Stratis

7 hours ago, vlaiv said:

Anyways, if you are guiding with faster guide cycle than a few seconds, in my opinion, you are doing it wrong.

Generally I would agree, but with the RST-135 in particular this seems to be a good thing. It has a very tight response and no backlash to speak of on either axis, but like most harmonic mounts does need to correct sometimes quite rapid drifts. 1sec is about as high as I like to go with it, 0.5sec improves matters if the seeing is decent, this is something many owners of the mount have noted and it's in the 'unofficial' manual for the mount.

Multi-star seems to mitigate chasing the seeing, and the subs are good at least at the shorter FLs I usually contend with. Part of my hopes in this upgrade plan are that I'll be able to make better use of my big scopes which tend to be relegated to visual work.

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To answer your question anyway, I'd say that ASI220 seems to be very good combination of factors for guiding. It has 4um pixel size which is good size for OAG (you might still need to bin if you guide scope with over one meter focal length), it has 91% QE and it has very low read noise of less than 1e (close to 0.6 and best gain setting).

Thank you for this @vlaiv, I had no idea this camera even existed and even if I had, I'd likely have dismissed it due to my overall poor impression of its predecessor. There's a review on FLO from a fellow 290MM user that clearly states the 220M is a superior camera specifically for OAG use, so that's fantastic news for me.

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But on the topic of guide sensors, the 220MM seems like a really good guide camera. 4 micron pixels, which im running binned x2, high QE, low read noise. SNR of a guide star is always very high, in the thousands or at least hundreds.

Thanks also @ONIKKINEN for the similar recommendation!

Can I ask which other guide cameras you've used in the past and how it might compare, especially in terms of faint star SNR in OAG usage? I've been working towards a truly portable but powerful imaging setup that can fit into hand luggage on a flight, and OAG/filter/rotator combo unit is a big part of that I'd rather not waste by falling back to a guide scope.

Also I've long held the dream of using an OAG to guide an SCT at f/6.3 or even f/10 instead of having to bolt an achro to the top rail with all that added mass. The 290MM works fine on a guide cam but haven't had the clear nights to test it on the OAG.

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HD mounts respond better to faster guiding but it doesn't necessarily mean the guiding is better (look at your subs as that is what matters). My hem responds to 0.5s guide exposures getting rms down to 0.6 or so, but it's pointless if the seeing is average (which is usually is) as it'll be correcting falsely to the seeing. Last session left mine on 1s exposures but usually guide at 2-3s.

@Elp you're right in what you say. Part of the plan for a more sensitive camera is the use of a broad NIR filter to reduce seeing interference with the guider. At present the 290MM just isn't sensitive enough for both fast exposures and NIR.

As topic really I have searched about this many times, even comparing QE curves as best as one can, but I've not been able to confidently select a guide camera capable of accurate, rapid corrections (0.25sec or less). Time was the SX Lodestar was the undisputed champ of guide sensitivity, but the CMOS era is more complicated it seems. I currently use an Altair GPCAM2 290M.

As the happy owner of an RST-135, the only complication of the mount is the necessity for rapid guiding exposures; the unofficial manual (authored by my mount's first owner, as it happens...) makes this clear and it's my experience that 0.5sec is as fast as I can achieve with the 290M / 30mm guidescope setup. As I plan switching to OAG and possibly longer focal lengths, I fear I will need a camera that can make better use of faint signal.

Are there any guide cameras that can be recommended on raw QE? Almost every other metric from expense to field of view to image quality to transfer rate I can work around... it's that 'ultimate' performance that I need.

16 hours ago, Adam J said:

Makes me wonder if its not using the internal memory and only working on USB 2

There was definitely a bandwidth issue. This is known with the RPi in that it doesn't really keep up with USB3 specs, especially not when all its ports are loaded as it's basically just a soldered-on £5 USB hub, but I had hoped that the internal memory buffer could compensate.

It seems it does compensate in that the data is delivered and somewhat faster than USB2 speeds, but the signal is somehow biased in the process. I used a USB-C hub with a Surface Go 2 tablet (I normally use it for focusing and polar alignment at the mount) to take frames with NINA and the output was way better.

Damn shame as the RPi is perfect in every other respect, sips power and super lightweight as remote control box.

15 hours ago, ONIKKINEN said:

But, an issue you should fix and it may actually have something to do with this is your offset which is way too low which results in 0 value pixels that carry no value and cannot be used for proper calibration. You should increase offset until no pixel values are at 0 in a bias frame. My camera had a default offset off 768 which is just fine and there was never a reason to touch it. I recommend increasing your offset to at least 500.

Offset was the setting I had the least idea about, thank you for clarifying.

Looking at the histograms you're clearly right, I was wondering why the values chopped off so abruptly like that.

If I may ask another newbie question, should I be using Bias frames or Dark frames with this camera? From what I've read a Dark frame includes the data the Bias frame would?

Thank you guys!

The only experience I have with OSC imaging has been a 585-based sensor (which produces profoundly different output) and my 6D which is hardly comparable. 

Really good to know it's normal behaviour. I shall have to learn how to process it out  

UPDATE: All issues fixed!

Rather than necropost and drag this back to the top of the forum, I'm editing this top post. Previously, I had concerns over a strange magenta gradient appearing in the darks and bias frames of an Altair 26C.

Initially the other forum members reassured me (see below for very helpful replies) that all is well, and indeed I was able to resolve the issue.

I was able to trace the problem to the use of a Raspberry Pi 4 as the imaging computer. There is something strange in the way the RPi's internal USB hub interfaces with the 26C, as by switching to a Windows PC and NINA the gradient (however mild) disappeared completely leaving beautiful even darks.

Reaching out to people who own both an Askar FMA 135 astrograph and a thread gauge!

The FMA135 is a neat bit of kit, but I'd like to be able to attach something to the front threads, for example a 2" filter or a stack of M48 spacers to use as an extended lens hood. 

The lens cap threads directly onto the lens cell; is anyone able to determine what thread size that is so I can get an adapter made up?

All the best,

Paul

20 hours ago, symmetal said:

Here's a nostalgic AE video

Wow.... does anyone know how to get in touch with Robert Dalby, he'd know  

On 07/09/2022 at 22:15, Elp said:

The AM5 as far as I've seen is the only HD one which has a 3/8 mounting thread, all the others are their own type. 

As a recent owner of an RST-135, I can attest this is not the case: the Rainbow Astro HD mounts have two mounting systems, 3/8" centre thread plus two M8s 45mm apart.

I literally just swapped my AZ-GTi (which I LOVE and has served as my travel AP mount wonderfully) for the RST in exactly the same mounting arrangement and It Just Works.

@Ags Happy to hear you fixed your GTi, I still use mine as my solar scope mount. In the future for you, I strongly, strongly recommend a 'Small Harmonic Drive' mount.... I've had a huge SCT hanging off the side of mine, it's practically the same size as the AZ-GTi but throws big scopes around like they're not there. Only concern is with legstrike, as these things have enough torque to physically bend and shear your equipment without pausing for breath...

Hi all,

Getting back into astro after a long break, and digging through equipment drawers I've stumbled across these two Astro Engineering ImageMate 4x units. I have no idea whatsoever how I ended up with two of these things... but more to the point, they are completely different!

You can see from the images, both are marked identically. One is physically over 5mm taller, but most significant is the clear disparity in optics, the shorter unit has a much smaller clear aperture and optic assembly so the prescriptions have to be different.

I would like to sell one to a good home as I hardly need two, but before that happens I'd like to know how these things differ. A comparison test would be ideal, but I don't really have the expertise to critically judge and clear nights are at a premium.

If anyone is experienced with these units or their manufacture, and can shed some light on these differences I would really appreciate it!

~Paul

I have three SCTs in the stable, a SkyWatcher 127XLT (so basically a blue-tube C5), a 2014 orange-tube 8SE OTA, and most recently a reasonably-new 6SE OTA.

All three are Synta-era XLT-coated.

Unfortunately between them I am missing several mounting screws that previous owners have lost over the years, so adding finder shoes is really difficult.

Does anyone know the appropriate finder shoe screw thread for these Synta OTAs? 

There's a post on here which states that the old USA models used an Imperial thread, but I feel like the chances of a Chinese manufacturer using Imperial sizing is very, very low.

Please help if anyone knows, or if anyone has any of these scopes and a way of determining thread pitch and screw size

Well it's been a bit of a rollercoaster since I started this thread, thank you to those offering advice  

My CEM60 has to have a full service as it turns out the previous owner (since learned he is a convicted criminal....) did some 'DIY' inside the mount and badly screwed it up. It's entirely correctable, but I want a proper repair warranty on it for the next owner so they can be assured it'll work as advertised. Til I get a new mount I am confined to my AstroTrac and baby refractors.

I've reviewed the payload specs for the CEM40 and it really should work fine with my largest imaging scopes, the C8 and the 115mm triplet. My biggest and most irritating scope to mount is a 152 f/5 achromat which is both long and heavy so that may be out of its comfort zone for imaging, but for visual should be ok. I recently sidegraded my imaging camera to a much lighter setup which should help a lot with its payload moment too.

I've confirmed I cannot use my beloved Avalon T-pod 110 with the CEM40 so if I go that route I will have to abandon it. Anyone in the market for a near-mint black Avalon T110 T-pod with free CEM60 adapter plate?  

Great news for us remote imagers (or people who just don't like having a laptop constantly open), the Raspberry Pi 4 has been released!

This model is WAY more capable than previous versions, and has some major upgrades which make it very attractive for AP;

• Up to 4GB of RAM - this speeds up platesolving and enables on-board stacking of larger subs
• Much faster CPU - everything runs better, at much greater power efficiency, and more high-demand apps at once
• USB3 - finally! So new CMOS cameras and large-chip sensors can rapidly transfer their subs to the Pi without amp glow
• Gigabit Ethernet - for people with observatories, this is critical for transferring subs 
• Onboard wifi - sadly the same as the old Model 3 but still, it enables remote control and connection to wifi mounts

We've seen the Pi and other SBCs used in things like the ZWO ASIair, I really feel the new Pi blows everything else out of the water though in its price range. The efficient CPU means you can effectively run this off a power bank and never have to worry about its draw, and the USB3 means you can link up numerous add-ons like long-range wifi adapters without impacting your sub downloads from the camera which was always a risk with the shared USB2 bus on the old Pi.

I've ordered mine and going to be setting it up as a remote Kstars-based imaging controller bolted to the scope itself; anyone else thinking of doing the same?

Hi all,

Coming back to astro after a long break, many ups and downs  

I recently hauled my trusty CEM60 out of limbo and realised I'd like to shift to a smaller mount, as I have never come near the incredible weight limits on this mount for imaging or visual. The new CEM40 really looks perfect in most regards, so I am hoping that will serve as my new main imaging mount. I image with apo refractors, from a 7kg 115mm TS triplet down to a little WO Megrez 72 at only 2kg on an AstroTrac, with both DSLR and QSI 583wsg (so about 2kg camera package). My greatest imaging load has never exceeded 12kg.

I already have an amazing Avalon T-110 T-pod underneath the CEM60, with an iOptron adapter atttached, this one in fact: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10799_Avalon-T-Pod-110-130-Adapter-Kit-for-iOptron-iEQ45-and-CEM60-Mounts.html

This adapter has the central peg for iOptron, plus mounting holes set around the perimeter for the alignment pegs. The adapter has two sets of two threaded holes; the first set is 130mm apart across the centre, and fits the CEM60 perfectly. The second set is 120mm apart across the centre, and I think is meant to fit the smaller iOptron mounts like the iEQ30. 

So the £1400 question; can the adapter accept a CEM40 mount? Can the two threaded holes set 120mm across the centre mate with the CEM40? If anyone has any info on the absolute mounting hole arrangements for the CEM40 alignment pegs it would really help  

I am quietly thrilled this thread is still going... I'd taken a long break from AP as a new job ate all my waking hours, but I have that under control now and I've got a few new scopes in the stable.

The ZS66 of course, remains

I have just completed construction of a 50mm finderscope conversion into a guidescope for my Lodestar; if early tests are any indication it will guide up to 1500mm no problem so everything in my lineup is catered for, with a guide load of barely 250g. Seriously, if nobody's tried guiding with one of these setups they should, once the guidecam is screwed into the finderscope it is like an iron bar, no flex whatsoever and no possibility of focus slip. 

I have a field flattener yes, it's the Altair Astro cheapo one that reduces to 0.8x and flattens, I believe. I haven't tried it with the ZS66 yet but perhaps I ought to. Is anyone familiar with the proper spacing for those?

I think what impressed me most was the clarity of the optics.... there's something beautifully sharp about the way the scope delivers the view.

It's often said that the best camera lenses are functionally invisible and just stay out of the way of the photography.... definitely true of this family of tubes.

Ok, I have to admit to a bit of uncertainty posting this initial review (and the follow-ups to come), on the grounds of relevance. William Optics have gone through a strange phase lately of pushing out numerous but ever-so-slightly distinct small refractors and then abruptly discontinuing the lines, only to release something yet again similar but different.

I am pressing ahead with this then, despite the fact that the entire ZS66 line is out of production, there don't seem to be suppliers holding any stock, and it has been superseded by a number of (technically) superior models. There is no reliable way of getting ahold of one, and finding this example was a total accident....

Thing is though, I love this tiny scope. Let me tell you why.

Background

I found myself looking for a scope this size out of a hatred of single-use parts. Some imagers I've met have a special little thing that is only used with this special other thing for this particular task and only on warm Tuesdays after Pentecost. Myself, I hate having 80% of my kit idle at any given time, instead I like to repurpose and re-use as much as possible. Sure I can buy a miniguider or an ST80 for a guidescope, but they're not really useful in any other role and that annoys me. 

Thus began my quest for a small, light, full-featured scope that could ride on the C8, guide the APO, and triple as a fully-fledged travel scope AND be useful for astroimaging on the road. A tall order indeed with a budget of low triple figures. Discarding the apparently awful TS Quadruplet, the definitely overpriced Borg Collective and the beautiful but just too damn heavy 80ED scopes from various sources, only the WO baby refractors remained.

When one came up on ABS, I had to have it; the William Optics ZS66 SD.

The Scope

The ZS66 was shipped in three versions; the 66 Petzval, the 66 ED Triplet, and the 66 SD Doublet. The Petzval variant was poorly received due to some serious CA issues unresolved by the Petzval elements, the Triplet is reported optically solid with a lack of contrast due to older coating technology and was too discontinued. The SD ('Special Dispersion') doublet hung in the longest, and after a few days with it I can see why. The coatings disparity is surprising, the ED version showing a bright purple reflection while the SD seems more neutral yet less visible. The 66 comes in lighter and almost two inches shorter than the other two designs.

First impressions, superb craftsmanship and a sense of style. This black-and-gold version has been dubbed "The Bling One" by my girlfriend and honestly looks more like something Iron Man should be using to defend freedom and justice. I know these scopes are marketed under many brands, but they must acquit each one with honour; the tube is light but substantial and heavy-feeling, the parts are smooth but solid metal, and not a single bit of anything moves unless you apply reasonable force. I would have preferred the option to mount a standard finder or RDS, but honestly this is not necessary.

The ZS66 has attracted some mixed reviews elsewhere, generally attracting terms like "Cracking little scope" but also mechanical complaints like "Loose focuser", "Rubbish focuser", and "Awful focuser". Naturally, upon getting the black-and-gold tube out of the case, the first thing I did was check this out. The scope comes with an SCT-threaded drawtube which is a lot more helpful than it may first sound; did I mention my love of dual-purpose? Well now I can re-use all my SCT-threaded accessories (which is nearly everything) to get a sound fixture without collimation trouble or slippage. Nice.

The focuser was absolutely fine, I attached a Canon 6D DSLR weighing nearly 1kg and pointed the scope skyward; not even 1mm of slip resulted. Perhaps I have just been extraordinarily lucky, but this one is solid as a rock and I wouldn't change it for anything. The axial rotation is perhaps a little over-stiff and requires a fair bit of muscle to turn, but I'd sooner have it that way around than too loose. The focus travel was butter-smooth both on coarse and fine, no grinding or sinusoidal characteristics. Attachment of some heavy accessories (see below) similarly revealed zero slippage despite over 1kg riding on an extended drawtube.

The Optics

This was my main concern when looking for a baby scope. I absolutely did not want an achro, I had an ST80 in the stable and honestly hated using it. I've also looked through some awkwardly-named 'semi-Apo' examples and hated each and every one. I'm very sensitive to CA and colour in general, blazing blue and green halos everywhere ruin the experience for me entirely. This scope is described simultaneously as 'Special Dispersion' and 'Apochromatic' by William Optics despite the doublet configuration and non-specified glass... I honestly expected the dreaded 'semi-Apo' performance, but was pleasantly surprised.

All of these tests were performed with a Revelation Quartz 99% Dielectric mirror diagonal and a TeleVue Ethos 8mm. The 8mm is a perfect match for this scope, pushing magnification to only 49x and serving a sharp and flat field. I'd have liked to push to 100x, but sadly I lack an appropriate EP and my only Barlow lens introduces colour into whatever it touches.

Strapping the oddly-proportioned WO L-bracket to a Vixen Mini Porta is a superb little combo that can be lifted with one hand easily; I learned that with most EPs, this scope doesn't need a finder, you should be able to eyeball anything. With typical refractor 'focus snap', I got a beautiful view of The Pleiades only slightly behind the Ostara 102 APO I image with, and honestly I feel that's a difference more in focal length. Limiting magnitude was an issue, the cluster looked distinctly empty compared to the same view in the large SCT, but the visible members were bright and hard pinpoints with minimal bloom and zero flare.

Swinging to Orion, the main stars showed their colours faithfully, Betelgeuse particularly a bronze-orange point demonstrating good CA control in the red spectrum. The view of the Nebula was rewarding, lack of light grasp rendered the core stars slightly dimmer but perhaps sharper than I am used to, with easily discerned nebulosity and colour despite poor seeing. I prefer the view through the 4" APO, but this entire setup weighs less than that OTA by itself. Compared to an ST80, it is obvious that WO have focused on coatings and careful spacing to make the most of the 66mm of aperture; I'd stand this scope toe-to-toe with an 80mm any day.

The standard CA torture test is Jupiter, so off we went, and here we see the inevitab.... oh wait, no, we don't. The giant which slew my ST80 and the SLT102 is himself brought low by this baby refractor, with beautifully resolved cloud bands and tiny little moons with only the very-barest-hint (did I mention I hate CA?) of faint far-violet. Once in focus, CA disappeared entirely, dominated many times over by atmospheric refraction. This view stood up to the 4" on sharpness and even made me glance sideways at the C8 for a moment, I honestly did not expect such a superb view from a scope I could fit in a camera bag and take hiking. It also reinforced my long-held impression that refractors rule the solar system, the much larger 1145P Newtonian cannot match the sharpness and contrast on display from the ZS66.

A series of star-tests revealed appalling atmospheric currents but crispy-black diffraction rings of even white colouration, perhaps a shade warm, with about half a rings-worth of glimmering violet outside of focus. This performance is inferior to the 4" Apo in only the strictest sense, it definitely belongs in the class of 'near-perfect doublets' and stands a breed apart from even fine, slow achromats. I tend to characterise CA by jibbing in and out of focus quickly by tiny amounts rather than examining diffraction rings; this revealed a slight tendency towards violet and none at all towards green, but nothing even remotely objectionable. Get your focus locked in and it's white little diamonds all the way.

The Verdict

As should be obvious, this scope is a keeper. Huge leaps forward in optical design notwithstanding, I can see no reason to ever deprive myself of this little gem. It is light enough to mount anywhere as a guidescope or super-finder, bright enough to view the better DSOs, sharp enough to image without vignetting a full-frame DSLR and small enough to lose in a large suitcase.

William Optics have a number of designs since this one, the ZS70 and Megrez 72 being the closest in aperture. These scopes have more furnishings, digital readouts and temperature gauges, as well as the bonus of better focusers and sturdier construction. All this however comes at a price of increased package weight, the 66 stays well under the competition here, although obviously can't compete with ultralights like the Borg.

Assuming I am not offered some remarkable Tak or TV mini-Apo at a superb price, I shall be making use of this scope for a good long while. I'd heartily recommend it to anyone as a travelscope, guidescope or light imager. It mounts on damn near anything and serves gorgeous views that belie its humble optical recipe.

Cracking little scope 

Clear skies everyone!

~Paul