Design or Optics quality?

[TareqPhoto]

Hi,

 

I want to know when buying a refractor, which is more important to have:

 

1. A triplet but with ED optics

2. A doublet but with Fluorite or FPL-53 optics

 

So if can't afford both or the type of scope i want can't be in both of above and only one of above, what will be the choice?

 

All scopes are well made and nice quality manufactured so no one will come to say that this build quality is better than the other one, so assume all are with excellent build quality.

[John]

Are you primarily interested in imaging, visual observing, or both ?

Actually the build quality does vary between the refractor brands / price ranges.

 

[John]

Ooops ! - I have just noticed that you have posted this in the imaging section so that answers my question !

I'm just a visual observer so I will duck out and let the imagers advise you

[TareqPhoto]

1 minute ago, John said:

Ooops ! - I have just noticed that you have posted this in the imaging section so that answers my question !

I'm just a visual observer so I will duck out and let the imagers advise you

It is ok, i want for imaging, don't mind for visual, so i can say both, but if i must choose then it is imaging all the way, thanks anyway

[vlaiv]

For imaging?

Design - go for triplet.

Optical quality is really not that important when discussing imaging scopes. What is important is color correction. Often, for imaging purposes (not the same for visual) - triplet with "lower quality glass" will give better color correction than high quality ED doublet.

There are a lot of imaging scopes that are not diffraction limited and would pass as rather poor optically. Optical quality is only important when doing planetary imaging, and there you want your telescope to be sharp, but for long exposure - atmosphere just dominates and if telescope is not sharp - no one will be able to tell at resolutions that we use for long exposure.

Take for example this scope (and line of similar scopes from Altair Astro or StellarVue):

https://www.teleskop-express.de/shop/product_info.php/info/p9868_TS-Optics-Doublet-SD-Apo-102-mm-f-7---FPL53---Lanthan-Objective.html

TS even markets this as being part of their PhotoLine series of scopes.

I've seen couple of images made with this scope - and it has very pronounced blue halo around stars. By the specs it should be excellent scope - FPL53 and Lanthanum glass. People that use it as visual instrument are very happy - no trace of CA, but for imaging purposes - it shows CA.

Now take this scope:

https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html

it has fpl51 glass, but it is triplet. It is proper imaging scope with no issues with CA.

 

[TareqPhoto]

1 hour ago, vlaiv said:

For imaging?

Design - go for triplet.

Optical quality is really not that important when discussing imaging scopes. What is important is color correction. Often, for imaging purposes (not the same for visual) - triplet with "lower quality glass" will give better color correction than high quality ED doublet.

There are a lot of imaging scopes that are not diffraction limited and would pass as rather poor optically. Optical quality is only important when doing planetary imaging, and there you want your telescope to be sharp, but for long exposure - atmosphere just dominates and if telescope is not sharp - no one will be able to tell at resolutions that we use for long exposure.

Take for example this scope (and line of similar scopes from Altair Astro or StellarVue):

https://www.teleskop-express.de/shop/product_info.php/info/p9868_TS-Optics-Doublet-SD-Apo-102-mm-f-7---FPL53---Lanthan-Objective.html

TS even markets this as being part of their PhotoLine series of scopes.

I've seen couple of images made with this scope - and it has very pronounced blue halo around stars. By the specs it should be excellent scope - FPL53 and Lanthanum glass. People that use it as visual instrument are very happy - no trace of CA, but for imaging purposes - it shows CA.

Now take this scope:

https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html

it has fpl51 glass, but it is triplet. It is proper imaging scope with no issues with CA.

 

So, for example, will you choose this one

https://www.teleskop-express.de/shop/product_info.php/info/p12929_TS-Optics-CF-APO-70-mm-f-6-FPL55-Triplet-with-Certificate.html

Or this one

https://www.firstlightoptics.com/askar-telescopes/askar-fra400-f5-6-quintuplet-apo-astrograph.html

Over this one?

https://www.firstlightoptics.com/takahashi-fs-60-series-refractor-telescopes/takahashi-fs-60cb-f5-9-doublet-fluorite-apo-refractor.html

 

Waiting your answer

[vlaiv]

Out of the three offered I would certainly go for first one if I was in market for 72mm refractor - but I would first look to see if there are people already using it and what is their experience like.

[TareqPhoto]

10 minutes ago, vlaiv said:

Out of the three offered I would certainly go for first one if I was in market for 72mm refractor - but I would first look to see if there are people already using it and what is their experience like.

Unfortunately, there is no one yet, it is a new product, with FPL-55, and the second one is almost new one just been out almost 1 year or so and not so so many reviews but it is not bad, not talking about the last one.

[vlaiv]

8 minutes ago, TareqPhoto said:

Unfortunately, there is no one yet, it is a new product, with FPL-55, and the second one is almost new one just been out almost 1 year or so and not so so many reviews but it is not bad, not talking about the last one.

Well, you can always go for tried and tested, like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html

Get Ricarrdi FF/FR for it:

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

and you'll get F/4.5 80mm imaging scope.

I've heard of only one sample being out of collimation and generally most people report it to be very nice scope. I also own one of these 80mm triplets and it is indeed very nice imaging scope.

[TareqPhoto]

13 minutes ago, vlaiv said:

Well, you can always go for tried and tested, like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html

Get Ricarrdi FF/FR for it:

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

and you'll get F/4.5 80mm imaging scope.

I've heard of only one sample being out of collimation and generally most people report it to be very nice scope. I also own one of these 80mm triplets and it is indeed very nice imaging scope.

I want focal length in this range only = 280mm - 320mm, i might go wider to 260mm maybe, but not longer than 320mm

[vlaiv]

1 minute ago, TareqPhoto said:

I want focal length in this range only = 280mm - 320mm, i might go wider to 260mm maybe, but not longer than 320mm

Well, you can then look at 60mm scopes with field flatteners / reducers, or you can do mosaics instead?

For example little RedCat has only 250mm of focal length:

https://www.firstlightoptics.com/william-optics/william-optics-redcat-51-v1-5-apo-f49.html

Or you can do mosaics.

Even simple 130PDS will act as 325mm FL scope if you:

- image 4 panels for 1/4 of the time

- bin 2x2 each of those panels to recover lost SNR because you imaged each panel for only 1/4 of the time. This also preserves total pixel count of your final image

It is a bit more involved - but it is cheap and it let's you go even wider - at 217mm (3x3 panel and x3 bin) or 162mm (4x4 panel and x3 bin).

[TareqPhoto]

1 minute ago, vlaiv said:

Well, you can then look at 60mm scopes with field flatteners / reducers, or you can do mosaics instead?

For example little RedCat has only 250mm of focal length:

https://www.firstlightoptics.com/william-optics/william-optics-redcat-51-v1-5-apo-f49.html

Or you can do mosaics.

Even simple 130PDS will act as 325mm FL scope if you:

- image 4 panels for 1/4 of the time

- bin 2x2 each of those panels to recover lost SNR because you imaged each panel for only 1/4 of the time. This also preserves total pixel count of your final image

It is a bit more involved - but it is cheap and it let's you go even wider - at 217mm (3x3 panel and x3 bin) or 162mm (4x4 panel and x3 bin).

Mosaic is not in my mind at all to do, i will not do it or go for it, so he only option is wider scope.

I saw RedCat or similar scopes, i don't want that much 250mm, i can use my Canon 70-200mm f/2.8 with 1.4x and can be at 250mm, but i prefer a scope, last results i made by Nov-Dec last year made me to change my plan entirely cause of FOV, in fact 300mm is the sweet focal length i want, so with my ST80 and 0.8x as test i've got 320mm and i was blown away with FOV, so something like Esprit 80mm and reducer will do, but those are expensive, i am trying to make the second scope below $1000 if possible, and if i buy reducer that will add to the price also.

Bottom line is, the options i gave i can all use at 275mm-400mm, Redcat is a fixed Petzval 250mm that i don't want, i prefer to be more flexible with many FL using flatteners and reducers, i ordered a new model of TS 90mm F/6 triplet which will be available after March since they changed the date from end of Jan this year, or even longer than March, it is 540mm scope, i am planning to buy APS-C color camera and buying 0.65x reducer so it can give me almost 300mm FOV, a second scope next to it as dual i can't buy exactly the same again, i already have 3 mono cameras all with 4/3", have to use them at least one of them, i already have 0.8x, but i don't mind buying their 0.7x to go much wider than 360mm or even wider than 300mm very slightly, but not 200mm-260mm.

[vlaiv]

2 minutes ago, TareqPhoto said:

it is 540mm scope, i am planning to buy APS-C color camera and buying 0.65x reducer so it can give me almost 300mm FOV

I highly doubt that x0.65 reducer will be able to properly illuminate APS-C sized sensor.

APS-C has diagonal of about 28mm and if you use it with x0.65 reduction - that is like using 28mm / 0.65 = 43mm sensor without reduction. That is almost full frame. Not many scopes can properly illuminate full frame sensor with decent star shapes

6 minutes ago, TareqPhoto said:

in fact 300mm is the sweet focal length i want

https://www.teleskop-express.de/shop/product_info.php/info/p10095_TS-Optics-PhotoLine-60-mm-f-6-FPL53-Apo---2--R-P-Focuser---RED-Line.html

+

0.8 FF/FR like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p5965_TS-Optics-REFRACTOR-0-79x-2--ED-Reducer-Corrector-fuer-APO-und-ED.html

[ollypenrice]

I think it's important to know that the mechanical quality is very important in imaging scopes. There are plenty of examples of focusers which slip or sag, introducing the problem of tilt. There also good and bad lens cells which can pinch the optics or be impossible to collimate. And refractors must be collimated to work well. (Takahashi have been guilty of supplying an alarming number of miscollimated FSQs in the last few years.)

In the real world, when you are on a budget, a simple optical design is likely to beat a complex one because it is more likely to be supplied in good working order. One of  our regular guests has a TEC140, which is obviously a brilliant scope, but he also swears by his TeleVue 85 doublet. On paper this is an old design but he takes great pictures with it. My general point is that you are far more likely to be limited in your success by mechanical problems, or collimation, than you are by what numbers define the glass type or by how many elements you have in the tube.

We don't take pictures with the spec sheet, we take them with the thing on the mount!  As Valiv says, look at the forums to see which scopes are consistently delivering good pictures.

Olly

[TareqPhoto]

26 minutes ago, vlaiv said:

I highly doubt that x0.65 reducer will be able to properly illuminate APS-C sized sensor.

APS-C has diagonal of about 28mm and if you use it with x0.65 reduction - that is like using 28mm / 0.65 = 43mm sensor without reduction. That is almost full frame. Not many scopes can properly illuminate full frame sensor with decent star shapes

https://www.teleskop-express.de/shop/product_info.php/info/p10095_TS-Optics-PhotoLine-60-mm-f-6-FPL53-Apo---2--R-P-Focuser---RED-Line.html

+

0.8 FF/FR like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p5965_TS-Optics-REFRACTOR-0-79x-2--ED-Reducer-Corrector-fuer-APO-und-ED.html

https://starizona.com/products/apex-ed-65x-reducer-flattener-for-apo-refractors-and-rc-acf-telecopes

^^^ Reading about it they mentioned up to APS-C, so i don't know if that mans it can be fine with APS-C or not, some said they managed to use it even up to full frame, and most likely i will crop the frame from edges anyway, but i have no choice rather than using this or another reducer and then i have to buy a full frame camera and not APS-C, that will cost me even more.

Well, that scope your linked is already a doublet with FPL-53, i posted a doublet with fluorite from Takahashi which blows away this one, and yet you didn't choose it for me, i also saw 60mm from Astro-Tech and also 60mm something from William Optics i think Zenithstar model, but i didn't mention them here.

[TareqPhoto]

3 minutes ago, ollypenrice said:

I think it's important to know that the mechanical quality is very important in imaging scopes. There are plenty of examples of focusers which slip or sag, introducing the problem of tilt. There also good and bad lens cells which can pinch the optics or be impossible to collimate. And refractors must be collimated to work well. (Takahashi have been guilty of supplying an alarming number of miscollimated FSQs in the last few years.)

In the real world, when you are on a budget, a simple optical design is likely to beat a complex one because it is more likely to be supplied in good working order. One of  our regular guests has a TEC140, which is obviously a brilliant scope, but he also swears by his TeleVue 85 doublet. On paper this is an old design but he takes great pictures with it. My general point is that you are far more likely to be limited in your success by mechanical problems, or collimation, than you are by what numbers define the glass type or by how many elements you have in the tube.

We don't take pictures with the spec sheet, we take them with the thing on the mount!  As Valiv says, look at the forums to see which scopes are consistently delivering good pictures.

Olly

I already posted few options that i think they can be fine, but i have a require by focal length more, then i try to match that focal length to whatever i can find affordable, i mean try to find a scope for me as refractor giving me between 280mm and 320mm, i don't want 360mm nor 250mm, the average is 300mm, so only like -+20mm from it, so find me that quality mechanical scope giving me that focal length and i will stop asking or searching, not interested in RedCat/SpaceCat models.

[vlaiv]

31 minutes ago, TareqPhoto said:

Well, that scope your linked is already a doublet with FPL-53, i posted a doublet with fluorite from Takahashi which blows away this one, and yet you didn't choose it for me, i also saw 60mm from Astro-Tech and also 60mm something from William Optics i think Zenithstar model, but i didn't mention them here.

Yep, my bad - I did not read carefully, I thought that it was triplet lens.

I don't really agree that Takahashi blows away that scope - but am happy to be corrected if you give me an example of that.

[ollypenrice]

32 minutes ago, TareqPhoto said:

I already posted few options that i think they can be fine, but i have a require by focal length more, then i try to match that focal length to whatever i can find affordable, i mean try to find a scope for me as refractor giving me between 280mm and 320mm, i don't want 360mm nor 250mm, the average is 300mm, so only like -+20mm from it, so find me that quality mechanical scope giving me that focal length and i will stop asking or searching, not interested in RedCat/SpaceCat models.

I don't know the latest scopes but that's a very short FL for a telescope. My point was a general one.

If we start by thinking of a 300-320mm FL then we have to factor in a realistic F ratio for a budget design.  F6 is potentially possible but that would mean a 50mm aperture. If you go to 60mm you'll need F5. Can anyone make well corrected F5 optics in a scope of adequate quality at a budget price? I would be surprised, I'm afraid, but I would be glad to be proven wrong. I'm wondering about prime camera lenses, possibly older, non-stabilized designs. If you make a simple front aperture mask you can stop them down without introducing multiple spikes from the diaphragm.

By the way, the Tak FS60, unless it has been greatly improved since the ones I tried about ten years ago, could not blow anything away. They were awful deep sky imaging scopes with dreadful blue bloat on hot stars.

Olly

Edited January 27, 2021 by ollypenrice

[jetstream]

4 hours ago, vlaiv said:

Take for example this scope (and line of similar scopes from Altair Astro or StellarVue):

Vlaiv, I have a 90mm Stellarvue Raptor, carbon fibre, FLP53 center element triplet at f7. I also have a niece that might want to try astrophotography.  The speed issue confuses me- would this scope be OK for DSO widefield eventhough f7? its 630mm fl.

I can start another thread if need be.

[vlaiv]

6 minutes ago, jetstream said:

Vlaiv, I have a 90mm Stellarvue Raptor, carbon fibre, FLP53 center element triplet at f7. I also have a niece that might want to try astrophotography.  The speed issue confuses me- would this scope be OK for DSO widefield eventhough f7? its 630mm fl.

I can start another thread if need be.

What is confusing you about the speed? I believe that scope will be very good at DSO AP - wide field or otherwise.

I mentioned above 102 F/7 scope as it is doublet with claimed excellent correction. I believe that to be true for visual - but not for imaging. I ran across that issue few times now - people having that scope (or same optics from another vendor) and results are not looking nice in some cases, for example see here:

Or if you don't want to go thru the thread - here is exact example:

Look at those stars and blue halo ...

Many people will not see this and will happily use their scope simply because they shoot mono + filters or do narrow band. This was taken with OSC sensor - where one shoots all wavelengths at the same time and any difference in focus is not compensated between filters (like in mono + filters).

Granted, above can be corrected with use of say Astronomik L3 luminance filters which cut furthest parts of spectrum - and that is fine for the price - you get 4" scope for half or third of the price of 4" triplet - but there are some things that you need to circumvent.

On the topic of speed of scopes - well, speed of optics - like F/ratio really does not have to do much with speed of image acquisition. "Slow" scopes can image fast and "fast" scopes can be slow when imaging. In fact - it is better to have slow scope in terms of F/ratio as aberrations are much easier to control with slow scopes. Less issues with filters / reflections / edge correction and such.

Just make sure that that you pair your scope with proper pixel and sensor size. Best definition of the speed is "aperture at resolution". You set your working resolution and then throw as much aperture as you can on it.

Slow scopes - or rather scopes with larger focal length - have only one drawback, they don't record very large field at once - FOV is function of focal length and sensor size. You can do mosaics in order to go around that.

[TareqPhoto]

33 minutes ago, vlaiv said:

Yep, my bad - I did not read carefully, I thought that it was triplet lens.

I don't really agree that Takahashi blows away that scope - but am happy to be corrected if you give me an example of that.

Ok, so in this case i shouldn't put Takahashi and Fluorite on top always unless it is a high end triplet or petzval design then, i thought that all Takahashi scopes even the doublet will outperform another doublets, because i keep reading that Takahashi is a well know manufacturer so they care too much for QC of all their scopes, but maybe that is not the case with all scopes from them, i am also happy to be corrected.

[TareqPhoto]

33 minutes ago, ollypenrice said:

I don't know the latest scopes but that's a very short FL for a telescope. My point was a general one.

If we start by thinking of a 300-320mm FL then we have to factor in a realistic F ratio for a budget design.  F6 is potentially possible but that would mean a 50mm aperture. If you go to 60mm you'll need F5. Can anyone make well corrected F5 optics in a scope of adequate quality at a budget price? I would be surprised, I'm afraid, but I would be glad to be proven wrong. I'm wondering about prime camera lenses, possibly older, non-stabilized designs. If you make a simple front aperture mask you can stop them down without introducing multiple spikes from the diaphragm.

By the way, the Tak FS60, unless it has been greatly improved since the ones I tried about ten years ago, could not blow anything away. They were awful deep sky imaging scopes with dreadful blue bloat on hot stars.

Olly

Ok, let's ignore that FC-60 then, although i heard that this issue is only if using the flattener that is not that good, while using their reducer will fix this issue, but let's not go deep to analyze this scope and compare it with others, i thought a fluorite will always make nice color corrected over any optics for refractor, as i did read an article which shows that fluorite will outperform all optics used in lenses or refractor and they make FPL-53 to be close to it, they even call FPL-53 or FCD-100 as "Synthetic Fluorite", but i think today i learn something new about it.

Well, don't exclude reducers, there are many people using reducers also with amazing results, so i didn't ask to have like 300mm as only native focal length, it can be from reducing the scope also, like from 360mm or 340mm to 270mm-300mm if possible, but sounds you didn't care much about reducers so you try to find super wide aperture to give ~300mm without reducer.

[ollypenrice]

1 minute ago, TareqPhoto said:

Ok, let's ignore that FC-60 then, although i heard that this issue is only if using the flattener that is not that good, while using their reducer will fix this issue, but let's not go deep to analyze this scope and compare it with others, i thought a fluorite will always make nice color corrected over any optics for refractor, as i did read an article which shows that fluorite will outperform all optics used in lenses or refractor and they make FPL-53 to be close to it, they even call FPL-53 or FCD-100 as "Synthetic Fluorite", but i think today i learn something new about it.

Well, don't exclude reducers, there are many people using reducers also with amazing results, so i didn't ask to have like 300mm as only native focal length, it can be from reducing the scope also, like from 360mm or 340mm to 270mm-300mm if possible, but sounds you didn't care much about reducers so you try to find super wide aperture to give ~300mm without reducer.

tHE THING 

[TareqPhoto]

5 minutes ago, ollypenrice said:

tHE THING 

What THING?

[michael.h.f.wilkinson]

Reducers can be fine, I get good results on an APS-C sensor with my APM 80 mm F/6 triplet (FPL-53), with a Tele-Vue TRF2008 0.8x reducer, yielding a very useful 384 mm focal length at F/4.8. I have tried a 0.6x reducer, which seems OK on the smaller ASI183MC's sensor, but  even there seems to give slightly eggy stars at the corners, so I haven't dared use it on an APS-C sensor