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Most Optically Perfect Fast Astrograph

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#1 LuxTerra

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Posted 05 December 2020 - 12:31 AM

The question is simple, but I suspect the answer is not.

 

If you were building a cost is no object fast astrograph (I don't have the funds to do this, I'm just curious), what would that telescope look like? The goal is the best realizable optical design, and since these are fast scopes, over a very large field/image circle; ideally diffraction limited edge-to-edge. To even ask that question, I have to define a few things and set some rules.

 

1) Fast = f/4 or faster. However, if you have a good argument for something slightly slower, I may allow it. smile.gif

 

2) Targeting a very large image circle; at least 55mm, but probably a nice corrected field closer to 70+mm to accommodate future sensors.

 

3) This must be a "reasonable" amateur telescope. Hubble, James Webb, etc. don't count. Thus, I'm going to say it must be under 500mm aperture. That's probably still a bit large/heavy, but it's a nice round number. The lightest 500mm fast scopes I can find are a f/2.4 Dreamscopes Hyperbolic Newton, which comes in at just under 90lbs. They do make a 500mm f/8 RC at 55lbs, which can be made much faster if you have the money. Generally, if it's technically possible to make it under 50kg, then it's good to go.

 

4) Must be ideally suited for todays and tomorrows fast CMOS sensors with small pixels (one of the reasons to go fast). I'm thinking something like the current awesome IMX533/571/461/411 family. i.e. image scales must make sense; CMOS pixels are likely to get even smaller.

 

5) No external constraints on support equipment such as computer process/storage, etc.

 

 

Some options I've seen out there:

 

1) Astro-Physics 305mm F/3.8 Riccardi-Honders Astrograph - AP claims it has virtually perfect color and all 5 primary aberrations corrected across the entire field.  https://www.astro-ph.../producthistory

 

2) Dreamscopes f/2.4 Hyperbolic Newton Astrographs - Notionally the 16" and 20" meet my requirements when configured as f/2.4 systems optimized for the IMX461/411. Sure are pricey though! Also, while their client list looks impressive, these are so expensive that I don't think I've ever heard of one in use for amateurs. http://dreamscopes.c...-astro-list.htm

 

3) Dreamscopes f/x.x RC Astrographs - Nominally these are f/7.5 with a field flattener which brings them to f/8 and they would not meet my definition of a fast scope. However, Dreamscopes claims to be able to make these as fast as f/3.375! Given how expensive #2 are, I imagine a super fast RC is going to break the bank, but the RC design is sound. http://dreamscopes.com/pages/DRC.htm

 

4) ASA H400 (hyperbolic newton) - To my knowledge, ASA dropped the smaller versions and this is the only thing they make that qualifies. I've heard of a few minor quality issues on the smaller scopes. No idea since they sort of dropped the amateur market. Just as pricy as Dreamscopes. https://www.astrosys...scopes/asa-h400

 

There are a lot of manufacturers with interesting designs that are a far better value and practically good enough, but don't really meet the qualification of absolutely the best. E.g. Planewave CDKs, Officina Stellare RH/RiFAST, etc. I know that may upset some people, but there are optical interferometer tests out there showing these are good, but not the best. Under practical seeing conditions, they're generally good enough and as I said, far better trade of usable quality vs money. It's just not the question I'm asking.

 

I certainly haven't listed all the possible options. There's some rare designs out there with amazing theoretical performance (bring them on!). There's certainly other custom, high-end manufactures as well. I'll leave you with this link for some practical considerations for a fast astrograph: http://hbastro.com/F...iderations.html

 

So what's your perfect fast astrograph?

 

Edit: if you care, reading about the AP 305mm RH got me thinking about this.


Edited by LuxTerra, 05 December 2020 - 12:32 AM.


#2 Kevin Thurman

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Posted 05 December 2020 - 01:17 AM

https://telescopes.n...astrograph.html


Similar to the first one. Within reach for a very rich amateur. I also like that it's a bit smaller than most of your list, making it portable and easy to set up.



#3 whwang

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Posted 05 December 2020 - 02:04 AM

Your #1 seems discontinued, unfortunately.  TAK's Epsilon pretty much checks everything except for the 55mm image circle requirement.  TAK's FSQ (106 and 130, the latter is discontinued) with 645 reducer also meets every requirement you list, but the aperture is substantially smaller than the others.



#4 leviathan

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Posted 05 December 2020 - 02:18 AM

Scopes that may look like optically perfect on paper often come with mechanical difficulties in supporting these fast systems. For example number 4 in your list, ASA H400. I've used it in remote configuration and it often lost collimation and had other problems.

 

Even FSQ-106, which is IMO perhaps the closest to be ideal, has a stock focuser problems for some.



#5 akulapanam

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Posted 05 December 2020 - 02:31 AM

Depends on how much patience the user has.  Everything about fast scopes is difficult: collimation, back-focus, tilt, reflections in light path etc...  Optical perfection comes down to your willingness to invest the time required.  Good mechanics certainly help but ultimately its on you.

 

Tak 106 - challenging focuser, Borg looks like a possible alternative

Tak Epsilon - focuser + no tip/tilt adjustment + collimation

AP RH and OS RH - arcs (fixable but AP does require you remove backplate) + tip/tilt (collimation is pretty easy on this guy)

ASA - your right only makes the big one, way over priced, poor support, actually saw one in person though and it looked pretty nice.  Used smaller ASA scopes are fine with Alluna mirrors, questionable with Orion UK

Harmer Wynne - I love my OS one, heard mixed things on AGO (and won't make them), Tak CCAs look nice

Dreamscopes / TS ONTC - Dreamscope is a bit over priced but built like a tank, TS ONTC looks like a really nice option BUT collimation and tip/tilt will be issues

Hyperstar / RASA - I loved mine.  Tip / tilt is an issue though and Baader never released their tip/tilt adapter to my knowledge

 

If your going to just buy an RC (saw Dreamscopes mentioned in your post) or CDK personally I would go Planewave, OS, CFF, or Alluna before Dream


Edited by akulapanam, 05 December 2020 - 02:32 AM.

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#6 alpal

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Posted 05 December 2020 - 03:16 AM

All those that I looked at from the list in the opening post

had secondary mirrors that were far too large.

Example:

 

1407632521_Comparisonsecondaryobstructio

 

That is from the pdf document here:

http://s000.tinyuplo...876064397637171

 

If you check out the astro physics one f3.8 Astro-graph here:

https://www.astro-ph.../producthistory

the secondary obstruction is 49%.



#7 whwang

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Posted 05 December 2020 - 03:41 AM

If your going to just buy an RC (saw Dreamscopes mentioned in your post) or CDK personally I would go Planewave, OS, CFF, or Alluna before Dream


RC is hardly fast. If there is a good focal reducer to bring it to F5-F6, then it becomes somewhat manageable. Does any one know an RC reducer that satisfies the 55mm image circle requirement?

#8 alan.dang

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Posted 05 December 2020 - 03:44 AM

Obviously the TEC 300VT.

Valery Terebizh‘a other design is mentions here

https://www.osti.gov...ts/purl/1561833

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

https://www.baader-p...-telescope.html

Edited by alan.dang, 05 December 2020 - 03:46 AM.


#9 leviathan

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Posted 05 December 2020 - 06:47 AM

All those that I looked at from the list in the opening post

had secondary mirrors that were far too large.

Example:

 

1407632521_Comparisonsecondaryobstructio

 

That is from the pdf document here:

http://s000.tinyuplo...876064397637171

 

If you check out the astro physics one f3.8 Astro-graph here:

https://www.astro-ph.../producthistory

the secondary obstruction is 49%.

I guess nobody is using fast astrographs for visual observation.



#10 alpal

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Posted 05 December 2020 - 07:06 AM

I guess nobody is using fast astrographs for visual observation.

But you can't get something from nothing.

When a system destroys contrast it's lost forever - even to a camera.



#11 spokeshave

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Posted 05 December 2020 - 07:39 AM

But you can't get something from nothing.

When a system destroys contrast it's lost forever - even to a camera.

That's less of a problem for deep sky photography than it is for planetary. But you're right. Fast reflecting astrographs have large central obstructions. TANSTAAFL. It is definitely a tradeoff.

 

I've always wondered why nobody builds a Schmidt-Baker telescope. It can be made very fast, has a very flat field that can be quite large, and both mirrors are spherical. l guess having to have a corrector near the ROC of the primary makes for a scope that is probably too long for most peoples' tastes. Still, I think it would make a fine scope.

 

Tim


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#12 42itous1

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Posted 05 December 2020 - 10:01 AM

That's less of a problem for deep sky photography than it is for planetary. But you're right. Fast reflecting astrographs have large central obstructions. TANSTAAFL. It is definitely a tradeoff.

 

I've always wondered why nobody builds a Schmidt-Baker telescope. It can be made very fast, has a very flat field that can be quite large, and both mirrors are spherical. l guess having to have a corrector near the ROC of the primary makes for a scope that is probably too long for most peoples' tastes. Still, I think it would make a fine scope.

 

Tim

:)  TANSTAAFL by Milton Friedman--my favorite economist.


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#13 Midnight Dan

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Posted 05 December 2020 - 10:06 AM

If you were building a cost is no object fast astrograph (I don't have the funds to do this, I'm just curious), what would that telescope look like? 

Not quite sure what you're looking for here.  Your initial question asks about building a scope, but then you list commercially available options.  

 

My perfect custom astrograph would be an 8" f/4 Petzval refractor.  Although personally, I think f/5 is fast enough.  Getting down into sub-f/4 territory brings a whole host of other issues as people have pointed out.  But a refractor has no central obstruction, and no worries about collimation.

 

I think it's interesting that this kind of scope does not appear to exist. I realize it would be extremely expensive.  But that never stopped companies from offering these kinds of extreme examples.  I've seen camera lenses that cost $50K or more.  Perhaps there's some physics that limits the capability to create larger refractors with fast f/ratios?  I've seen 8" achromats available, but not Apo's or Petzvals.  

 

-Dan


Edited by Midnight Dan, 05 December 2020 - 10:07 AM.


#14 spokeshave

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Posted 05 December 2020 - 10:34 AM

smile.gif  TANSTAAFL by Milton Friedman--my favorite economist.

I'm not sure who originated the acronym, but Heinlein popularized it in "The Moon is a Harsh Mistress".

 

Tim


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#15 Gregory

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Posted 05 December 2020 - 10:42 AM

Tim wrote:

 

 

I've always wondered why nobody builds a Schmidt-Baker telescope. It can be made very fast, has a very flat field that can be quite large, and both mirrors are spherical. l guess having to have a corrector near the ROC of the primary makes for a scope that is probably too long for most peoples' tastes. Still, I think it would make a fine scope.

 

At least one company is inviting inquiries:

 

http://www.knaeble-e...elescope-2-1-1/

 

 

Gregory



#16 akulapanam

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Posted 05 December 2020 - 11:06 AM

Tim wrote:

At least one company is inviting inquiries:

http://www.knaeble-e...elescope-2-1-1/


Gregory


They are actually owned by OS at this point. No intention to build that.

#17 LuxTerra

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Posted 05 December 2020 - 11:11 AM

https://telescopes.n...astrograph.html


Similar to the first one. Within reach for a very rich amateur. I also like that it's a bit smaller than most of your list, making it portable and easy to set up.

Nothing says it must be large. I simply wanted to cap it to avoid silly answers to buy the ESOs ELT! Portability is a very good requirement for many people (myself included, I have to travel to do anything buy solar/EAA).

 

Officina Stellare is a good scope, but my limited reproach looking around for actual testing data on just how well they build their scopes indicated that while they're generally better than the mass produced lines, they really are just held to good enough. This is just all for fun, so I'm going to be lazy here and post a link to a OS scope report, not the same design, to give you an idea. 

 

Take this with a huge gain of salt. It's a sample of 1.

https://airylab.com/...015-12001-a.pdf



#18 WadeH237

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Posted 05 December 2020 - 11:12 AM

All those that I looked at from the list in the opening post had secondary mirrors that were far too large.

That is not a coincidence.  You cannot have a fast reflecting telescope without a large secondary.  Based on which forum we are in, and the requirements by the OP, it's pretty clear that he's looking for a deep sky imaging scope, not a planetary one.  The requirements are dramatically different.

 

Also, if you think that a scope with a 31% obstruction cannot produce exceptional planetary images, you should look up Damian Peach or Christopher Go.  They've both routinely produced "Hubble-like" planetary images, mostly with C14's, which have a large obstruction.


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#19 Gregory

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Posted 05 December 2020 - 11:19 AM

AG Optical continues to advertise their f/3.9 Harmer-Wynne

on their web site:

 

http://agoptical.com...g-harmer-wynne/

 

However, it does have a significant secondary obstruction.

 

Gregory


Edited by Gregory, 05 December 2020 - 11:36 AM.

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#20 LuxTerra

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Posted 05 December 2020 - 11:19 AM

Your #1 seems discontinued, unfortunately.  TAK's Epsilon pretty much checks everything except for the 55mm image circle requirement.  TAK's FSQ (106 and 130, the latter is discontinued) with 645 reducer also meets every requirement you list, but the aperture is substantially smaller than the others.

TAKs optics are good, but objectively perhaps not as good as I'd want for this hypothetical? Discontinued doesn't bother me since this is just meant to be a fun hypothetical. Here's an TAK FSQ106 interferometer report and while it's not bad, it's definitely not "perfect."

 

Again, sample size of 2, but I'm being lazy and just using this one site. You can find reports elsewhere. The second unit looks a lot better than the first; spotty QC/QA?

https://www.airylab....014-20002-a.pdf

http://airylab.net/c... 2014-41002.pdf



#21 LuxTerra

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Posted 05 December 2020 - 11:22 AM

Scopes that may look like optically perfect on paper often come with mechanical difficulties in supporting these fast systems. For example number 4 in your list, ASA H400. I've used it in remote configuration and it often lost collimation and had other problems.

 

Even FSQ-106, which is IMO perhaps the closest to be ideal, has a stock focuser problems for some.

That's a great point. The scope is a system and the best mirror ever made in a poor mechanical design is useless. That's one thing Dreamscopes harps a lot on in their marketing. Now, do they do any better? I don't know. Never seen one.



#22 akulapanam

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Posted 05 December 2020 - 11:30 AM

But you can't get something from nothing.
When a system destroys contrast it's lost forever - even to a camera.

It really has zero impact for deep sky. The midline transfer function is really only impacted in the low frequency area. This is a problem for planetary on things like bands on Saturn. On deep sky any contrast loss in extended objects is swamped by the contrast enhancement in post processing. Take a look at Astrobin or professional telescope images where the obstruction can be up to 60%.

https://www.telescop...obstruction.htm

Edited by akulapanam, 05 December 2020 - 11:33 AM.


#23 LuxTerra

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Posted 05 December 2020 - 11:32 AM

Depends on how much patience the user has.  Everything about fast scopes is difficult: collimation, back-focus, tilt, reflections in light path etc...  Optical perfection comes down to your willingness to invest the time required.  Good mechanics certainly help but ultimately its on you.

 

Tak 106 - challenging focuser, Borg looks like a possible alternative

Tak Epsilon - focuser + no tip/tilt adjustment + collimation

AP RH and OS RH - arcs (fixable but AP does require you remove backplate) + tip/tilt (collimation is pretty easy on this guy)

ASA - your right only makes the big one, way over priced, poor support, actually saw one in person though and it looked pretty nice.  Used smaller ASA scopes are fine with Alluna mirrors, questionable with Orion UK

Harmer Wynne - I love my OS one, heard mixed things on AGO (and won't make them), Tak CCAs look nice

Dreamscopes / TS ONTC - Dreamscope is a bit over priced but built like a tank, TS ONTC looks like a really nice option BUT collimation and tip/tilt will be issues

Hyperstar / RASA - I loved mine.  Tip / tilt is an issue though and Baader never released their tip/tilt adapter to my knowledge

 

If your going to just buy an RC (saw Dreamscopes mentioned in your post) or CDK personally I would go Planewave, OS, CFF, or Alluna before Dream

Great info. Just to be clear, this is a fun hypothetical. I would say, assume, that if the user spent this kind of money, they'r is willing to put in the time to configure the system correctly. See rule #5. However, anything that's beyond merely configuring, and that's a bit of a grey line, would be a mechanical issue that shouldn't exist in a "perfect" system. Calibration difficulty is a good metric.

 

I mentioned Planewave and OS in my post, and to be clear, I'm not claiming anything is bad in practical terms. I'm simply saying that the optics, when measured objectively, are not top shelf. They generally are good enough for practical purposes though.

 

I posted about an OS scope earlier. Airylabs doesn't have a link up, so I did a quick Google for a report I recalled seeing a while ago and found it for a Planewave 12.5": http://r2.astro-fore...kham-astrograph That's a rough mirror...



#24 Gregory

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Posted 05 December 2020 - 11:33 AM

And, not to forget Telescopi Italiani's astrograph line:

 

https://www.italiant...-65-astrograph/

 

Gregory

 



#25 LuxTerra

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Posted 05 December 2020 - 11:35 AM

All those that I looked at from the list in the opening post

had secondary mirrors that were far too large.

Example:

 

1407632521_Comparisonsecondaryobstructio

 

That is from the pdf document here:

http://s000.tinyuplo...876064397637171

 

If you check out the astro physics one f3.8 Astro-graph here:

https://www.astro-ph.../producthistory

the secondary obstruction is 49%.

My list wasn't meant to be complete. Just to get things started. Absolutely, a large secondary obstruction (a common feature of fast astrographs), acts a a low pass filter and cuts out high frequency content. If you're doing widefield that's less of an issue. As you show, it's rather bad for planetary. No one scope can do it all, there's always a compromise.




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