15 replies to this topic

[HCR32]

Not long ago I ask Markus what it would take to get a 7” doublet Refractor on his to do list and he said he needs 100 buyers. Seeing how good his 152mm APM doublets is I’m wondering if there is enough show of hands to show APM that there is interest in this scope. If it doesn’t work out then a small number of us can let the idea go then.

[junomike]

Focal Length, Cost?

[HCR32]

The cost will be obviously cheaper than the triplet on offer if it ever materializes. Focal length I think will be something APM will determine based on what is required to achieve respectable performance.

Edited by HCR32, 07 September 2020 - 04:38 AM.

[junomike]

The cost will be obviously cheaper than the triplet on offer if it ever materializes. Focal length I think will be something APM will determine based on what is required to achieve respectable performance.

IMO, It's difficult enough to get people to commit with a posted price, let alone without

[Jim Curry]

interested

[Rick Runcie]

Oh I'm very interested, a decent quality, affordable 7" is long over due.
Best regards, Richard

[bobhen]

It would be a pretty big scope. I’m just guessing but what – a 7-inch doublet with FPL 55 at say F9.5? Probably more than a G11 could handle.

 

Bob

[Kunama]

If I had a permanent observatory mount to put a big refractor on I would be interested in a 7" F8 or F9.... 

without the observatory I think it would be too much hassle compared to a good 6"F8........ 

[RogerLaureys]

Interested

[CHASLX200]

I would fine with a 7" for under 6k. But the mount is the problem once you get past 6". Big and heavy and cost a ton.

[ris242]

If you stayed at a 1200mm length you would be at f/6.7..........probably a little fast for visual?

 

What weight would you assume a 7" doublet would be?

 

 

I'd be more inclined due to weight - for a long 130 doublet.........but I can also guess why there isn't one in the range yet too.

[cam1936]

I don't know if the market would be there. Who is the buyer? At any reasonable focal length you're going to have enough colour that a comparable reflector is going to outperform, and be easier to mount and deal with.

[Jimmy462]

Would a well-corrected (read: semi-APO) doublet be possible at f/9–f/10 (i.e. keeping the OTA under 6-foot)? Could make for a load of fun on a Berry-style mount!

[noisejammer]

I already have a 6", so the increase in light gathering would be (7/6)^2 - 1 = 36%. This translates to an threshold improvement of about 0.36 magnitudes or 50% more stars.

 

That's quite easy to obtain by taking a trip to a slightly better location.

 

Sorry - it's not enough to warrant the cost.

[CHASLX200]

Meade did it cheap and included a mount 20 years ago. But we all know how bad they were due to cells that could never hold center. But a few that did work gave a good image.  FL would need to be around F/9 to keep cost down.  Just gonna need a mount bigger than a G11 so that is where the money comes into play. Big mounts are not cheap or easy to move around.  Gonna need a Titan, AP1100 or something in that class.

[Jeff B]

Just scaling the 152ED doublet to 175mm gets you an F9-ish focal ratio.   

 

But I would not want an FPL51 based objective which would be a rehash of the Meade even though there appears to be a much better understanding of cell design.  

 

I would much prefer an FPL55 or similar type glass with a good mating element, but still at F9.  This should make it a bit less sensitive to the cell design, construction and thermal ballistics.

 

Regarding weight, I found the Meade 178ED to be really rather light weight for the aperture.   I've made 8" F9 achromats that are "light weight" too and just barely standard G11 friendly, but that depends very much on you tolerance for vibration. 

 

But it can certainly be done.

 

Jeff