180 replies to this topic

[jerry10137]

Had an opportunity to borrow an AP160 scope for a little while and decided to do some side by side testing with the SW150 Esprit I have on hand.  I've read in the past where a few folks were interested in such a test.  The visual part of this test will come later.  Hopefully by the end of the month.  I am a photographer and have devoted all of my time to photography so the visual results will be performed by some other unbiased individuals.  I would like to detail my experience with this side by side setup in a photographical sense.  Please understand that both scopes are engineered very well and that I am a strong supporter of Sky Watcher due to their price point.  That being said.......I am also a very honest person and nothing here has been fabricated as some may quickly assume.  The AP160 scope is, without a doubt, an extremely nice scope to have.

 

I attended the Okie-Tex star party recently in Oklahoma and rented a private ranch about 2 miles from Camp Billy Joe.  Okie-Tex skies are very nice and I had a nice opportunity to stretch the legs on both of these scopes photographically.  The results may surprise you and they may not but I am here to detail what I personally found.  First......here is the setup....

AP160:  The AP is an F 7.5 scope with a monster Feather Touch focuser.  The extra $1200 field flattener bumps this to F7.7.  I also used a $135 DSLR adapter for the flattener.  For this reason, it may appear just slightly dimmer in contrast than the SW picture.

 

SW150:  Out of the box F7....no extra items needed.  It comes with rings, a flattener and a t-ring for Canon cameras.

 

Cameras:  I used two identical Canon 60Da cameras.  All menu setting were matched and both cameras have a very low shutter count.

Capture: Backyard EOS Premium on one laptop.

Guiding:  Orion SSAG using AstroTech AT65EDQ

 

Mount: A single Astro Physics 1200 was used with a dual saddle configuration.  Guiding was real good as long as the wind was calm.  Wind was my biggest battle in general.  I took so many subs though that the bad ones were used in stacking and just got rejected.

 

 

 

Processing:  I use PixInsight and the same basic workflow was used to process both pictures.  The only real abnormal modification is that I scaled down the AP160 picture to match the SW150 picture exactly.  PixInsight has a real nice tool for this.  Both pics could then be processed side by side using the exact same techniques once cropped identically.  124 lights from each camera were used at 360 seconds each (6 minutes).  56 darks were collected and used on both rigs separately.  I threw in 100 bias frames to scale the darks.  No flats at all.

 

Target:  NGC891 is an edge on galaxy in the constellation Andromeda.  It is very similar in nature to NGC4565.  You might ask why this target?  Well, I chose this target as a runner for the comparison for several reasons.  It's color is a bit deep and abnormal for most galaxies and it has a ton of dust that can be used for detail comparison.  Just look at all the streaming dust in its lane!  It also has a number of nice little galaxies in the background.  Being as this was shot with a DSLR, I am very happy with the result and think it looks very good.  I can only imagine what a nice CCD would have shown here.  Stars in both scopes were pin point all the way out to the far corners on both sensors.  So both flatteners did their job very well.  The amount of chromatic abberation I am seeing around stars is no better or worse than what is seen in the other photo.  This came as a surprise to me but you can see for yourself.

 

SW 150 PIC

 

Link to a high res SW version

 

AP 160 PIC

 

Link to a high res AP version

 

The only real differences I was able to distinguish is that the SW pic is slightly brighter (I am certain this is due to the slight difference in F ratio) and the stars are slightly cleaner around the edges on the AP picture.  The CA seems to be about the same and I see no difference in detail whatsoever.  Now....I know what you are thinking.....there is no way this is a good comparison!  This is why all images were taken at exactly the same time (same sky quality) using the same mount and guiding (same error if any).  The number of exposures are identical.  I even went as far as to babysit the rig 100% because I dithered every 3 images.  Therefore, I had to pause one of the sessions a few seconds while the other dithered the mount.  The sensor from one camera sat and cooled NO longer than the other.  I can thank Backyard EOS for the ease of use.  Their software really helped in making this as simple as possible.

 

Bottom Line:  Is one scope really better than the other?  Only you can be the judge.

[JMKarian]

Thank you for a very nicely planned and executed comparison - 

John

[psandelle]

Now THAT'S a solid comparison piece. Nicely done!

 

Paul

[orlyandico]

There's one variable that could throw all the comparisons out of whack... Focusing.  in the end the difference would come down to stellar FWHM across the field...  And slight mis-focus would wreak havoc with FWHM.

 

I'd assume both scopes were autofocused with a robofocus or similar to eliminate that variable. 

 

On the other hand I suspect for not so critical imaging either would do.  If you need to measure FWHM to tell the difference then perhaps the difference is really not that significant for 99% of users. 

[BillP]

Wow...impressive (both the setup and the extent you went to to control the experiment).     And yes, how clean the brighter stars appear looks to be the only difference, and not a major difference either.

[jerry10137]

There's one variable that could throw all the comparisons out of whack... Focusing.  in the end the difference would come down to stellar FWHM across the field...  And slight mis-focus would wreak havoc with FWHM.

 

I'd assume both scopes were autofocused with a robofocus or similar to eliminate that variable. 

 

On the other hand I suspect for not so critical imaging either would do.  If you need to measure FWHM to tell the difference then perhaps the difference is really not that significant for 99% of users. 

 

Thanks for pointing this out......I forgot to really mention it.  Since I was dithering every 3 images......I refocussed every 9.  Scope was slewed to a bright star and refocussed manually with a focus mask.  Both scopes were done at the same time on a star.  Again, Backyard EOS made this very quick and easy.  The I would set up another 9 exposure session and dither every 3.....the 9th exposure did not need to be dithered because when slewing back to the target, I always had to reframe slightly.

[Zoomit]

A wonderful comparison--thank you for taking the time.  I wonder how the edge performance would compare with two full frame cameras.  

[mwc11]

Thanks Jerry for setting up this comparison and the informative writeup!

[GJJim]

Thank you for the considerable effort you put into this experiment. The AP test looks like a draw and I have to admit I'm a little surprised by the result. Your visual test/comparison will be interesting indeed. Looking forward to it.

Edited by GJJim, 12 October 2014 - 12:42 PM.

[schang]

When photographs show similar performance, I wonder what visual assessment/comparison will tell us

[mark8888]

Thanks for the excellent and entertaining comparison!

 

To me, as I compare the high res versions of both pictures... in the AP pic, many of the bright stars are looking cleaner and rounder.  I'm also seeing spikes on the brightest stars in the SW picture, which I'm not seeing in the AP picture.  So I'm feeling like the AP photo is showing slightly sharper, cleaner stars.  Both photos are fantastic.  The galaxy is really (really) gorgeous and looks very similar to me in both pics, a little brighter in the SW one.

Edited by mark8888, 12 October 2014 - 01:11 PM.

[rockethead26]

Thanks for the excellent and entertaining comparison!

 

To me, as I compare the high res versions of both pictures... in the AP pic, many of the bright stars are looking cleaner and rounder.  I'm also seeing spikes on the brightest stars in the SW picture, which I'm not seeing in the AP picture.  So I'm feeling like the AP photo is showing slightly sharper, cleaner stars.  Both photos are fantastic.  The galaxy is really (really) gorgeous and looks very similar to me in both pics, a little brighter in the SW one.

 

 

Yes, the spikes are definitely visible on the SW brighter stars.

[BillP]

Those look like the spacers may be in the light path a bit.  A minor mask will probably clean them to round.

[Bill Barlow]

I can't see much difference between the two images.  What is the cost difference between the two scopes?

 

Bill

[BillP]

What is the cost difference between the two scopes?

 

$$$$$$$$$$$$$$$$

[neptun2]

Very good an detailed comparison. I personally plan to get the esprit 150 as future upgrade of my current 120ED. Looking at the high res versions i also see the so called spikes on the brightest stars. I wonder if that can be explained entirely with the brighter picture of the esprit due to the faster f-ratio or there is something else contributing also. 

[Aquatone]

Thanks for an excellent comparison. I can also see a number of stars are a little bit tighter and spike-less in the AP image but it is marginal. 

 

One variable not yet mentioned is that (from the pictures) the guide-scope/camera is mounted on the rings holding the SW150, and not the AP160 which is in a position more prone to flexure over time. The short 6-minute individual exposures are too short for much flexure to show in this configuration, but considerably longer individual exposures would have been an interesting comparison. (Say in Ha) Mounting the guide-scope on each telescopes focuser would also have been more accurate. (Though that would eliminate parallel exposures)

 

Another aspect is that a DSLR is a really extremely light loading for the AP tube and 4" focuser. When I owned an AP160, it could handle really heavy 16803 chip cameras with large 65mm filter wheels over very long exposure durations. The focuser of the Skywatcher does not look quite that robust. Long duration exposures at a heavier loading would be a relevant comparison.

 

However, the test shows how good most telescopes are these days (or at least these samples) and the differences are all about that extra last few percent.

 

Chris

[Josef1968]

Wow, superb Setup!

Fine report!

[rainycityastro]

Excellent comparo!

Since you are running Pixinsight, could you run the FWHMEccentricity script on the subs produced by each of the scopes? And post the results here?

I think the results might shed more light than just looking at the final images.

 

Best regards,

--Ram

[samovu]

Jerry, thank you for sharing your report. Nice job.

 

Clear skies,

John

[jerry10137]

Excellent comparo!

Since you are running Pixinsight, could you run the FWHMEccentricity script on the subs produced by each of the scopes? And post the results here?

I think the results might shed more light than just looking at the final images.

 

Best regards,

--Ram

Are you wanting me to run this against the raw stacks?  All of the RAW subs each (There are some bad ones)?  Or a single RAW sub from each setup (I would assume some good ones after a focus routine)?  Default script settings?  Please let know and I will happily give you this.

[jrbarnett]

In full resolution mode, the AP image is considerably better, albeit slightly less well focused.  But how much of that is because of the extra 10mm of aperture, rather than something else?  It looks like the Sky watcher has either some pinching or some spacer intrusion on star images to me (AP has nice round stars; SW has some spiking on bright stars).  But in the CN condensed shots, both look really nice, and (1) how long do you wait for a SW vs the AP and (2) how much does the AP cost vs. the SW?  It's all relative.

 

I think what your test proves is that you can get decent results without waiting 10 years and spending $10k.  Well done.

 

Regards,

 

Jim

Edited by jrbarnett, 12 October 2014 - 07:02 PM.

[BillP]

In full resolution mode, the AP image is considerably better.  

 

 

Really?  I downloaded them both and looked and didn't think so at all!  Other than that little artifact on the brightest stars, CA, details, everything was just the same IMO.  About as subtle as subtle can be.  So much so that would really characterize as identical.

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[rainycityastro]

 

 

Are you wanting me to run this against the raw stacks?  All of the RAW subs each (There are some bad ones)?  Or a single RAW sub from each setup (I would assume some good ones after a focus routine)?  Default script settings?  Please let know and I will happily give you this.

 

 

Hi Jerry,

Try to run the best ones (maybe the ones right after the focus routine) with the default script settings. Moffat4 and other default settings are fine.

Run a few of the RAW subs just to get a sense of how they vary. Basically we want to find out how the FWHM and eccentricity varies across the scopes during the same seeing conditions. 

PI returns the results as a value in pixels. It would be great to convert this to arc secs to get a sense of how tight the stars are with each scope. 

Regards,

--Ram

[Scott99]

very nice DSLR pics!   amazing to see what they can do these days.