Lets play a game! Pick a refractor Part I

[Lady Fraktor]

I'd be happy to hear more on about oil spaced lenses , are there still advantages with them? Even in current day technology, coatings, triplet scopes ectt?

Yes the advantages are still there even with modern technology but the extra costs and time to do it would be expensive.
I will elaborate a bit more in a day or two.

[turboscrew]

So this is not "how to keep morons thrilled"-game? :-D

No, we are discussing refractors not newtonians

Still, I've been waiting for the words of wisdom. (It doesn't harm to know about things even if they are not to be applied immediately.)

[Lady Fraktor]

I have most of it written now but the 14-16 hour work days are turning my mind to slush by the time I sit down.
One more day and then it will be posted.

Many of these principles are the same between different styles of telescopes as well.

[Thefatkitty]

I have most of it written now but the 14-16 hour work days are turning my mind to slush by the time I sit down.
One more day and then it will be posted.

Many of these principles are the same between different styles of telescopes as well.

Oh good, time enough to respond... Well, for what it's worth, I'm like AbbN; all I have are achro's. Some nice long f/l Vixen-mades as well, but achro's nonetheless.

Given the equipment I have (read mount) I'd go for option #2 as it's lighter, comes with a dovetail, and might not kill my CG4 (at least for the mostly visual I do). I know nothing better than an achro, so even an ED would be a huge leap forward. Though I wish it had the 10:1 focuser....

With my budget, #2 would be the best for me since it would probably be OK with my mount and I could save/trade up for a better focuser.

Now Gabby, are you perchance giving one of these away...????

Looking forward to tomorrow....

[Lady Fraktor]

Alright, let us compare the two refractors and some of the terms used.

#1 - 150mm triplet f/6.7 refractor
Description:
* 3-element objective lens system minimizes colour aberration
* Fully multi-coated optics ensures superior light transmission
* Finely CNC machining aluminium body and 1:10 focuser
* Deluxe high-glossy anodizing finish
* Retractable dewshield
* Accept all 1.25” universal telescope eyepieces

Specification:
Optical design: Refractor
Aperture: 150mm
Focal Length: 1000mm
Focal Ratio : f/6.7
Lens: Triplet , ED APO S-FPL51 Fully Multi-Coated
Attachments: 2.7“ Rack & Pinion Focuser, Steel lens cell, 2"-1.25" Brass Compressing Adapter, 1 set (2pcs) Tube Rings, Aluminium Carry Case

#2 - 150mm f/6.7 refractor
Description:
* 2-element objective lens system minimizes colour aberration
* Fully multi-coated optics ensures excellent light transmission
* Finely CNC machining aluminium body, lens cell and focuser
* Deluxe high-glossy anodizing finish
* Retractable dewshield
* Accept all 1.25” universal telescope eyepieces

Specification:
Optical design: Refractor
Aperture: 150mm
Focal Length: 1000mm
Focal Ratio : f/6.7
Lens: Doublet , ED APO S-FPL53 Fully Multi-Coated
Attachments: 2.7“ Rack & Pinion Focuser, 2"-1.25" Brass Compressing Adapter, 1 set (2pcs) Tube Rings, 8” Vixen style extruded dovetail
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


The one thing I was surprised about when reading responses was the topic of the focal ratio of the telescopes, it was only mentioned in passing by few people.

Focal Ratio is the first thing that I will look at as this will define if it is fast or slow, widefield or narrow. It also mentally gives me a list of things that should or should not be there.

(In the wild rule): A telescope of FPL-51 even as a triplet is going to show more colour then the FPL-53 doublet at f/6.7.
The focal ratio cutoff is around f/7. At f/7 a 51 triplet will match a 53 doublet for correction.
Faster than f/7 and FPL-51 will show some colour.

Coatings: The difference between superior and excellent can be a bit speculative.
There are only a few ways to apply coatings and if done properly will work effectively.
When you read about someone evaluating coatings on a new telescope they mention colour and such.
For us we are looking for smooth consistent colour, with no blotches or fading from one location to another.
When viewing is colour rendition accurate or shifted?***

Coatings of themselves are colourless and it is the type of coatings and applied thickness that give us a visible reflected colour.

Focusers: At f/6.7 I would definitely want a two speed focuser for visual or astrophotography.
The depth of focus becomes increasingly smaller as the focal ratio decreases.

This is sometimes referred to as focus snap, it happens as you are focusing and the object immediately goes into focus with a small margin of in/ out of focus.
With telescopes longer than f/7 a two speed focuser becomes a not really necessary luxury as the depth of focus can be a large range.

Crayford focusers are very smooth in operation but normally have less capacity than a rack & pinion though well made R&P focusers today can be just as smooth.

Lens Cells: A quality cell needs to expand and contract with the lens and this is why a steel cell is best. Glass and steel have very close coefficient of expansion so thermal shock, pinched optics and lens misalignment are not likely to happen.
This is referred to as a temperature compensating lens cell.

Aluminium absorbs/ dissipates heat very quickly so can expand or contract faster than the glass.
This can cause lens misalignment or pinched optics until the glass/ aluminium equalise in temperature.
Most mass produced telescopes use aluminium.

Plastic works well and is cheap to manufacture, once or twice a year you may need to loosen the retaining ring and tap the cell to realign the lens.
Usually in low to mid priced beginner telescopes, occasionally in novice level.

Dewshield: Sliding is best for storage when travelling but if used at home or permanently set up they become a non-issue for me.

Accessories such as cases and dovetails I tend to not worry overly much about.
Majority of supplied cases are adequate for vehicle travel, premium retailer cases are usually goof for air travel in overhead/ underseat locations and to get one good enough for going in the airplane hold will require a specialized case.

I personally do not like or use extruded dovetails and I change them out for solid ones.

In conclusion, strictly going by the listed specifications, telescope #1 has the best mechanical options while #2 has the advantage for colour correction.

Future discussions so far:

1. Advantages of air spaced and oil spaced
2. Can a doublet be apochromatic
3. Effects of coatings


Ask questions if not sure about something or if I has missed or not explained something.
I have listed 3 future topics but let me know if there is something you want covered

[John Baars]

(...) Though I wish it had the 10:1 focuser....
With my budget, #2 would be the best for me since it would probably be OK with my mount and I could save/trade up for a better focuser.

Even a standard R&P focuser can serve planetary targets flawlessly visually if a helical attachment is used with it. Any image shift is of course not resolved.

[turboscrew]

Alright, let us compare the two refractors and some of the terms used.

#1 - 150mm triplet f/6.7 refractor
Description:
* 3-element objective lens system minimizes colour aberration
* Fully multi-coated optics ensures superior light transmission
* Finely CNC machining aluminium body and 1:10 focuser
* Deluxe high-glossy anodizing finish
* Retractable dewshield
* Accept all 1.25” universal telescope eyepieces

Specification:
Optical design: Refractor
Aperture: 150mm
Focal Length: 1000mm
Focal Ratio : f/6.7
Lens: Triplet , ED APO S-FPL51 Fully Multi-Coated
Attachments: 2.7“ Rack & Pinion Focuser, Steel lens cell, 2"-1.25" Brass Compressing Adapter, 1 set (2pcs) Tube Rings, Aluminium Carry Case

#2 - 150mm f/6.7 refractor
Description:
* 2-element objective lens system minimizes colour aberration
* Fully multi-coated optics ensures excellent light transmission
* Finely CNC machining aluminium body, lens cell and focuser
* Deluxe high-glossy anodizing finish
* Retractable dewshield
* Accept all 1.25” universal telescope eyepieces

Specification:
Optical design: Refractor
Aperture: 150mm
Focal Length: 1000mm
Focal Ratio : f/6.7
Lens: Doublet , ED APO S-FPL53 Fully Multi-Coated
Attachments: 2.7“ Rack & Pinion Focuser, 2"-1.25" Brass Compressing Adapter, 1 set (2pcs) Tube Rings, 8” Vixen style extruded dovetail
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


The one thing I was surprised about when reading responses was the topic of the focal ratio of the telescopes, it was only mentioned in passing by few people.

Focal Ratio is the first thing that I will look at as this will define if it is fast or slow, widefield or narrow. It also mentally gives me a list of things that should or should not be there.

(In the wild rule): A telescope of FPL-51 even as a triplet is going to show more colour then the FPL-53 doublet at f/6.7.
The focal ratio cutoff is around f/7. At f/7 a 51 triplet will match a 53 doublet for correction.
Faster than f/7 and FPL-51 will show some colour.

Coatings: The difference between superior and excellent can be a bit speculative.
There are only a few ways to apply coatings and if done properly will work effectively.
When you read about someone evaluating coatings on a new telescope they mention colour and such.
For us we are looking for smooth consistent colour, with no blotches or fading from one location to another.
When viewing is colour rendition accurate or shifted?***

Coatings of themselves are colourless and it is the type of coatings and applied thickness that give us a visible reflected colour.

Focusers: At f/6.7 I would definitely want a two speed focuser for visual or astrophotography.
The depth of focus becomes increasingly smaller as the focal ratio decreases.

This is sometimes referred to as focus snap, it happens as you are focusing and the object immediately goes into focus with a small margin of in/ out of focus.
With telescopes longer than f/7 a two speed focuser becomes a not really necessary luxury as the depth of focus can be a large range.

Crayford focusers are very smooth in operation but normally have less capacity than a rack & pinion though well made R&P focusers today can be just as smooth.

Lens Cells: A quality cell needs to expand and contract with the lens and this is why a steel cell is best. Glass and steel have very close coefficient of expansion so thermal shock, pinched optics and lens misalignment are not likely to happen.
This is referred to as a temperature compensating lens cell.

Aluminium absorbs/ dissipates heat very quickly so can expand or contract faster than the glass.
This can cause lens misalignment or pinched optics until the glass/ aluminium equalise in temperature.
Most mass produced telescopes use aluminium.

Plastic works well and is cheap to manufacture, once or twice a year you may need to loosen the retaining ring and tap the cell to realign the lens.
Usually in low to mid priced beginner telescopes, occasionally in novice level.

Dewshield: Sliding is best for storage when travelling but if used at home or permanently set up they become a non-issue for me.

Accessories such as cases and dovetails I tend to not worry overly much about.
Majority of supplied cases are adequate for vehicle travel, premium retailer cases are usually goof for air travel in overhead/ underseat locations and to get one good enough for going in the airplane hold will require a specialized case.

I personally do not like or use extruded dovetails and I change them out for solid ones.

In conclusion, strictly going by the listed specifications, telescope #1 has the best mechanical options while #2 has the advantage for colour correction.

Future discussions so far:

1. Advantages of air spaced and oil spaced
2. Can a doublet be apochromatic
3. Effects of coatings


Ask questions if not sure about something or if I has missed or not explained something.
I have listed 3 future topics but let me know if there is something you want covered

VERY informative!
And just in time. I was about to kill someone.

[John Baars]

One is never too old to learn.
-I suspected the equal CA, but didn't know about the cutoff.
-I knew steel coefficient of expansion matches glass better than aluminium. But I assumed that the difference would have little effect. Until I read your answer. I just realised that I just had polished the inside of my aluminium cell to give the 6" lens somewhat more space and it is very likely that this action contributed to a better performance during cooldown. A difference I noticed.

Thanks for this topic!
Chapeau!

[Lady Fraktor]

The polishing may have given the lens some expansion room, the important thing is it works for you

[John Donne]

Interesting thread Lady Fractor.
Thank you for the work you have done to bring this conversation.

[Lady Fraktor]

Thought there would be a few questions after posting my thoughts though...
I think the next section will be on can doublets be apochromatic.

[turboscrew]

I'm waiting...