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Old 19-12-2007, 11:50 AM
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g__day (Matthew)
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Advice on next years outlays - I see an apo - what is it?

Next year if I save my pennies well I was considering buying an apo. I have this vision in my mind where a sturdy mount is bravely carrying two rigidly secured scopes, my 9.25” SCT and a 4” – 5” apo. My naïve thinking is this might suit a range of targets well, so with this in mind I’d thought I’d raise a few questions to reality check this dream.

So what I would in effect be doing is keep my SCT with its piggy backed 80mm refractor, but replace my 5” MAK with a 4” – 5” apo. This rig would probably be used to be 20% - 30% (max) visual versus 80% - 70% imaging. Auto-guiding could be off any of these three scopes whilst one of the remaining two takes a picture and the third is there if I want a glance to see what is happening (being careful not to knock things in the middle of an imaging run) or put a video CCD camera in it to do the same. Eventually late next year (budget permitting) or the year after I would replace my main imaging camera (Canon DSLR) with a auto-guiding S-BIG camera.

So them’s the plans and I live in light polluted Sydney suburbs.

Now I’d love to reality check this thinking. So far of all things I’ve snapped planetary nebulae have taken my fancy for the SCT. I haven’t ever taken an DSLR image from my Megrez 80mm. So I have a few questions.

How do I decide on aperture? Is the range 100mm to 130mm a sensible choice for what I’ve just described?

Do I want to opt for an apo where you can add an electronic focuser as a must have, nice to have, or not really needed?

Should I lean towards a general purpose quality apo or go for one that is more specific to imaging (e.g. the televue IS series)?

Should I eschew aperture and go for quality (would a 4” Televue be better for my desires than a 5” Williams Optics)?

Am I going to have horrible problems matching final imaging cameras to this very different range of SCT and apo? (meaning sorry its two not one S-BIG or FLI or whatever needed)?

I say I like planetary nebulae on the SCT but an apo is more wide field – your thinking is wrong and you haven’t taken into account… whatever

* * * * *

I’d really appreciate people’s thoughts here because if I say allocate $3-$5K for an apo and say the same for a CCD – what are my best options given what I have already and where I am trying to head all this?

Many thanks,

Matthew
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Old 19-12-2007, 01:50 PM
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Starkler (Geoff)
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How about a nice 115mm Vixen ed apo
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Old 19-12-2007, 02:02 PM
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Once I have the hang on how it all fits together I'll progress, but it also raises a question when you say "ED Apochromatic Design" does that mean its APO, APO like, APO near, ED? I read it as following the design guidelines of APO - but using only ED lens making it semi APO - if that is the right language?
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Old 19-12-2007, 02:02 PM
Karls48 (Karl)
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Mat, I may be totally wrong but if I have a luxury to afford new scope and camera my first consideration would be to match the scope to the camera. Using formula 206 X camera pixel size in microns / focal length of scope = resolution arc sec per pixel. I would choose scope that would give me resolution 2 to 4 arc sec per pixel for chosen camera. I’m sure that other will come up with different ideas but this would be my first consideration for imagining setup.
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Old 19-12-2007, 02:15 PM
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I"ll zip in and mention the Pentax range of refractors {in hope of feedback about them }
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Old 19-12-2007, 08:18 PM
jase (Jason)
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Matt,
I'm not sure if I'm following you. You mention you enjoy imaging planetary nebula which on average are small in angular size, thus requiring a considerable focal length. This would rule out a wide field APO. Have your imaging goals changed? Do you want to do more wide field work? Typically anything under 1000mm is wide field (some would say 1500mm). I'd consider deep space going beyond 2000mm (equipment demands of DSO imaging are considerably higher). Of course this is open to interpretation.

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I"ll zip in and mention the Pentax range of refractors {in hope of feedback about them }
I've had the pleasure of using the 100 SDUF Quadruplet APO. At F/4 its a very quick scope and fun to use. It provides a large flat field and vignetting is keep to a minimum with its 88mm image circle. Actually, its optical configuration is a copy of the legendary Takahashi FSQ created by Yuyama-san, thus has similar characteristics. The SDUF is poor scope for visual work, but outstanding for imaging.
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Old 19-12-2007, 08:55 PM
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Quote:
Originally Posted by g__day View Post
but it also raises a question when you say "ED Apochromatic Design" does that mean its APO, APO like, APO near, ED? I read it as following the design guidelines of APO - but using only ED lens making it semi APO - if that is the right language?
APO, semi apo etc are simply marketing terms as no refractor is truly apochromatic to the strict scientific definition.

I think it is an ed doublet, and a very very good one. The only time I see colour in mine is if the scope is out of focus. in fact i find it a good indicator of precise focus on bright objects, red fringing on one side of focus, and green/violet on the other side. I certainly have no complaints about its colour performance and am selling it simply because I dont have a need for a refractor amongst my scopes.

In terms of apo-ness its better than i remember the ed80 to be that i used to own.

Heres a review http://www.astromart.com/articles/ar...article_id=555
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Old 19-12-2007, 09:08 PM
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g__day (Matthew)
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Geoff - thanks - its in consideration then!

Jase - Taa, the SCT is for DSO - the apo would be for wide field, but being that I am totally unused to wide field I am unaware of its specific requirements - whatever they may be. So if someone would say - flatness of field is an issue with apo's that you don't get with SCTs - you need X to correct it - it would be lessons learnt. It was a catch all query!
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Old 21-12-2007, 01:55 PM
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It depends on the APO - some like the TV127is and Pentaxes which are designed to be used primarily for imaging produce a very flat field without correction, but most others need a field flattener to get it flat right to the edges - I know there are flatteners for the FLT110, TEC, TMB and the Tak scopes for example.

There are a large number of options in this area, you can get scopes that are designed for imaging, scopes that are designed for visual and scopes that are good for both. If you want good for both then you are probably looking at scopes in the f/6.5-7ish range. You would generally expect a triplet APO to be better for CCD imaging because it has a wider range of correction across the spectrum than a doublet.

I have an FLT132 which is an f/7 triplet FPL-53 APO, and I know some people use it to produce very high quality CCD images, but I only use mine for visual and webcam imaging so I can't really add much more. For visual a good 5" is definitely superior to any 4" but for CCD imaging its more complicated.
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Old 22-12-2007, 10:22 PM
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g__day (Matthew)
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Glancing up I appreciate all of the thoughts - but a quck check shows none of the five specific questions I asked answered so far

Still hopeful though
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Old 23-12-2007, 05:17 PM
jase (Jason)
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Quote:
Originally Posted by g__day View Post
How do I decide on aperture? Is the range 100mm to 130mm a sensible choice for what I’ve just described?
As an imaging scope, aperture is only one component. Equally important is the focal ratio. Seriously, I don't think you'd notice a huge difference between the output between a 100mm and 130mm when you're attaching a high QE chip to the imaging train. There is a marked difference between 100mm and 150mm. You'd obtain more faint fuzzies and background galaxies in the 150. Potentially improved resolution, however this will vary depending on arcsec/pixel combination etc.

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Do I want to opt for an apo where you can add an electronic focuser as a must have, nice to have, or not really needed?
Indeed. You'll want an electronic focuser for any imaging set up. I don't know how others manage without it. You can mount a stepper motor such as a robofocus to just about any scope. I wouldn't purchase a scope that I could not fit a motorised focus on to it - seriously. When you've used a motorised that is computerised, you will never turn back and you can be assured you're always imaging in the CFZ of the telescope, even due to temperature contractions/expansion.

Quote:
Originally Posted by g__day View Post
Should I lean towards a general purpose quality apo or go for one that is more specific to imaging (e.g. the televue IS series)?
You'll want a APO that has the flattest field possible so head towards a dedicated imaging scope. Most are good for visual work as well.

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Originally Posted by g__day View Post
Should I eschew aperture and go for quality (would a 4” Televue be better for my desires than a 5” Williams Optics)?
Depending on what you want to image, but aperture isn't everything in the imaging world. Sure if you're trying to hunt down and image 5'x8' 20th mag. galaxy, you'll want aperture. However, you'll never be doing this with an APO. APOs are wide field instruments as I mentioned in my previous post. If you want deep sky (+2000mm) you need a different scope. I'd go for a quality 4" over a mediocre 5" any day. Also keep an eye on the required accessories too. If the scope has an inherent flat field into the design (Petzval), then the imaging train is simplified.

Quote:
Originally Posted by g__day View Post
Am I going to have horrible problems matching final imaging cameras to this very different range of SCT and apo? (meaning sorry its two not one S-BIG or FLI or whatever needed)?
Can't really make a call on this. Get a camera between the 7.4u and 9u pixel size. On the SCT you can bin the pixels to get a suitable arcsec/pixel combination. On the APO, simply go 1x1 (no binning). Once you've selected the telescope, come back to us regarding camera selection.

Quote:
Originally Posted by g__day View Post
I say I like planetary nebulae on the SCT but an apo is more wide field – your thinking is wrong and you haven’t taken into account… whatever
Yep, you need to work out what you want to image. The wide field APO isn't going to be the best to hunt down the faint Sharpless objects. If you're passionate about your planetary nebula goals and still want to use the SCT, perhaps look into an AO unit to counteract seeing. Alternatively sell up and get something else suited to your goals.
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Old 23-12-2007, 05:24 PM
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g__day (Matthew)
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Jase,

Huge thank you for your insights! Can I ask one other question:

How would you rank the brands / models in this price range - if you had to give a top five brand / model by price performance - what would make your list?

Matthew
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Old 23-12-2007, 08:03 PM
jase (Jason)
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Matt,
$3k-$5k price range does limit selection. I’m not going to produce a list without causing a commotion amongst other members, so I’m merely providing three well known imaging ready scope options. Note, I place an emphasis on imaging ready – just about all APO’s you can image with however with these models you will not need to purchase a separate field flattner/focal reducer. They’ll handle the big CCD chips (35mm+) “out of the box”. I know you don’t have a big chip at the moment, but you’ve got to think about future proofing your purchase unless you want to upgrade gear on a regular basis (or as you advance).

Pentax 100 SDUF II – You should be able to pick one of these up for around the $4 to $4.5k mark. Can only get them through Optcorp at this stage. A few were going cheap on Astromart not long ago.

Claude from AEC is selling the FSQED for ~$5800. Note: there is also an EDX edition now. Takahashi realised the CAA that was integrated on the OTA can’t be used for remote imaging purposes so they simply installed the old CAA from the FSQ-N and called it the EDX. I think it takes up approx 15mm of backfocus – nothing major. Optics are identical to the ED.

The NP101is is also around $5,200 from Bintel. If you’re going to head the Televue path, make sure it’s the NP design.

Today, if I was on the hunt for a real value for money 5” refractor, I’d make it the Takahashi FS-128. They are now superseded by the TOA-130 and in my opinion the best keep secrets in the 5” market. They’re only a doublet, but have extremely good colour correction – some say just as good as the TOA-130 triplet, though I haven’t personally had the opportunity to compare the two. The “F” version comes with a 4” focuser and works well with big chips. The 2.7” focuser version is good for small chip imaging. There was one being sold in the buy/sell forum about a month ago. Keep a look out for them as they provide great value for money.

I know someone will make comment around Astrophysics. Sure, great refractors, but certainly out of the price range. The smaller AP Traveler and AP Stowaway demand a hefty price even on the seconds market. In some cases more than what they were worth when they originally came out. Talk about return on investment!

EDIT: Clarification of the FS-128. The FS-128N comes with the 2.7" focuser. There are other third party focusers available to upgrade it. I have not had confirmation that the TOA-130F 4" focuser screws onto the FS-128N mountings. Perhaps someone else could clarify. I'm aware a TOA-130S can be upgraded to a TOA-130F.

Last edited by jase; 23-12-2007 at 08:42 PM. Reason: EDIT.
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Old 25-12-2007, 12:15 AM
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I notice some APO exotics (e.g. A&M) also come with carbon fibre tubes - which I presume really helps avoid thermal expansion / contraction - so keeps focus razor sharp.

http://www.cloudynights.com/item.php?item_id=1544

"Some have asked if the carbon fiber is a good or bad feature for a refractor. It is much more rigid and stronger than an aluminum tube but that is not a big issue in the small cylinders used in a scope. Some have questioned the performance of the scope for maintaining focus as the temperature changes. Well...those questioning it have never had the actual scope to test...like me (I believe there are only a couple of the A&M refractors in the US and mine may be the only 6"). First, the scope uses a temperature compensating lens cell. As I understand it as the temperature changes the cell compensates for the change in the glass keeping it at the null point. I do know that the earliest TMBs had a problem with pinched optics in cold weather...but the new setup cures that. As an engineer (now retired) I do know about the temperature coefficient of carbon fiber that is much better than aluminum. That being said...all this is theoretical and only hands on use will test these features. I have done several all night imaging sessions here in Michigan. I do recognize that at f8 there is more of a zone of focus than at a much faster optical system. Still I have been very impressed with the lack of temperature-induced change in focusing. I did one test run with the scope operating at f5 through my home made focal reducer and imaging through Astrodon Narrow Band filters from about 11:00pm to sun up. I intentionally didn't touch the focus for any of the filter changes and the image is sharp and there aren't out of focus colored rings around stars. I plan on doing some additional testing using the auto focuser to give me some sort of quantitative test. But based on my comparison of a SCT with aluminum tube and an identical scope with a carbon fiber tube I expect there to be less focus change with the carbon fiber. On the aluminum tubed SCT it would only go 2 degrees max before the focus would change. With the carbon fiber it would go for at least twice that before it changed. In any case it may be academic since most of us normally always focus with each filter change and confirm focus throughout the night."


Do you have any experience or views on this set-up?
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Old 25-12-2007, 01:29 AM
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Indeed, I’ve heard of two manufacturers coming out with carbon fibre (CF) APO tubes, being A&M and WO. Though the WO models are limited edition. Interestingly, both A&M and WO use TMB lense sets – perhaps one of them negotiated the supply of the tubes. There might be more in the future, but it would appear that this is not the current trend. The high-end APO manufacturers continue to use aluminium. CF does look rather cool though.

I think the snippet of information you provided covers the key points;
A faster scope will have a shorter CFZ and as such will take some refocusing work to remain in the CFZ as the temperature changes. This would also apply to CF tubes, however you would expect the shift would be less over time. I seriously don’t believe you’d avoid refocusing all together especially on fast scopes. To give you an example of the CFZ size, the Takahashi FSQ-ED operating at F/5 has a zone size of only 55 microns. Thus, it doesn’t take much of a contraction/expansion to the focus out. This also depends on how rapid the temperature changes are.

The last sentence makes a valid point – “In any case it may be academic since most of us normally always focus with each filter change and confirm focus throughout the night." This is the reason why I indicated in the previous post that a motorised focuser that can be computer controlled is a major leap forward in acquiring quality data – I wouldn’t purchase an imaging scope without it. I can’t imagine you’ll want to manually refocus the camera due to different filter changes or every hour during an imaging run to compensate for temperature changes. If done manually, you’ll waste too much time validating you’re in the CFZ. However, if you can automate the procedure, life becomes a whole lot easier. Free software like FocusMax is a “god sent” for astro imagers. It will provide repeatable focusing ensuring you hit the CFZ every refocus, typically under 70 seconds. Mine is around 90 seconds, but I tell FocusMax to do more – It first takes an image, plate solve it, then find a 4-7 mag. star (based on the plate solve info) that is +70 degrees to focus on, perform the auto focus routine, then slew back to the original target. As you could imagine, refocusing the telescope every hour to compensate for temperature changes is no longer a tedious task with motorised/computerised focuser. You can refocus the telescope every 20 minutes if you really wanted. Alternatively, you can also calibrate the focuser to compensate for ambient temperature changes. Many possibilities are available.

Don’t let “flashy” APO tube assemblies dictate your final APO decision. Ultimately when buying an APO you’re paying for its optical configuration/performance and associated diffraction limited specs and strehl ratio. I’d much prefer to buy a scope with quality optics that needs to be refocused regularly, than one with mediocre optics that holds focus all night.
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Old 25-12-2007, 10:23 AM
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g__day (Matthew)
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Jase,

Totally agree with that - and glad I thought to ask how importance is computerised, automatic focus! It seems apo's require significant equipment and deployment techniques to get the most out of them - expecially compared to CF SCTs!

I reviewed your site again - awesome gear producing shot mate - I aspire to that within a few years!


Matt
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