14 replies to this topic

[RossW]

I'm going to open a tab here on CN to catalog my experience with this new astrograph by Askar (a Sharpstar subsidiary). 

 

 

The homepage for this refractor can be found here: 

http://www.askarlens...class2/182.html

 

And the optional 0.7x reducer here:

http://www.askarlens...class5/192.html

 

The specs and accessories are listed as:

• Aperture: 72mm
• Focal Length: 400mm
• Focus Ratio: f/5.6
• Objective Type: Quintuplet Dual ED Air-Spaced APO
• Overall length: 317mm
• Weight: 2.56kg
• Options Weight: 2.88kg
• Back-end Adapter: M68x1 Male Thread, 2" Adapter/1.25" Adapter
• Back focus: 140mm

Accessories:

1. Pair of Tube Rings
2. Handle
3. Dovetail Plate
4. M68-M48 adapter, 2” adapter, 1.25”-2” adapter
5. Inspection List
6. Manual

Michael Fong of Sharpstar listed this new astrograph in the Vendor's forum here:

 

https://www.cloudyni...apo-astrograph/

 

The timing was perfect, because I was interested in a new refractor that could handle full frame sensors, and which had a wider field of view than my Esprit 100 reduced to 412mm focal length. With the optional Askar 0.7x reducer, the focal length of the FRA 400 reduces from 400mm to only 280mm. The price point of $1,100 plus $250 for the reducer seemed more than reasonable for an f/3.9 Petzval astrograph, but just how good are the optics, especially spot size and CA at the edges of the field of a full frame sensor? The docs suggest typical spot sizes @ native f/5.6 of 4.5um RMS at centre, increasing to 6.2um @ 22mm radius. With the reducer added the spot sizes are reported to increase to 5.7um and 13.2 respectively. I'm not sure how that compares to something like a Tak or other high-end refractor, but for this price point the numbers do seem quite good.  

 

After a short debate in my mind (to buy or not to buy) I contacted Michael via email, and after receiving example images taken with the scope at both f/5.6 and f/3.9, I purchased the FRA 400 and reducer, with payment via PayPal. I received the tracking link the following day, with the shipment's arrival estimated to be in seven days time.

 

(more to follow)

 

Disclaimer: I know very little about scope optics, so I will not be able to offer much technical discussion in this thread.

[RossW]

Quite amazingly the shipment arrived within three days (thanks FedEx). The reducer and scope were shipped in separate packages, with the reducer being a little light in protective packaging (double-boxing may be more appropriate), but in any case neither package showed any sign of damage:

 

 

 

Let's take a look at the reducer first. Yes, the instruction manual is written in both English and Chinese.

 

 

This reducer is heavy (similar to a large, quality 2 inch eyepiece), and has an M48 x0.75 male connector on the camera side, covered by a protective metal thread-on lid:

 

 

The focal reducer fits up into the FRA 400's focuser and screws onto the focuser's M72 male connector. This reduces the imaging train by around 6cm. Back focus is the standard 55mm, so things are cramped when using both an OAG and a filter wheel (a little more BF would be nice), but the result is a compact setup that will fit well in cramped home observatories. The reducer's optics have an iridescent pink hue.

 

 

 

[RossW]

Now to unboxing the OTA:

 

 

 

 

The OTA is well protected from rough handling. At bottom right is the M68 female to M48 male adapter that attaches to the focuser. The accessories include a selection of stainless Allen screws for attaching hardware like a focuser motor bracket to the bottom of the focuser.

 

 

An English/Chinese instruction manual is included, along with a warranty card and a packing check list:

 

 

 

Finally the OTA is revealed:

 

 

 

The carrying handle is a nice addition but it can be removed for mounting accessories to the top of the OTA if necessary. The OTA mounting rings have several tapped holes on the side, making it convenient to attach further accessories or balance weights. It's certainly nice to see some thought has gone into the design of this product.

 

 

 

[RossW]

The optics and dew shield. Not much I can say at this stage, other than the main cell has a green hue and is immaculately clean. The dew shield is a quality snug fit and it takes some pressure to move it. Its movement is smooth and with no slop (unlike my Esprit's dew shield).

 

 

 

[zjc26138]

Looks like a beautiful scope!

[RossW]

Focuser

 

Thank you Askar! The 3 inch focuser is a rack and pinion, not a Crayford. As an astrophotographer I've never been impressed with Crayfords and their potential to slip, resulting in less accurate autofocus. Drawtube movement is very stable with no slop. From memory the drawtube extends 6cm.

 

 

 

 

There are two small red knobs on the focuser. One (right of centre) on the focuser body is a drawtube brake, while the other at the end of the drawtube is for camera/imaging train rotation. Camera rotation adjustment is firm yet smooth with the feel of quality machining.

 

 

Coarse focus is also firm yet smooth and I feel not an ounce of backlash. The microfocus is smooth and light to the touch. Focuser movement is perhaps not as slick as the moonlight I have, but that is neither required nor desired by me; for a scope of this price point I am more than satisfied with this focuser (though admittedly it has not seen much operation yet). Fine focus also has a clutch, so if you're driving the microfocus axle with a stepper it is still possible to manually focus using the coarse focus knob (though for astrophotography at this f-ratio I think it would be better to only use the coarse focus control for autofocus, to avoid the potential for clutch slip). 

 

For autofocus stepper axle coupling we have the choice of a 4mm diameter microfocus shaft, or a 6mm diameter coarse focus shaft.

 

 

 

For mounting the stepper motor to the focuser body there are a choice of threaded holes in the focuser pinion assembly. Askar recommends using holes 4, 6, and 8 in the image below. I temporarily retro-fitted a stepper mounting I already had and mounted it using short bolts into holes 7 and 8 (I'm using the microstepper axle until a NEMA 11 stepper arrives, for coarse axle coupling). In reality, many of these holes have Allen screws in them, but they are deeply set, so there is still plenty of thread remaining to use these holes for mountings.

 

 

 

[telfish]

Nice scope, it will be great to see your first light images and if this scope has the "Iron cross" on bright stars or need a step down mask to get really round stars. And thanks for a great initial in depth quality report.

[Bob_Huerbsch]

Looking forward to your results with OSC! Would be interesting to see with mono the results focusing with a B mask with L filter then changing RGB filters and seeing how they are. 

[RossW]

Well, I now have a scope, but the parts required for imaging are probably months away from arriving. Chroma 2-inch filters: no ETA. ASI1600MM Pro and 2" filter wheel: on backorder. Ummm. Itchy fingers. Can I jury-rig an imaging train with parts that I already have available? I have a few spare ZWO imaging cams but they all have tiny sensors (ASI 178MC, ASI 385MC, ASI120MM), so it's close to pointless using them to test the optics of this widefield scope. I also have an old 2004 vintage Canon 20D with APS-C sensor, but it's less than ideal to do any real testing: no live view, so focusing will be a pain. Non-cooled sensor = noisy images. Needs an EOS to M48 adapter when I only have an M42 adapter. What to do ....?

 

As luck would have it my replacement Esprit 100 arrived a day after the Askar (the Esprit problems are a story for another thread), and blow me down if it didn't happen to have an EOS to M48 adapter in the box! Coincidence or divine intervention?! That gets the camera connected to the reducer and provides 55mm of backfocus with no other adapters required. I had a spare SkyWatcher AZ-EQ5 mount already, which conveniently has a Vixen saddle (rather than Losmandy) as required by the Askar's Vixen dovetail. While not necessary, I also had a spare home-brew Arduino myFocuserPro stepper controller and stepper mount attachment lying around that would conveniently mount to the focuser pinion assembly with little modification. Guiding? I had an unused TS Optics finder scope that I modified to accept the spare 120MM guide camera (this required a small 1.25" barlow to get it to focus though).

 

So in the end I had an imaging rig set up within the day. And despite being monsoon season here with seemingly endless nights of cloud, there was a prediction of a few hours of clear sky that night. Who says new equipment attracts cloudy weather?

 

Apologies for the poor quality images, and the mess of cables, but this was all thrown together at short notice.

 

 

 

 

 

The 0.7x focal reducer is sitting inside the focuser draw tube, so the camera sits very close to the end of the focuser. This is advantageous for minimising tilt-inducing sag, a huge positive when working at this focal ratio.

 

 

 

I attached the robo-focus motor to the microfocuser axle using holes 7 and 8 on the focuser pinion assembly. The tiny 5 volt 28byj-48 stepper motor easily drives the microfocuser axle and can be driven directly from a computer USB port (no need for external power).

 

 

 

[RossW]

So that night I had a small window of opportunity to get the mount roughly polar aligned and the scope imaging. I figured 60 second subs should be sufficient at this focal ratio with the large 6.42 µm pixels of the 20D. I didn't know the focal ratio of the finder scope so I first needed to upload a sub from the finder + ASI120 to Astronony.net, throw the FL and pixel size data into PhD2, calibrate, and slew to a random point in the sky that had a star field (remember, no easy image download and plate solving possible with the 20D - all images are stored on the 20D's flash drive). I think I ended up pointing close to NGC 7000. 

 

Next to focus. Once again I realise I have been spoilt by automation; it's back to manual trial-and-error focusing. Bhat mask focusing without a live view is quite tedious, and in the end I have little idea as to how close to optimal focus I achieved. None-the-less, I then set up the intervalometer to take an hour's worth of 60s subs and left it to do it's work. Later, captured some flats, dark flats, and darks before the ambient temperature changed too much.

 

Next day, calibrate, star align, and integrate the subs in PixInsight. Extract the luminance from the OSC master, throw it into FWHMEccentrifity and AberrationInspector scripts, and Bobs-your-uncle, I have my first qualitative and quantitative FRA400 performance measures.

 

 

The field looks surprisingly flat for an f/3.9 scope on an APS-C-sized sensor, with only 0.2 pixel variation in FWHM from centre to corner. The optics look well centred. Eccentricity also looks great across most of the field, and if it is possible to remove that slight tilt from the system, eccentricity should further reduce. Three points to note:

 

1. These measures are from an integration of about 60 subs, not a single frame;
2. I have no idea how far off focus I am. When I get autofocus working, these numbers can only improve.
3. No filtering used. Not even a sky glow filter. Unfiltered light going directly to the sensor.

 

 

 

 

 

[RossW]

OSC processing

 

I have zero experience with processing one-shot colour images, so I simply following my usual mono processing steps. I kept it simple: no background extraction, no noise reduction, no colour correction, etc. Just a stretch using a combination of arcsinh stretch and masked stretch. The lack of colour correction may explain the brown'ish background in the following images. First, the final minimally-processed image:

 

 

Here is a link to the full image file

 

And the aberration inspector mosaic:

 

 

If you pixel-peep you'll see a little colour fringing in the periphery stars: blue on the inside edges and red on the outer edges. I also saw this in example subs I received from Sharpstar. To me, this amount of CA seems reasonable for a scope of this price range, particularly given the f/3.9 focal ratio. I also need to consider what effect only guessing focus has had on this CA; being able to optimise focus via software autofocus may reduce it. It may also be possible to reduce it by extracting the individual RGB frames, star aligning with distortion, and then re-integrating in PixInsight.

 

As I have no experience with imaging at such a short focal length I would be very interested in hearing the opinions of my fellow imagers regarding this scope and the test image results. I am particularly happy with the tiny spot size at the edges of this APS-C-sized image frame. What do you think, colleagues?

 

 

 

 

[ZL4PLM]

Focuser

 

Thank you Askar! The 3 inch focuser is a rack and pinion, not a Crayford. As an astrophotographer I've never been impressed with Crayfords and their potential to slip, resulting in less accurate autofocus. Drawtube movement is very stable with no slop. From memory the drawtube extends 6cm.

 

 

 

 

There are two small red knobs on the focuser. One (right of centre) on the focuser body is a drawtube brake, while the other at the end of the drawtube is for camera/imaging train rotation. Camera rotation adjustment is firm yet smooth with the feel of quality machining.

 

 

Coarse focus is also firm yet smooth and I feel not an ounce of backlash. The microfocus is smooth and light to the touch. Focuser movement is perhaps not as slick as the moonlight I have, but that is neither required nor desired by me; for a scope of this price point I am more than satisfied with this focuser (though admittedly it has not seen much operation yet). Fine focus also has a clutch, so if you're driving the microfocus axle with a stepper it is still possible to manually focus using the coarse focus knob (though for astrophotography at this f-ratio I think it would be better to only use the coarse focus control for autofocus, to avoid the potential for clutch slip). 

 

For autofocus stepper axle coupling we have the choice of a 4mm diameter microfocus shaft, or a 6mm diameter coarse focus shaft.

 

 

 

For mounting the stepper motor to the focuser body there are a choice of threaded holes in the focuser pinion assembly. Askar recommends using holes 4, 6, and 8 in the image below. I temporarily retro-fitted a stepper mounting I already had and mounted it using short bolts into holes 7 and 8 (I'm using the microstepper axle until a NEMA 11 stepper arrives, for coarse axle coupling). In reality, many of these holes have Allen screws in them, but they are deeply set, so there is still plenty of thread remaining to use these holes for mountings.

 

 

The hole layout is such that a ZWO EAF fits directly using the parts out of the box 

 

agreed its a lovely focuser!

[telfish]

If you pixel-peep you'll see a little colour fringing in the periphery stars: blue on the inside edges and red on the outer edges.

 

That looks more like Atmospheric Dispersion to me. Were you imaging low in the sky?

[RossW]

That looks more like Atmospheric Dispersion to me.

 

I would never have thought of that, though it does make sense; thanks. Altitude of around 40 degrees, so not so low, not so high either.

[maxmir]

The stars look round enough but the CA is bit disappointing.

I was hoping an ABS frame would look much better with the F3.9 reducer.

You might try registering the RBG frames in PI using distortion correction.

 

It is not going to do well on a full frame camera with this amount of CA.

 

I am processing my first set of image with the scope at F5.6

I will post the results when I am done.

 

Max