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Anttler’s Optics 8” f/5 HPN Newtonian

Posted by Craig Stark   07/25/2006 07:00AM

Anttler’s Optics 8” f/5 HPN Newtonian
Repeat after me -- "No one telescope can do it all. Every scope is a balance of constraints and compromises." There -- I feel better now. No one scope can do it all, so I really can justify having a garage full of telescopes. Or can I? What if we ignored what we just said? Or rather, what if we accept the fact that everything is a compromise and try to come up with a really good balance of these constraints in a single scope? Could there be a great "general purpose" scope that is the one that gets the job done and done well 95% of the time?

For me, the goals or constraints for such a scope are:
  1. Moderately portable / lug-able. My urban back yard is not well suited for much and I must take my gear out to darker skies (even 15 minutes away) to do much of anything. When almost every session involves packing up the car, heading out, setting up, breaking down, heading back, and unpacking you start thinking lots more about weight and ease of setup.

  2. Well-suited to astrophotography. I’m primarily interested in DSOs but enjoy a bit of lunar and planetary astrophotography.
  3. #2 can’t come at the cost of being a poor visual scope.
  4. Sharp, contrasty optics. After all, this is a telescope right? Mushy views need not apply.
  5. Easy to setup and use. The best telescopes are the ones that get used the most and the ones that get used the most are generally the ones you like to use – the ones that are easy to use and don’t interfere with your enjoyment.
  6. Cost. Even for people with very deep pockets, less expensive is always preferred over more expensive with all else being equal. These are incredible times for amateur astronomers as features and quality on telescopes have gone up while costs have gone down. Quality optics won’t come cheaply, but if every time I use the scope I think about how long it will take to pay it off or what my child could have done with a better education, I won’t enjoy these nights out as much.

These goals are probably shared by a number of readers. A telescope that hit these goals well would certainly be a great "general purpose" telescope. These goals already do put some hard constraints in place and rule out a number of kinds of scopes. Now, before I permanently annoy a number or readers, a bit of background. I’ve been doing this long enough to go through a dozen scopes by now and have been doing CCD astrophotography for a few years now. I currently own and use a 12" Dob (converted from an DSH-12 into an Albert Highe style 3-tube truss, putting me in the "ATM" and "Dob-guy" camp), a 4" TMB APO refractor (putting me in the "small high-end APO refractor" camp), a 66 mm William Optics ZS 66 SD doublet refractor (putting me in the "really small APO refractor" camp), an 8" Celestron C8 SCT (and formerly a Meade LX-90 putting me in the "SCT-guy" camp).

Turning back to these goals, we find that Dobs are pretty much ruled out. The DSO astrophotography aspect (#2) is very tough on a Dob. Yes, tracking platforms can be made and you could use digital setting circles at the same time to help you find things. Oh, and it could be a dual-axis tracking platform to help you compose your shots. Oh, and it will have to have a nice sliding balance setup to deal with you attaching and removing the camera… Possible, but we’ve added a lot of expense (#6) and it’s getting tougher to use (#5).

Small refractors are another option. They do very well on portability (#1) and offer very sharp contrasty views (#4), are easy to setup and use (#5) and can be quite affordable, especially in the 80mm range these days (#6). They’re very good for wide-field astrophotography, but aperture rules the signal-to-noise equation for CCD and it sets the limit on the details that can be resolve. In CCD imaging, once your signal is well above the read noise, the aperture takes over from f-ratio as the dominant factor in SNR. (See the excellent F-ratio myth page by Stan Moore). Likewise, planetary detail goes up considerably with aperture with what I consider to be rewarding views starting at 6-8" (See the excellent examination of optics for planetary images by Damian Peach). As aperture goes up with refractors, these constraints work hard against each other. An 8” refractor with sharp optics would violate #1 (portability), #5 (ease of setup), and #6 (cost) at the very least.

In talking with my good friend Michael (who was going through the same mental balancing act for his setup), we made these constraints a bit more concrete by saying the scope should be at least 6” in aperture, weigh under 20 lbs and was either natively or could be converted into about a 1000 mm focal length. The weight limit was imposed by the available mounts and how well they fit these constraints. I have a Takahashi EM-10 and he was in the market for a new mount. There are a number of choices for mounts than can do well with < 10-20 lbs on them but start to be unsuited for photography at more than this. The focal length constraint was imposed by the CCD cameras for photography. Many of today’s CCD cameras have pixel sizes in the 6-9 um range. If we assume an average value of 7.5um, a 2000 mm focal length yields 0.77"/pixel of resolution. That’s a bit "oversampled" for many of us as our seeing will blur our stars 3-4" disks and guiding the scope without error at this kind of tolerance is a challenge. Besides, even a 1/2" format chip such as on the Starlight Xpress MX-7C, the SBIG ST-7, or the Meade DSI-II only covers about 8x12' - barely enough to fit M51 and not even enough to frame M101. If we halve that to about 1000mm or less we mate well at about 1.5"/pixel. We can always extend the focal length with a Barlow as needed for planets or planetary nebulae, but 1000 mm will give a nice 16x24' FOV on a 1/2" format CCD chip.

The 8" HPN
With these goals and constraints in mind, I asked Michael the rhetorical question, "What couldn’t you do with a 1000 mm focal length, high quality scope? Say an 8-inch f/5. It should be light enough to ride on a reasonable mount, give you 8 inches of light grasp, and have a 1000 mm focal length to mate well with the camera. All we need is one that has sharp, contrasty optics and doesn’t cost an arm and a leg." With this all as the background and setup, enter the Anttler's Optics 8" f/5 HPN.

Many ATM'ers know that a fine way to make an excellent and affordable scope is to begin with a mass-market scope (generally of Asian origin) and to re-work it to a much higher standard. This is often a lot more inexpensive than buying all the parts and assembling from scratch. The HPN line consists of telescopes that Scott at Anttler's Optics has done just this to. The line includes both Dobs and EQ mounted Newtonians that all have base upgrades but can be custom configured to meet the customer's specific needs. You’ll pay more than you would for the mass-market scope but less than you would for a custom-built "premium" scope.

The 8" HPN in question has been converted from a mass-market scope (the Antares 8" f/5) into a prime example of what Newtonian optics can be. As of this writing, the OTA was $899 ($1395 on a dual-axis ASGT mount and $1599 on a GOTO ASGT mount, both upgraded with polished worms). The base upgrades include:

  • The primary mirror has been refigured by Raycraft to 1/10th λ or better (it comes with a certificate – this one certified to 1/13th wave)
  • The secondary replaced with a 1/20th λ or better 92% reflectivity mirror
  • The spider replaced by a curved 1800 Destiny spider
  • A Crayford focuser is installed (if not on the base scope already)
  • The steel tube has been flocked with Protostar flocking material at both ends
  • A fan is included for the primary
  • The cosmetics generally improved by nice screw covers, etc.
  • Fully checked and certified prior to going out to the customer.

Anttler’s Optics offers this scope in three flavors: Basic ($899), Planetary ($994), and Deluxe ($1039). The difference across these comes down to included upgrades and the size of the secondary mirror. The Planeary has an undersized 1.52" and the other two have 1.82" mirrors. For DLSR imagers, an oversize 2.14" is available too for a larger fully-illuminated field of view. Both the Deluxe and the Planetary have two-speed Crayfords with a 10:1 fine focus knob and a variable-speed baffled fan. The Deluxe also includes studded rings to easily attach a guide scope.

I got a chance to evaluate both the Basic and the Deluxe configurations (I started off with a Basic and ended up with a Deluxe). In my opinion, the upgrades offered by the Deluxe are must-haves for astrophotography. The stock Crayford is very nice but focus is a lot easier to achieve with the two-speed setup. I'd already purchased the same two-speed focuser from him and simply swapped mine in place. The stock rings, while perfectly functional don't provide a means of attaching a guide scope. The stock cooling system is certainly effective at cooling the mirror, but the baffled variable speed one is a clear step up. It does wonders to cut out problems associated with tube currents, sharpening the views clearly in the process.

Finally, I should note that Anttler's Optics recommends a Baader MPCC coma corrector for all astrophotography. I'd previously purchased (and used) the same MPCC coma corrector having searched the web and seen numerous "before" and "after" shots turning the coma-induced "seagulls" into nice round stars across the field. This is typically the fatal flaw associated with using a fast Newtonian for astrophotography. The MPCC is spec'ed to give 12 micron stars across a 35mm film frame and I have no reason to doubt this. Baader lists it has having been tested as the "sharpest coma corrector available, both on-axis and off-axis, and provides the best edge of field illumination" as tested by ITE. I've never compared it to other coma correctors out there (e.g. the TeleVue ParaCorr) but I can certainly say it does a fine job.

Tests and Comparisons
So, how well does it work? How well does it work as a general-purpose scope? Is it a great scope for the astrophotographer without astoundingly deep pockets?

I’ve had the 8" HPN out a number of times now and am quite impressed. There are few things I can find "wrong" with the scope as it consistently delivered clear, sharp, views and images. When directly compared to a fine example of an 8" SCT (a very nice Celestron C8), the HPN always performed as well if not a touch better. Jumping to the conclusions for the impatient reader:

  • Superb optics. Excellent star tests, sharp and contrasty views. Flocking did its job as did the refigured primary and upgraded secondary.
  • Stock is a very good, smooth focuser and the two-speed is excellent.
  • Very effective cooling fan without any vibration issues I could ever find (and yes, I looked closely.)
  • Highly portable and mount-friendly for an 8" Newtonian (bare OTA weighs 17 lbs, is 36" long, and 9" in diameter)
  • Easy collimation (helped by the short tube and the EQ mount) that held very well across sessions.

  • Less portable and mount-friendly than the 16.5" 12 lb C8. With rings, dovetail, and finder the HPN is pushing 22 lbs while the C8 is still only 14 lbs loaded and ready to mount (no rings needed).
  • EQ-mounted Newtonian-induced neck strain when you decide it’s not worth loosening the rings and rotating the tube to bring the eyepiece back to a comfortable position. In contrast the SCT’s relatively constant eyepiece location and rotating diagonal were clearly more user-friendly.

Star Test and Jupiter
First light was a star test and visual test on Jupiter in late June. I am not a seasoned planetary observer but I have spent a bit of eyepiece time on the moon and planets. The seeing was above average here in the Mid-Atlantic - not the absolute best night for a test, but at least it wasn't raining (which is more than I can say for most of the days around this one). The star tests looked quite good with very similar views inside and outside of focus and a Ronchi at the eyepiece (JSP EasyTester) showed clean straight lines. Given the seeing, I used the test mainly to get at least close to collimation and for a basic test only. Nothing I saw raised any alarms or gave any reason to doubt the excellent quality of the optics but without a very stable night, I would not want to be more detailed.

Most of my time with the HPN was spent on Jupiter with a Burgess/TMB 6mm Planetary (~170x) with some time spent on a Televue 11mm T6 Nagler both with and without a Celestron Ulitma 2x Barlow. The views were very nice and about what I could ever expect out of 8” on an average night – a bit of detail in the Equatorial Zone and a bit in the Temperate Belts.

To serve as a comparison, the C8 was alternated with the HPN several times. It did take a few minutes to swap OTAs on the mount (Takahashi EM-10 with Losmandy dovetail setup), meaning the comparisons were not perfectly direct, but they were close. Most of the time on the C8 was spent with the TeleVue 11mm T6 Nagler (~180x). Differences between the two were not night and day, as both are fine examples of 8" scopes and both were well collimated. That said, the HPN did offer a touch sharper view and a touch more contrast.

I have spent a few nights in the yard comparing the HPN + MPCC to the C8 with a Meade 0.33x reducer set to f/5. The yard is not the best place to take showcase shots (I live within the city limits) but test shots work well. Comparison tests were done one night with a SAC10 camera (9 mm diagonal, 0.7" per pixel) aimed at M3 on one night with an Orion StarShoot camera (6 mm diagonal, 1.3" per pixel) on a second. The distance between the Meade reducer and the CCD was adjusted to equate the focal lengths of the two setups, yielding f/5 on the C8 (various configurations led to a 3%, -7% and 11% difference in focal lengths across tests).

Test shots were taken with high gain and short exposures (15 seconds) to reduce mount-induced errors and only the sharpest frames were stacked (~20 frames per setup). All processing was identical and histograms matched as best as possible to create comparison shots. In addition to rating the images myself, I enlisted the help of Michael and several others to judge the shots blindly (they did not know which shot came from which scope). Having a curved spider on the HPN avoided the clear give-away of diffraction spikes.

The results were a draw. With the largest chip I have (the SAC10 - a bit larger than the 1/2" format chips discussed above) both setups produced round stars to the corners. Push the Meade reducer down to below about 0.4x or so and you can’t say the same thing but with careful alignment it works well at this level. The MPCC does a fine job as well, yielding nice round stars (that should be found for a full 35 mm frame). The two globs gave a chance to evaluate just how well the close core stars could be resolved and neither scope came away a clear winner. Differences could be seen but varied more from test to test than across scopes.

Such performance similarity should come as little surprise. Both are 8" f/5 setups that were well collimated. Both have been corrected to remove the dominant aberrations and both used a relatively small, central portion of the field. At 15 seconds of exposure, atmospheric blurring would reduce very fine differences in performance anyway.

In addition to these direct comparisons, I’ve spent some more time using the HPN visually, taking in the sights so to speak without the hassle of direct comparisons to the C8. Every time, I've been rewarded with crisp, clean images. In short, it behaves as a textbook 8" f/5 Newtonian should behave. Stars are wonderfully tight and clean (and I appreciate the lack of diffraction spikes resulting from the curved secondary) and the 8" brings out DSOs and details better than the excellent 4" scope on hand. None of this should come as a real surprise. The primary has been reworked and certified (complete with test data) to a very fine level, the secondary is also of very high quality, and the flocking and cooling systems work very well. The mechanics are solid so that everything remains in alignment to provide these crisp views. In short, there's nothing in the execution of the scope that would keep it from performing as well as an 8" Newtonian can. In sum, what's not to like?

The Anttler’s Optics 8" f/5 HPN does a great job at hitting the goals and constraints I had at the start of this. If I could change anything, I'd make it a few pounds lighter to tax my mount a bit less, but that’s about the only thing I could see improving and it's already doing well weight-wise. (Note: most of the weight comes from the steel tube.) Every telescope represents choices made in a number of trade-offs. In my opinion, the 8" HPN has had these choices made very well and is an excellent all-around scope well suited for either visual or photographic work.

If I did not already own a nice 8" setup I would keep the HPN without hesitation and it would likely be my most-used scope. In the end, Michael is trading out his 8" SCT for the HPN and I look forward to being able to use the excuse of "well, your scope is a touch sharper than mine" whenever he makes a better image than I in our monthly online imaging contests.