I have been looking for a fast wide-field astrophotography setup for fullframe DSLR imaging around FL 300mm, as well as ASI CMOS imaging.
Commercial range
Actually, within a reasonabe price range there are not that many telescopes available on the market meeting these criteria . I had some satisfying test results with the TecnoSky 70AG telescope a few weeks back. There is another alternative, with assumably very good image quality: the Esprit 80 F5 at a focal lenght of 400mm. And then you have this new model of the Sharpstar 61EDPH, the 76mm model.
The 300-350 focal range was important to me. Shorter focal lenghts would bring me too close to my regular DSLR telelenses of good quality (eg. the 180mm ED Nikkor F2.8). So Redcats and whitecats, with FL=250mm, dropped of the list for this reason. Also the Esprit 80mm F5 with 400mm focal length brings me already too close to the Esprit 120mm F5.5 with 663mm focal length.
High-end alternatives cost as much as 3 times more, like the Takahashi 85FSQ or the Vixen 100mm F3,8. That is more then I’m willing to spend on this instrument for these purposes. The Vixen gives a perfect image circle of 70mm, if you only use fullframe sensors what’s the point?
I took a small gamble and ordered the Sharpstar 76EDPH after seeing one full frame result on Astrobin. The gamble is that I could not find at this time any reviews or examples of uncropped full frame RAW frames.
There is one ED element in the triplet, it’s apparently not Lanthanum or FPL53. See here for a discussion on this topic. The Teleskop-Service website (where I purchased), mentions FPL-53, however the Shaprstar factory site does not mention it. It only discloses “ultra-low dispersion element’. In the discussion there is a mention that the 76EDPH does not use FPL-53 but glass with very similir properties. Anyhow, whatever the glass might be, color is very well corrected. I could not detect blue halo’s or fringes, even not in the very corners.
Specs
Specs – in combination with the F4.5 reducer – can be found (including spot diagrams) on the website of SharpStar. Spot Diagrams here. The instruction manual can be found here
Choice
Reasons to prefer the Sharpstar above the TecnoSky were the slightly shorter focal length, closer to my target, and the slightly faster F-ratio. I could test a specimen of the TecnoSky, and would be happy owning that instrument. However there is room for slight improvements in the corners of full frame image. These are by no means defects that would rule out using such an instrument, as a slight crop would solve everything. For APS or any format slightly smaller then full frame it does not matter at all. As I have now more images with the 76EDPH, also there star images are not pinpoint into the very corners. So it’s by no means perfection. That’s also not what I expected. The TecnoSky has a bit of a blue fringing in the corners, with the 76EDPH it’s more on star shape. Color is not an issue.
Unboxing
I ordered the telescope with Teleskop Service (Germany) on Monday and on Friday the instrument arrived.
It was well packaged, with a double cardboard box, the box of the scope itself being inside a larger box that contained additionally the Bahtinov mask (ordered in addition) and the seperately packaged F4.5 reducer.
Inside the box was also an inspection checklist of Teleskop Service itself, checking a number of technical elements of this telescope including the collimation (which needs to be perfect for this F/ratio in order to have good stars in all FF corners). It was clear the box was not closed with the original tape. For a moment I feared that this speciment might have been a reject, sent back by a previous customer. I assume the inspection of TS explains the re-taping of the boxes.
When the instrument came out, it looked obvious that this material was all brand new, and not used in any way. The finishing is very elegant, the construction very solid. One is surprised by the weight of this little scope (almost 3kg).
Mechanics
I attached the finder shoe, and unboxed the heavy reducer, screwed it IN the focusser (the reducer fits inside the focusser for about 41mm). For that I needed to remove the 2″ to 1 1/4″ reduction ring, the M74 to 2″ adapter. The reducer/flattener has M48 threading to accept camera connections.
I you want to use a 2″ M48 filter, also that is possible: the reducer can be split in two parts. The camera-side-part has internal M48 thread that accepts 2″ filters. I did not try that yet.
The reducer comes with two metal caps, nicely machined in aluminum, that serve as dust covers. One is M48 female, the other I guess M74 male.
Rotation of the camera can be done by a “giant” (in respect) 3″ rotator, that holds the complete rear-end assembly of focusser, reducer and camera. A tension knob allows you to unblock the rotator. Also the rotation friction can be adjusted using a few screws, however this is not recommanded. As small as this scope is, you can also slightly unscrew the scope collar itself and rotate the entire scope assembly. And on top of that you can easily rotate everything also by a second rotation ring at the end of the reducer.
Daylight shooting
When using a DSLR on this scope, the image you get is upright. That means it’s easy to use it also for bird imaging or other daylight applications.
First frames
The sky only briefly cleared that evening, still within twilight. It clouded out again completely by the time a partial penumbral eclips of the Moon set in, so I missed that.
I could capture a few frames with the Nikon D750, putting the scope on an EQ8, with a 8×50 finder attached, using an MGEN2 for guiding. I limited the exposure to 10 or 15 seconds to avoid any influence of bad tracking.
I could also snap some shots of the extremely bright Venus (all shots unprocessed, uncropped FF NEF files converted with View NX to high-quality JPG):
Venus centrally imaged
Reflections
Reflections can be an issue in all kinds of optical setups. The optical layout of this setup counts 6 optical lenses. What happens with reflections? I have an example where an Optolong L-Pro T-Ring was used (filter integrated in the T-Ring) So the reflective surface of this filter is as close as possible positioned towards the DSLR sensor (apart from a clip-in). There are some spots in the image that are suspected reflections / I’m not entirely sure of it as they seem more fuzzy.
Next
After doing some more imaging, see the results in the post here and here, I can come to a preliminary conclusion. Within this price range, comparing it to the Esprit80mm and the TecnoSky 70AG, it’s for sure not a bad choice.
The TS76EDPH or Sharpstar 76EDPH has no perfect stars to the outer edges and corners. I need a short exposure high-ISO image to confirm that, some of the dodgy star shapes in my results might have been linked to bad polar alignement combined with a 5-minute exposure and hugh fields. So the field rotation shows? Color is not an issue at all. See the following 30 second (less field rotation) single frame on FF, which shows one corner out-of-focuse but round stars in the corners:
Both the TecnoSky and the 76EDPH have their issues in the edges, so a bit of cropping is required: there is no shame in that! The 76EDPH has a slight advantage of being a bit faster, and slightly more sky coverage, closer to the 330mm to fit the gap between my DSLR lenses and the Esprit120mm F5 at 663mm FL.
The Esprit 80mm F5 with it’s 400mm FL, has assumed better optical quality all-over also in the image corners. However, after cropping the final image of the 76EDPH you get an image with the same netto field coverage as the Esprit, also with pinpoint stars into the corners, but taken at F4.5 instead of F5. Ofcourse, the resolution of the final cropped image will be smaller, but that ‘s a difference – in case of the Nikon D750 – of
Out of the box (inspected by TS) I can rightfully say that this instrument is properly collimated, and did not show evidence of obvious image defects. The star shape -especially of Venus – was not perfect but I can imagine that the extreme brightness of this object would bring out the worst.
Adding a ZWO EAF Focusser
This autofocusser is the less expensive between the available commercial offering. It was a quick fit with two screws and everyting basically worked from the first night. The backlash setting in SGP was 100 steps. Also the “reverse” setting needs to be switched on. A nice feature is that it will beep when at the start and the finish. At least you know something is happening. Also it will beep twice for an overload and stop.
Sample images taken with this instrument:
