. What kind of scope is this and why does it have this single blockage? Nicolàs

Hi David, thanks for the explanation. My friend does not use a flattener, so that could make a difference. The first images have shown that the focal length is about 22mm more than the design value. So I suggested to give it another try, see if we can get closer to the design value without the Ronchi lines becoming curved. When we started, collimation and mirror distance were both quite far off due to a previous attempt. At least the spider diffraction patterns are razor-sharp now! 🙂 Nicolàs

Recently I collimated a RC8 for a friend using the methods described here. The first images showed great improvement, but still show oval stars in the corners like in Kookoo's image, all pointing diagonally in the direction of the centre. Is this normal for a RC8 or does this mean that the mirror distance is not correct yet? Nicolàs

Hi Steve, very nice image. Do you guide and, if so, what type of guide scope do you use for the 150ED? Nicolàs

The whole idea behind this is to avoid stress in the pier and the anchors. If indeed the surface on which the pier stands is uneven a lot of stress is created when bolting down the pier and what basically happens is that the pier tries to pull the bolts out of the floor. Having the pier 'floating' above the floor reduces the stress to a minimum. So what I did is first level the pier, then turn up the lower nuts until they were all flush with the baseplate and then tighten the upper nuts to hold the pier. In this way there is only stress between the nuts and in the section of thread between them. The idea was not mine but I saw it mentioned in a brochure of the Euro EMC Star Observatory Pier (see p.2). I know levelling up the topside of the pier is not a requirement, but it sure is beneficial to the polar alignment. If the topside of the pier is at an angle any change in azimuth will result in a change in elevation, so each azimuthal change requires a change in altitude. Having the top plate near to level avoids this. Nicolàs PS: no I am not going to remove the washers... 😉

If you would do that you probably end up with a stability that is equal to the 6" pole. The pier should be fastened to the floor using the bolts around the circumference, fastening in the centre would not add much stability. Here are some pictures of my pier's maiden voyage and its landing place shortly before touchdown: The pier is 3.4 metres high and has a baseplate of 80 centimetres in diameter, which is fastened to the concrete floor using 16 M12 threaded anchors. They all have a nut and washer below and above the pier's base-plate and were finally shortened and finished with cap nuts. Recently I did some stability tests which showed the pier is rock solid, no measurable movement. Nicolàs

The focal ratio should be more than 3 x pixel size (for a monochrome camera). The ASI178 has a pixel size of 2.4 microns, so the focal length of the scope should be at least f/7.2. A lower focal ratio results in undersampling, a higher in oversampling. It is better to over- than to undersample, the only downside of oversampling is that the exposure times go up. The 60mm Lunt has a focal ratio of f/8.33, so that is just fine it is slightly oversampled. The ASI174 has a pixel size of 5.86 microns, so the focal ratio should be at least f/17.58. My 80mm Lunt is f/7, so capturing the full disc without a Barlow results in severe undersampling. I therefore image details with a 4x Barlow (my other Barlow is 2x, I do not yet have a third), making the scope f/28. As light is in abundance I do not mind oversampling this much. Maybe I should get myself a 178MM, it is an almost perfect for the 80mm Lunt. Nicolàs

Hi Nicos, these images are better than what I achieve with the 174MM, even though I am using a 80mm scope. This is a full -disc image of the 174MM: And this with a 4x Barlow: Nicolàs