Altair Astro 102 super ED or 115 triplets?

[kingboya]

Hi all advice required!

I have narrowed my search to these two scopes, primarily for imaging with my Atik 314L, but also some visual solar system work too.

The 102 uses FPL53 glass, and the 115 uses FPL51 glass. Olly Penrice did reviews of both scopes and loved them at this price bracket.

I am confused as to which to go for, both being the same price, both F7 etc etc.

What are your thoughts?

[DirkSteele]

Way too many individuals get hung up on the glass type of the ED element in a refractor as though it is some sort of optically badge of quality.

Ohara makes FPL-53 in a variety of quality levels and will sell to any optical maker who will pay. Lower quality will tend to have more bubbles, occlusions and striae (if you read the minimum spec requirements for LZOS lenses used in APM refractors which use OK4 it is not just about figure accuracy but also defects in the glass substrate itself), and will cause the lenses that it ends up in to be not as good as they would have been had a higher grade of FPL-53 been used. I may be corrected on this, but I have not seen a disclosure from any manufacturer on which grade is used in a scope and hence you will never know. It allows all makers to use a bit of marketing spin to make you believe the scope will be optically excellent. The price difference between the best and worst grades of FPL-53 is very large (4x plus I understand).

Glass quality is more important than glass type when it comes to refractor performance. Similarly, the quality and selection of mating glasses is every bit as important as the type of ED glass used (you never hear any marketing nonsense about the other elements dispite being very important). A skilled optical maker can make an FPL-51 triplet with a much better figure and much better colour correction than an FPL-53 triplet of the same aperture by making more sensible mating glass choices on the former than the latter, and by extending the focal length of the instrument (needed because FPL-51 does have a higher dispersion characteristic than FPL-53).

In their price bracket both scopes seem to be well regarded and both will likely serve you well. The colour correction (assuming both have been figured well, using good quality elements etc as described above) will be slightly better in the 102 as they are both F/7 but I would guess (having not used the 102) that they would be virtually indistinguishable (A club member has the Altair 115 and I was impressed). “IF” my assumption is correct about comparable performance in colour correction, you will have more resolving power in the 115 which will prove invaluable when planetary observing (plus if you decide to do a bit of visual deep sky, the extra aperture will be noticeable – I can certainly see the difference between my 105 and 115 on deep sky). The 115 will have a slightly longer focal length (91mm) so your guiding will need to be a touch better but I am not sure that should be enough of a difference to push you one way or the other.

Hopefully Olly will see this thread and offer his informed opinion as he has extensively used both scopes. And I am sure other SGL members who know the scopes will have more thoughts to offer as well.

Clear Skies,

[damnut]

A great considered reply above. Both of the scopes you mention have their following. Whilst only ever had achromats in 120, 127 and 152mm. The 120mm was more versatile and I would tend towards the 115mm. Just popped on to altair astro website and both 102 mm & 115 mm f7 scopes are listed at £1395!. With the 102mm F6.5 at £1049.

[ollypenrice]

In response to your PM I did like them both, as I said in Astronomy Now. I would base the decision on focal length alone. Which would give you the framing you would like best?

There was a small difference in the star test. A slight change of colour inside and outside focus on the 115 suggested a hint of spherical aberration while the 102 star tested perfectly. I had a professional optical engineer staying for both tests and this was his opinion too. He's far better than I am at optical testing.

I agree entirely with DirkSteele on the glass business. What matters is what the scope does. For all its marginally better star test, real life use showed discernible advantage from the 102. Nice to know it tested impeccably but this would not sway me. I'd choose based on what I wanted to fit in the frame.

Olly

[kingboya]

In response to your PM I did like them both, as I said in Astronomy Now. I would base the decision on focal length alone. Which would give you the framing you would like best?

There was a small difference in the star test. A slight change of colour inside and outside focus on the 115 suggested a hint of spherical aberration while the 102 star tested perfectly. I had a professional optical engineer staying for both tests and this was his opinion too. He's far better than I am at optical testing.

I agree entirely with DirkSteele on the glass business. What matters is what the scope does. For all its marginally better star test, real life use showed discernible advantage from the 102. Nice to know it tested impeccably but this would not sway me. I'd choose based on what I wanted to fit in the frame.

Olly

Thank you Olly for your help. Just to clarify, (maybe I am having a slow day), you felt like the 102 had a real life advantage over the 115?

[kingboya]

Way too many individuals get hung up on the glass type of the ED element in a refractor as though it is some sort of optically badge of quality.

Ohara makes FPL-53 in a variety of quality levels and will sell to any optical maker who will pay. Lower quality will tend to have more bubbles, occlusions and striae (if you read the minimum spec requirements for LZOS lenses used in APM refractors which use OK4 it is not just about figure accuracy but also defects in the glass substrate itself), and will cause the lenses that it ends up in to be not as good as they would have been had a higher grade of FPL-53 been used. I may be corrected on this, but I have not seen a disclosure from any manufacturer on which grade is used in a scope and hence you will never know. It allows all makers to use a bit of marketing spin to make you believe the scope will be optically excellent. The price difference between the best and worst grades of FPL-53 is very large (4x plus I understand).

Glass quality is more important than glass type when it comes to refractor performance. Similarly, the quality and selection of mating glasses is every bit as important as the type of ED glass used (you never hear any marketing nonsense about the other elements dispite being very important). A skilled optical maker can make an FPL-51 triplet with a much better figure and much better colour correction than an FPL-53 triplet of the same aperture by making more sensible mating glass choices on the former than the latter, and by extending the focal length of the instrument (needed because FPL-51 does have a higher dispersion characteristic than FPL-53).

In their price bracket both scopes seem to be well regarded and both will likely serve you well. The colour correction (assuming both have been figured well, using good quality elements etc as described above) will be slightly better in the 102 as they are both F/7 but I would guess (having not used the 102) that they would be virtually indistinguishable (A club member has the Altair 115 and I was impressed). “IF” my assumption is correct about comparable performance in colour correction, you will have more resolving power in the 115 which will prove invaluable when planetary observing (plus if you decide to do a bit of visual deep sky, the extra aperture will be noticeable – I can certainly see the difference between my 105 and 115 on deep sky). The 115 will have a slightly longer focal length (91mm) so your guiding will need to be a touch better but I am not sure that should be enough of a difference to push you one way or the other.

Hopefully Olly will see this thread and offer his informed opinion as he has extensively used both scopes. And I am sure other SGL members who know the scopes will have more thoughts to offer as well.

Clear Skies,

Thank you very much DirkSteele for your help also, lots of superb help in there!

[PARTY MARTY]

I have the TS 115mm Triplet APO F7 Refractor, I have no idea what type of glass it is made, but I don't care to be honest as long as it performs to high standard it's completely fine, I've had no hint of false colour at all in my images but one thing I would suggest is getting a APO Refractor nor matter what size it is 102mm or 115mm both scopes will perform really well, but as cost mounts up, also the sheer weight also adds up, My 115mm APO is really quite heavy with the triplet lens system adding weight to 8kg, I wanted to adopt for the TS 130mm Triplet, but I'm glad I didn't go for it, this thing is a beast even though it's slightly larger in apeture of an extra 15mm the cost almost doubles the 115mm and the weight being 10Kg might be to much for my NEQ-6 mount, especially if you're adding guide scopes, CCD's Etc.. Etc... You'll run deep into balance problems or overloading on the mount!!! So think about portablability and use of the scope, then thinking of the size of apeture! I believe that the Altair astro are similar to Telescope-Service branded scopes! For me I wish I could afford a TAK or a Borg, or a Televue! But the prices are just simply out of this earth for me, so I've adopted for a cheaper brand, but even they are much cheaper they offer very good performance and do the job for DSO imaging, For many other members might disagree with my say in this matter!!!! But I don't carry a spare £4000 in my wallet that's for sure!!!

[ollypenrice]

Thank you Olly for your help. Just to clarify, (maybe I am having a slow day), you felt like the 102 had a real life advantage over the 115?

Sorry, a typo made my post confusing. No, I didn't feel there was a real life advantage to the 102 but I'm getting on a bit and it is not impossible that someone with younger eyes might prefer the 102 visually. I couldn't see a difference once in focus. Speaking as an imager I'd say that there was nothing in it. Just a choice of focal lengths.

Olly