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What key features do you look for when purchasing a refractor?

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#1 thelosttrek

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Posted 14 November 2019 - 12:30 AM

I'm just getting into DSO astrophotography (AP) and I'm trying to figure how you all go about purchasing a new scope. Based on my novice research, I've come up with the following list. The first category is the standard features that you look at based on how you want to use the scope. The second category is the features that set scopes apart from others, which in most cases might help decide about the quality of the scope you're looking for. I am personally looking for two types of scopes up front based on the standard features. One scope to capture larger DSO and another to capture smaller DSO.   

 

What are some key features that you look for when buying a refractor telescope for AP? Are there others besides what I listed below? Finally, how do you rate these features? Meaning, what makes that feature great, good, ok, and bad?

 

Standard features:

  • Focal Ratio (speed of the telescope; the lower the number the faster you can collect data, but you sacrifice magnification)
  • Aperture (diameter of the objective lens; larger numbers provide better resolution, but less data can be collected)
  • Focal Length (longer focal length the higher the magnification)
  • Customer Service
  • Price

Features that set scopes apart from others:

  • Sharpness (are some scopes softer than others?)
  • Glass Type (Ohara FPL-53, Hoya FCD100, fluorite, any others? Is there one better than others for AP?)
  • Optical Design (triplet, doublet, petzval, apochromatic, achromatic. From my understanding, apochromatic triplets are the way to go for AP. Is that correct?)
  • ED (Extra-low dispersion) and SD (special dispersion) (both used in the removal of chromatic aberration, but is there more to this?)
  • Strehl Ratio (not exactly sure how to find this on the specifications or what it does, but I've seen it mentioned)
  • Focuser (Are stock focusers ok? I've heard the term feather touch and the company Moonlite. What should we be looking for and can they be upgraded?)
  • Brand (do some brands have an edge over others? If so, how does the community grade each brand?)
  • Manufacturing (does it matter which country they are manufactured in? I've heard negative comments about China, but it appears a lot of them are made there under a company called Synta. How are manufacturers graded?) 
  • Premium line vs entry level (I've seen some that state premium, but it's difficult to tell what makes them premium other than a premium price tag)
  • Build quality (how do the different brands compare?)
  • Cooling time (I wasn't aware this was a thing)

As a landscape photographer, this is a lot more to think about than simply purchasing a wide angle and telephoto. I appreciate any advice you might have. Thanks!


Edited by thelosttrek, 14 November 2019 - 01:57 AM.


#2 SeattleScott

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Posted 14 November 2019 - 01:32 AM

No one really cares about cooldown for AP with a refractor. It will reach ambient half an hour or more before you finish setting up everything and alignment. Most refractors have temperature compensated cells that maintain focus as temperatures drop.

Country of origin doesn’t really matter, other than personal preference. Price is a much better indicator of quality.

Strehl is important, but rarely published, so you buy a brand known for high quality, or you buy what you can afford and hope for the best. I think the obsession with strehl is more from visual users but it doesn’t hurt AP.

Focuser: the opening (2.5”, 3”, 3.7”, etc.) can be an indicator of how well the focuser will accommodate full frame cameras without vignetting. F ratio plays a part too, the lower the F ratio, the larger you want the opening. Load is another factor. How much load is the focuser rated for? Intended use has to be considered also. Are you going to stick a Mallincam on it? Or a CMOS camera with eight position filter wheel?

Glass type-you mentioned the good ones.

SD is really just a marketing term, but it implies better correction than a standard ED. Not all vendors use the term. Vixen likes to be secretive about their glass types so they use the SD term to say they use excellent glass, without actually saying what glass. With their reputation they can get away with it.

Brand is more personal preference. Again, to me, price is really a better indicator as some brands, like Vixen and SV, are known for having higher end and lower end stuff. And ES and WO aren’t normally thought of as top tier brands, but they each now have a large, very expensive flagship Apo.

Generally for AP people look for mid to low F ratio, like F5.5 to F7. A reducer can help. For example a Vixen Apo is typically F7.7, a bit slow for AP, but the reducer takes it down to F5.4.

Of course, all that being said, for many DSO imagers, purchasing the refractor is almost an afterthought. The mount is really the critical part.

Scott

Edited by SeattleScott, 14 November 2019 - 01:34 AM.

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#3 DLuders

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Posted 14 November 2019 - 07:31 AM

We don't know if you already have a decent Mount on which to place the Refractor.  One can get the finest Refractor telescope, but if it's on a wobbly mount, your astrophotography will be affected.   undecided.gif   Are you looking to get a Mount too, as a standalone purchase or as part of a package deal with the Refractor telescope?


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#4 Ishtim

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Posted 14 November 2019 - 07:52 AM

  • Flat field across entire image circle for the intended sensor size
  • Option to reduce or extend focal length (and maintain sufficient sized flat image circle when reduced)
  • Weight (and sufficient mount as DLuders mentioned above).  Are you looking into portable or "permanently" mounted scopes? 
  • Availability.  Some offerings may take considerable time (years) to make it into your hands.

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#5 zakry3323

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Posted 14 November 2019 - 09:56 AM

 

  • Option to reduce or extend focal length (and maintain sufficient sized flat image circle when reduced)

 

This is a priority for me as well. It's how one can define the versatility of the refractor. I don't have money to spare on anything extraneous in this hobby- everything must be a great value, and versatility adds a great amount of value to a refractor.  



#6 bobzeq25

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Posted 14 November 2019 - 11:14 AM

The most important part of an imaging setup is the mount.  The best scope is utterly wasted if the mount can't keep it precisely on target for long exposures.  Choose the mount first.  Anything less than 50% of your total budget is very likely a mistake.

 

Learning imaging is different than doing imaging.  The key features in a scope for learning imaging on are:

 

Short focal length.  600mm max, 480 is better.

 

Light weight.  10 pounds max, 5 is better.

 

Speed.  F7 max, F6 or less is better.

 

A common mistake is to try to buy your "forever" scope up front.  You just don't know enough about imaging (it's very complicated) to make the best decision for you, and others choices may not be yours.  A $469 AT72EDII is fine scope to learn on.  You'll make a much better choice with some imaging knowledge/experience under your belt.


Edited by bobzeq25, 14 November 2019 - 11:16 AM.

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#7 thelosttrek

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Posted 14 November 2019 - 01:37 PM

Hey all,

 

Thank you very much for your input. I understand the importance of a good mount and will put a majority of my budget into that. Over the past year I've been looking at the Sky-Watcher EQ6-R Pro, but I'm still in the process of researching other options. I'm open to suggestions. My goal is to buy the mount separately and not as a package. I guess I was wanting to know more about scopes first because they seem more complex than a mount, so I was trying to understand all of the terms surrounding the key features I mentioned above. But to everyone's point, I am on board with getting a solid mount. smile.gif

 

Also, good point about "doing" vs "learning." Thanks for pointing out what you consider a short focal length. Again, open to any and all suggestions. I might just post the setup I was thinking of getting and get overall feedback on that. That sounds like it might be the better approach so you all have more information. Thank you!



#8 YAOG

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Posted 14 November 2019 - 01:44 PM

I'm just getting into DSO astrophotography (AP) and I'm trying to figure how you all go about purchasing a new scope. Based on my novice research, I've come up with the following list. The first category is the standard features that you look at based on how you want to use the scope. The second category is the features that set scopes apart from others, which in most cases might help decide about the quality of the scope you're looking for. I am personally looking for two types of scopes up front based on the standard features. One scope to capture larger DSO and another to capture smaller DSO.   

 

What are some key features that you look for when buying a refractor telescope for AP? Are there others besides what I listed below? Finally, how do you rate these features? Meaning, what makes that feature great, good, ok, and bad?

 

Standard features:

  • Focal Ratio (speed of the telescope; the lower the number the faster you can collect data, but you sacrifice magnification)
  • Aperture (diameter of the objective lens; larger numbers provide better resolution, but less data can be collected)
  • Focal Length (longer focal length the higher the magnification)
  • Customer Service
  • Price

Features that set scopes apart from others:

  • Sharpness (are some scopes softer than others?)
  • Glass Type (Ohara FPL-53, Hoya FCD100, fluorite, any others? Is there one better than others for AP?)
  • Optical Design (triplet, doublet, petzval, apochromatic, achromatic. From my understanding, apochromatic triplets are the way to go for AP. Is that correct?)
  • ED (Extra-low dispersion) and SD (special dispersion) (both used in the removal of chromatic aberration, but is there more to this?)
  • Strehl Ratio (not exactly sure how to find this on the specifications or what it does, but I've seen it mentioned)
  • Focuser (Are stock focusers ok? I've heard the term feather touch and the company Moonlite. What should we be looking for and can they be upgraded?)
  • Brand (do some brands have an edge over others? If so, how does the community grade each brand?)
  • Manufacturing (does it matter which country they are manufactured in? I've heard negative comments about China, but it appears a lot of them are made there under a company called Synta. How are manufacturers graded?) 
  • Premium line vs entry level (I've seen some that state premium, but it's difficult to tell what makes them premium other than a premium price tag)
  • Build quality (how do the different brands compare?)
  • Cooling time (I wasn't aware this was a thing)

As a landscape photographer, this is a lot more to think about than simply purchasing a wide angle and telephoto. I appreciate any advice you might have. Thanks!

As a professional photographer it really is just like buying lenses and an appropriately sized sensor but on steroids. I used to shoot motorsports, equestrian, American field sports, architecture and products. This required an arsenal of glass and bodies because they were all so different. This is the same thing you will encounter with astrophotography of DSOs with the wide range of sizes, brightness and very different structures.  

 

There is an intimate relationship between sensor size, photosite (pixel) size, objective size and focal ratio. Buy a couple of books and read up before you buy anything, yes this will cost you a couple hundie but if this puts you off imaging is not for you. Trust me it is a lot cheaper buying books and being well educated on the topic before you buy than after buying stuff you don't need. Her are some of my favorites: Astrophotography by Legault, Deep Sky Imaging Primer by Bracken and Astrophotography Manual by Woodhouse. If you plan to use a DSLR there are a couple of okay books for starters that are older but generally still useful, Budget Astrophotography by Hall and DSLR Astrophotography by Covington. 

 

A telescope is the last thing you need to buy first. 


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#9 NikhilJoshi

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Posted 14 November 2019 - 01:48 PM

If you’re just getting started you’ll be much happier with a smaller, wide-field refractor in the 60mm-80mm range. Lots of big DSOs and forgiving in terms of tracking.

The big thing is finding the right mount. Astrophotography needs good tracking and that means a reliable and predictable mount. Unless you are thinking about a premium mount, use the 50% rules - for AP don’t load a mount being 50% of its advertised capacity. Most people would say allocate 50% of your budget to the mount.

If you are thinking of a bigger optic down the road I’d go with a bigger mount now so you can learn its behavior before moving to longer and more challenging focal lengths. Over-mounting will make you happier in the long run.


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#10 thelosttrek

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Posted 14 November 2019 - 01:50 PM

As a professional photographer it really is just like buying lenses and an appropriately sized sensor but on steroids. I used to shoot motorsports, equestrian, American field sports, architecture and products. This required an arsenal of glass and bodies because they were all so different. This is the same thing you will encounter with astrophotography of DSOs with the wide range of sizes, brightness and very different structures.  

 

There is an intimate relationship between sensor size, photosite (pixel) size, objective size and focal ratio. Buy a couple of books and read up before you buy anything, yes this will cost you a couple hundie but if this puts you off imaging is not for you. Trust me it is a lot cheaper buying books and being well educated on the topic before you buy than after buying stuff you don't need. Her are some of my favorites: Astrophotography by Legault, Deep Sky Imaging Primer by Bracken and Astrophotography Manual by Woodhouse. If you plan to use a DSLR there are a couple of okay books for starters that are older but generally still useful, Budget Astrophotography by Hall and DSLR Astrophotography by Covington. 

 

A telescope is the last thing you need to buy first. 

Thanks for the advice and book recommendations. I will look into it.  



#11 thelosttrek

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Posted 14 November 2019 - 04:28 PM

If you’re just getting started you’ll be much happier with a smaller, wide-field refractor in the 60mm-80mm range. Lots of big DSOs and forgiving in terms of tracking.

The big thing is finding the right mount. Astrophotography needs good tracking and that means a reliable and predictable mount. Unless you are thinking about a premium mount, use the 50% rules - for AP don’t load a mount being 50% of its advertised capacity. Most people would say allocate 50% of your budget to the mount.

If you are thinking of a bigger optic down the road I’d go with a bigger mount now so you can learn its behavior before moving to longer and more challenging focal lengths. Over-mounting will make you happier in the long run.


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Thank you for the response and the note about the 50% rule. What would you consider a premium mount? I'm currently looking at getting a Sky-Watcher EQ6-R Pro. Does that fall within the premium range?



#12 MrRoberts

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Posted 14 November 2019 - 04:59 PM

Although I am an Ioptron user, I have heard many good things about the EQ6-R Pro. I suspect it would work very well with anything in 102mm and under range.

I cut my ap teeth with an ES 102/FCD100 on a CEM25P (I was definitely under mounted for this scope, but it was what I had at that time, and it performed rather well).

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#13 MalVeauX

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Posted 14 November 2019 - 05:11 PM

1) Can I afford it?

2) Can I mount it?

 

Very best,


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#14 Nippon

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Posted 14 November 2019 - 06:59 PM

Although I am an Ioptron user, I have heard many good things about the EQ6-R Pro. I suspect it would work very well with anything in 102mm and under range.

I cut my ap teeth with an ES 102/FCD100 on a CEM25P (I was definitely under mounted for this scope, but it was what I had at that time, and it performed rather well).

Sorry to sidetrack the topic but is that the shell of a voting booth you are using for your laptop?



#15 NikhilJoshi

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Posted 14 November 2019 - 08:11 PM

Thank you for the response and the note about the 50% rule. What would you consider a premium mount? I'm currently looking at getting a Sky-Watcher EQ6-R Pro. Does that fall within the premium range?


I think most folks would say brands like Astro-Physics, Paramounts, and 10Micron are premium. You’re talking about mounts starting at $7,000 new. For mounts in this class you can believe the stated capacity for imaging gear.


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#16 dron2015

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Posted 14 November 2019 - 08:32 PM

My two cents:

 

1. From where you will be shooting? If you have you observatory - ignore this moment, if you do not have... well it does take time to assemble and disassemble the set up for each session. I put my scope on the wheels. It helps a lot!

2. I also Just recently started - well, I already spend perhaps 2 times more money and time than inially planned to get ready. AP - it is constant loop of figuring out that you need something else...

3. Stability and precision of the mount is paramount for AP.

4. Quality of glass - the most important for the telescope. Look for good abbe ratio. Good glasses are Fpl53, ok-4, etc based triplet. manufacturer of lens cell is also important no name from China vs stellarvue.

5. Focuser - it should be sturdy and take the charge of the load weight but flexible to allow reducer flattened and perhaps automation. Cheap Chinese focuser, however looks nice, might require posi drive for automation which is 5x more expensive than said focuser... if you decide to automate focuser later. You have mentioned FeatherTouch and Moonlite - any perfect for AP. Anyway, there several types of them and should consider carefully which one fits best for you reducer/flattened, etc

6. Aperture is less important 4,5,6 inches does not perhaps that much matter for a beginner vs glass quality... large but cheap lens cell is inferior to good glass but smaller aperture in terms of picture quality. for a person who already tested the water - the large aperture - the more contrast and details you will have. Even 80 mm refractor is considered just great for AP. Check for instance LOMO 80 mm triplets - legendary,  impossible to buy. but not produced any more frown.gif today somebody just sold it after a couple of hours after posting. No doubts whoever bought it - did for AP.

7. Also budget a lot of funds for good camera and best filters you can buy, especially if you live in a city.

 

Best Luck!

 

best,

Andrey


Edited by dron2015, 14 November 2019 - 08:41 PM.

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#17 YAOG

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Posted 14 November 2019 - 11:36 PM

Thank you for the response and the note about the 50% rule. What would you consider a premium mount? I'm currently looking at getting a Sky-Watcher EQ6-R Pro. Does that fall within the premium range?

Most of the popular mounts under about $2,500 or so fall in the 50% of rated payload when used for imaging. Based on web posts the EQ6-R Pro is a 50% mount. There are exceptions to this guideline, the new iOptron CEM40 and GEM45 are both specified with a PE +/- 7 arcseconds but don't expect to see this in the real world but they are better than most of the other consumer mounts within the 40-45 pound payload class. The iOptron CEM25P is also a good low cost entry level imaging mount. Buy the 2" tripod version and you will have a decent small scope imaging mount up to  maybe 20 pounds or so. This is not a bad way to get started, just add a decent fast 80mm f/6 FPL-53 or FCD100 triplet apo with a field flattener and say one of the Sony rear illuminated 183 sensor cameras and you will have excellent resolution out of an 80mm scope.    



#18 bobzeq25

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Posted 14 November 2019 - 11:44 PM

 the new iOptron CEM40 and GEM45 are both specified with a PE +/- 7 arcseconds but don't expect to see this in the real world  

My CEM60 was specced at +/- 5 and did +/- 4, new.  It's a bit worse now, probably needs some adjustment.  But it guides out well, so I'm not motivated.


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#19 dron2015

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Posted 15 November 2019 - 12:52 AM

Gents, let's do not start yet again another 'Holy War' re 'premium' vs' mass produced' or 'guided' vs 'EC'.

 

We have been asked about a refractor after all.

 

Just a general rules

1. The larger diameter of refractor lens 4 - >6- >10 - >? the heavier it is. Light capturing is proportion to r^2, while its weight is proportional to r^3. in other words, Wight of refractor goes faster than it diameter. Price of a good refrctor goes exponentially with its diameter.

2. The heavier the scope, the more capable mount you need.

3. The longer focus length of the refractor - the more and more precise mount you need! especially if you are going to shoot in narrow bands - exposure times 10-30mins are regular there. And your objective is restraint all deviations in one pixel size. It becomes quite arduous task with a long focus...

 

Combining 2 and 3 - price of a mount can be at least comparable to than of the scope frown.gif or much more frown.gif

 

thelosttrek, you said that you are photographer - perhaps you have already have good lens like 300-400mm - perhaps this lens and your current DSLR is a good start to test the water - all you need - just a mount! After all DSO objects can be large - Andromeda is very large!  Of course, uif you are target planets - it is a different story   -perhaps some mirror based system would be better than a refractor. 

 

If I would start from the beginning, I would probably prefer to have smaller refractor and save some monies and time. Also, this system would serve better in going out to dark sites. And it would be easy to re-sell it.

 

Best,

Andrey


Edited by dron2015, 15 November 2019 - 01:13 AM.


#20 ezwheels

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Posted 15 November 2019 - 01:08 AM

For AP, I stay away from doublets. After that:

 

1) Focal length (Probably start short. Big refractors are expense and more challenging to mount)

1) Aperture (probably start small. again big refractors are expensive and more challenging to mount)

2) Size of focuser (bigger is better)

1) Back focus length should be high on your radar as well. (The more the merrier. There are inevitably going to be configurations in your imaging train that this critical distance will rear its head.)

1) Available Flatteners (preferably native)

1) Available reducers (preferably native)

2) Consistency in manufacturing

3) Availability (I will wait to get what I want if it checks off all the boxes)

 

And yes my numbering is funny, but basically at this point I am looking at very specific criteria based on past experiences. After these criteria are met, you may or may not have a few options from different manufacturers at a desired FL. At this point check reviews and A-Bin for examples.


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#21 thelosttrek

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Posted 15 November 2019 - 01:49 PM

thelosttrek, you said that you are photographer - perhaps you have already have good lens like 300-400mm - perhaps this lens and your current DSLR is a good start to test the water - all you need - just a mount! After all DSO objects can be large - Andromeda is very large!  Of course, uif you are target planets - it is a different story   -perhaps some mirror based system would be better than a refractor. 

 

If I would start from the beginning, I would probably prefer to have smaller refractor and save some monies and time. Also, this system would serve better in going out to dark sites. And it would be easy to re-sell it.

 

Best,

Andrey

Hi Andrey, thanks for the reply. I currently have a 100-400mm lens with a 1.4x extender. I can use this on my full frame (Canon 5DMIV) or cropped sensor (Canon 7DMII) and have experimented with both over the past year. My range is anywhere from 16mm for super wide angle all the way to 400mm (full frame dslr), 560mm (full frame dslr with 1.4 extender), or 896mm (cropped sensor dslr with 1.4 extender). The extender adds two extra stops, which adds time to my exposure. My mount is the iOptron Skyguider Pro, which is great for wide field, but gets trickier, and more sensitive, as I add focal length. I have had success capturing Orion and I'm pleased with the wide field Milky Way shots I can get in combination with a landscape shot. However, I can see image degradation when using the extender or the cropped sensor. And the 5DIV isn't all that great for quality in low light.

 

With all of that said, I do like using that setup, but would like to increase the quality of my shots with a more suitable upgrade. I'm also seeing issues with star trails with my current setup even with a really good polar alignment when I go beyond 90 seconds. The iOptron seems to easily get knocked off of alignment when positioning the camera as well. Anyways, not to get off topic, but I thought I would describe why I'm looking to upgrade.


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#22 thelosttrek

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Posted 15 November 2019 - 01:51 PM

For AP, I stay away from doublets. After that:

 

1) Focal length (Probably start short. Big refractors are expense and more challenging to mount)

1) Aperture (probably start small. again big refractors are expensive and more challenging to mount)

2) Size of focuser (bigger is better)

1) Back focus length should be high on your radar as well. (The more the merrier. There are inevitably going to be configurations in your imaging train that this critical distance will rear its head.)

1) Available Flatteners (preferably native)

1) Available reducers (preferably native)

2) Consistency in manufacturing

3) Availability (I will wait to get what I want if it checks off all the boxes)

 

And yes my numbering is funny, but basically at this point I am looking at very specific criteria based on past experiences. After these criteria are met, you may or may not have a few options from different manufacturers at a desired FL. At this point check reviews and A-Bin for examples.

Good to know about the back focus and available flatteners/reducers. I'll keep that in mind. Thank you!



#23 dron2015

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Posted 15 November 2019 - 05:00 PM

. I currently have a 100-400mm lens with a 1.4x extender. I can use this on my full frame (Canon 5DMIV) or cropped sensor (Canon 7DMII) and have experimented with both over the past year. My range is anywhere from 16mm for super wide angle all the way to 400mm (full frame dslr), 560mm (full frame dslr with 1.4 extender), or 896mm (cropped sensor dslr with 1.4 extender).

I would say that you have quite good experience. This changes actually everything.  I would recommend to look for

1) scope: 130mm+ aperture of the good triplet scopes such as Takahashi, TEC, Stellarvue, AMP (LZOS lens), TMB (if you luck to find one), Astro Physics. DO not run for large FL - 2000+mm - perhaps mirror based systems would be better to start with. refractors in this segment exist but will cost a fortune. if you are buying pre-owned refractor - my advise would be to check collimation.  Canon lenses are well collimated and generally refractors also good at keeping the collimation - but it is better to check it.

2) Focuser: most probably the focuser that will come with these scopes will be great for AP.  FeatherTouch and Moonlite that you have mentioned are very good ones.

3)Reducer/flattener: shall be, as ezwheels recommended, native - specially designed for the selected scope. Note! that the best way is to submerge the reducer into focuser - so you will have min length of your optical train and will give you a lot of flexibility - it will be easier to focus. Several Focusers and reducers allow doing so whole some are not.

4) Mount: this is the most perhaps most important... talk to the Mount forum. There are a lot of good mounts and here is never ending holy war - what mount is the best :) so - selecting the mount w/ help of CN is going to be fun! do not be cheap here :(

5) Other things:

5.1) Polar alignment. Even the best Mount will not help if you polar alignment is bad. I use qhyccd polemaster - but there are other solutions as well.

5.2. DSLR is good but I personally switched from DSLR to astro camera. talk to several people. there all ranges - from working horse ZWO and up to Finger lake instruments.  it should coupe with the scope you select.  Also, filters can cost a fortune, but if you live in an urban area - it is required to have good filters other wise you won't be able to use your costly system... only going to dark places...

5.3. consider setting up permanent pier and shed in some future.

 

Best,

Andrey


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#24 LLEEGE

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Posted 15 November 2019 - 10:03 PM

The key feature I look for is: Made in USA.
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#25 Suavi

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Posted 16 November 2019 - 12:31 AM

What key features do you look for when purchasing a refractor? (for DSO astrophotography).

 

#1 - aiming for about 1-1.3 arcseconds per pixel (matching the focal length with the camera)

#2 - a relatively fast focal ratio and a large well-corrected imaging circle that covers more than the intended sensor, for a couple of reasons, one being decent star shapes for the OAG's guide camera

#3 - robust quality motorised focuser designed for astroimaging, ideally FLI, Optec or Nitecrawler

#4 - sufficient backfocus for OAG +FW +camera (+AO - optional)

#5 - must be a triplet with a quality corrector or Petzval


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