Thinking about a Frankenscope...

[Macavity]

I know that a Frankenscope (qv) can be quite a complex
thing re. construction and SAFETY issues, but thinking...

Re. the safety thing, I'll BEGIN by buying a Baader ERF
anyway! I can only afford a 70mm at first (maybe) but!
For a TEST, I have the "remains" of an ST102 to give a
500mm f/5 Achromat doublet (in lens cell) as objective.
(This will be stopped-down by the ERF/cell to give f/7!)

I KNOW that *everyone* starts by using a PST mod for
this stuff, but I envisage using "collars" and inserting
my (objective-less) Lunt 50 OTA in the (stopped-down)
ST102 OTA! I will get the ERF cell + collars "made up".

ASSUMING I *know* what I'm doing (Noting safety!)
I have a couple of questions re. the basic principals...

o The Lunt 50 innards (Etalon etc.) are designed for
   use with an f/7 (350mm) objective in the original!

o I only have (in the foreseeable future!) the "B600"
   Blocking filter with my version of Lunt50 scope...

Question 1: The f/7 limit is surely a *minimum* limit
for f-number re. the Etalon of the Lunt? I could e.g.
ultimately use e.g. say an f/10 Achromat instead?

Question 2: MY present (B600) blocking filter implies
a maximum focal length 600mm for any(!) objective.
Or could I stretch this limit a bit... 600... 700 etc?

Still wondering if this is a *worthwhile* experiment?
I CANNOT imagine a major size upgrade in HA scope!
But I often sense my Lunt 50 is *somewhat* limited
aperture... A modest increase in both this and the
focal length might make a *useful* difference??? 

[xtreemchaos]

why not just get a quark Chris, it would be cheaper in the long run. charl.

[Macavity]

6 minutes ago, xtreemchaos said:

why not just get a quark Chris, it would be cheaper in the long run. charl.

Well, that is a perfectly VALID option! (A fair lump of dosh despite tho) 

One has to be wary of the "hidden costs" of all this frenetic DIY stuff...

[Peter Drew]

My "expertise" lies in PST modding but I see no basic reason why a similar approach shouldn't have similar benefits. A couple of caveats, firstly both the Coronado and Lunt etalons have additional reimaging lenses to produce parallel light beams configured for their respective focal ratios, straying far from these parameters results in adverse field angles and loss of performance. Secondly, the Lunt units, I believe, have single lens objectives and further corrections downstream might not be compatible with a donor objective. A quark is a very tempting alternative and will work with a good range of apertures, they have gone up a fair bit in price recently, if that reflects better consistency all well and good but there is still an element of caution in the air.   

[GreatAttractor]

2 hours ago, Macavity said:

Question 1: The f/7 limit is surely a *minimum* limit
for f-number re. the Etalon of the Lunt? I could e.g.
ultimately use e.g. say an f/10 Achromat instead?

F/7 is recommended. If you couple it with an objective with larger F-number, the linear size of sweet-spot will be smaller (and if you use a Barlow to stretch it to cover your camera's FOV, you'll be likely oversampling).

If you use an objective with smaller F-number, the incoming light cone will be clipped (and with it the effective aperture). I use a 90/660 (so f/7.33) achromat in my mod.

 

2 hours ago, Macavity said:

Still wondering if this is a *worthwhile* experiment?
I CANNOT imagine a major size upgrade in HA scope!
But I often sense my Lunt 50 is *somewhat* limited
aperture... A modest increase in both this and the
focal length might make a *useful* difference??? 

It is absolutely worthwhile, just keep in mind the trade-offs. Unlike a Lunt 80 etc., a Lunt 50 with larger objective will have a limited sweet-spot; you won't be able to view/image the whole disk at once. For small FOV high-resolution imaging (say, a 1/3" sensor) it's fine.

[xtreemchaos]

I went through all the options before buying a quark, and it didn't matter how I looked at it the quark ticked all the boxes , of course there is risk, but the newer ones which use less power seam "and this is just my view" to be more relieable, its deff the most bang for your buck and makes HA imaging and processing very easy because of the amount of detail you get too play with. ive only tried a lunt for a short time on a day when the seeing wasn't so good so carnt say which is better.  charl.

[Merlin66]

If you check out the solar chat site, you'll see many successful Lunt mods.

with larger aperture mods you can always save some cost by using a sub-aperture ERF in the OTA.

My PST mod cost much less than a Quark......

[Macavity]

Thanks for the great contributions above! And, not least, the patience
to read through (understand!) my questions above. lol. I suppose I was
also looking for a few like-minded "enablers", if the truth be told... 

Indeed there are also some great designs on "Solar Chat" - A forum to
which I have sadly been unable to contribute. And I sensed there was
local opinion too! "That's good enough for me" (Morcambe & Wise)

SAFETY issue are paramount! I think I have most of the rest sussed?
I did recall later that there is an aspect of TILTING the ERF as well...
But, with reasonable thought, there is no "Magic Pixie Dust" needed?

I have a surplus of Tube Rings (various sizes)! Aluminium "Box Girder"
(Two Metres plus - Arranged not so prettily along my stairway) So the 
the idea of consuming some of my "resources" also figures in this! 

It is my downfall that I like ALL sorts of Astronomy? Among these HA.
No objections to the Quark! But it would be yet another "technology"?
"Details to be Finaliased" (lol) -- But I think will give THIS "a go"... 

[Peter Drew]

Ready to help if needed.