Solar mod ERF suggestions

[spaceboy]

OK so donor scope has been acquired but sadly the original plan for a 110mm Baader D-ERF has gone bosh due to the internal dimensions of the 120mm scope being only 113mm. Not only is an externally mounted Baader too expensive it is also pointless as I will only be using a B600 filter.

 

Any suggestions?? I want to steer clear of absorption filter if possible so daystar and the majority of lunts alternatives are not an option unless a full aperture Lunt would work better than an internally mounted Baader D-ERF ???? I am of course open to any suggestions of getting the 110mm filter to some how fit ? I had considered some how mounting it to the scopes baffle although an ERF isn't cheap so I'd rather doing a bodge job.

[JamesF]

The problem is that the cell for the D-ERF is too large to fit inside the OTA even though the D-ERF itself is small enough?

James

[Stu]

Why not go with an internally mounted 75mm? Much cheaper and in many ways neater as it is protected inside the scope. Or do you already have the 110?

[Macavity]

[I'm] "Boring for Britain" on this Frankenscope thing. lol
But glad you picked up my thoughts over on Solarchat.

https://solarchatforum.com/viewtopic.php?f=9&t=23650#p213125

Whatever my erudite reasons, I found it easier to mount
the ERF filter externally on this, the prototype Version 1.0! 

As a postscript, I am reminded of my poor old DAD... up the top
of a ladder struggling with a pane of glass. An elderly neighbour
asking him if he had "remembered to allow for expansion..."? 

 

[spaceboy]

18 hours ago, JamesF said:

The problem is that the cell for the D-ERF is too large to fit inside the OTA even though the D-ERF itself is small enough?

James

That's correct James. I wanted the 110mm because it puts the filter as close to the objective as possible but sadly the ota itself is only 113mm internal diameter. My problem here is the Baader's next offering is the 90mm which is going to send it a considerable way down the OTA so as to not clip aperture. I don't want to do this for several reasons.

1) Call me fussy but I don't want to hacking the OTA, and I was only ever going to feed the cell down from the objective end with maybe one small hole in the OTA for a fixing screw to ensure the cell remained in situe.

2) I have been informed that an ERF should be no further than 1/3 the distance from the objective for best results anyway.

3) Again I didn't want to start removing and re-installing more than one baffle if possible

4) Obviously performance is also a factor, and I wanted to get the best results I could for my budget. I have experience with hot summer days and the havoc it can play with internal heat. While the Baader is highly rated by many if an alternative that is equal in performance can be used that is a more ideal size then I would be willing to go with that. The trouble is here many claim to be energy rejection filters but are often are just absorption filters.

18 hours ago, Stu said:

Why not go with an internally mounted 75mm? Much cheaper and in many ways neater as it is protected inside the scope. Or do you already have the 110?

As above mate.

18 hours ago, Macavity said:

[I'm] "Boring for Britain" on this Frankenscope thing. lol
But glad you picked up my thoughts over on Solarchat.

https://solarchatforum.com/viewtopic.php?f=9&t=23650#p213125

Whatever my erudite reasons, I found it easier to mount
the ERF filter externally on this, the prototype Version 1.0! 

As a postscript, I am reminded of my poor old DAD... up the top
of a ladder struggling with a pane of glass. An elderly neighbour
asking him if he had "remembered to allow for expansion..."? 

 

Externally for me mate is another £250, 15mm of which would go to waste  

And I was hoping joining solarchat would have given me more feedback than it has TBH. Seems great if PST mod advice is needed but Lunt mods are thin on the ground even on a dedicated solar site.

My hope was the OTA will expand at a similar rate to the filter not that the filter should get too warm anyway if it's doing its job correctly but a good point all the same mate. Although again the 110mm filter seems a poor choice. Ideally something around 100mm seems the way to go. Unfortunately the lunt filter you linked to on solarchat is only 3.5mm (possible absorption filter) thick which if I have researched correctly, could possibly with heat / expansion create optical errors.

 

[Peter Drew]

I've built dozens of mods from 60mm to 220mm aperture. Although without exception PST mods, the ERF considerations would be the same with a Lunt mod. I've tried external and internal ERF configurations and have had no observable problems to date from internal heating. On a good day the images are always good and on a bad day they are bad. I would certainly recommend the Baader D-ERF variety, not only are they of excellent quality but the reflective aspect reduces internal heat significantly. A 90mm D-ERF is perfectly adequate for mods up to 150mm aperture as they can be positioned further downstream without problem. I once used a 125mm Daystar 3.5mm thick non reflective ERF externally and it gave very satisfactory results.  

[JamesF]

How about trapping the D-ERF between two separate rings, perhaps with some shims (or even just a few "teeth" on the rings) to centre it in the OTA and stop it rattling?  If the rings were, say, 10mm diameter (so 3.5mm overlap on the D-ERF all the way round) then I don't think they'd intrude on the light cone if it were 200mm to 300mm from the objective which I think should meet your requirement of not being more than a third of the way down the OTA?

Or are you no longer going down the route of 3D printing your cell?

James

[Macavity]

My apologies if there is something I'm not "getting" here, but an
external ERF doesn't have to MATCH the objective diameter... 
Better if it doesn't (probably). Stopping down may help things.

Solar Chat can be a bit quiet? Some "don't suffer fools gladly"? 
Occasionally "language barriers" lead to perceived abruptness?) 
Or maybe... As someone joked with me, "Stick with US, son..."! lol

A while back, I started experimenting with the ubiquitious (DIY)
Hole Saws! You can make all sorts of weird & wonderful things
with "Wood Technology" --- Odd bits of PVC pipe and Duct etc. 

 

[spaceboy]

7 hours ago, JamesF said:

How about trapping the D-ERF between two separate rings, perhaps with some shims (or even just a few "teeth" on the rings) to centre it in the OTA and stop it rattling?  If the rings were, say, 10mm diameter (so 3.5mm overlap on the D-ERF all the way round) then I don't think they'd intrude on the light cone if it were 200mm to 300mm from the objective which I think should meet your requirement of not being more than a third of the way down the OTA?

Or are you no longer going down the route of 3D printing your cell?

James

Sandwich came to mind earlier today and if I could get the same top baffle as is already in the scope I think I would go with that method. Some sponge type draft proofer to allow tilt on the ERF and flock to offer some sort of protection to the filter from the ota wall and baffles. I need only put the ERF in then slide a second baffle in and job done. Trouble is finding a baffle. We all know if astroboot ain't got it there's no getting it. That said I doubt even if spares were regularly available a baffle may still be a tall ask.

3D printing was the preferred way to go but I would be relying on a fellow member not having a printer myself. Mention solar mod and a lot of people have concerns of people blinding themselves and them not wanting to have that on their conscious. I know it is only plastic but I had thought there would be no real heat involved being so close to the objective and rejecting any energy anyway but I can understand peoples concern as solar does come with its risks.

8 hours ago, Peter Drew said:

I've built dozens of mods from 60mm to 220mm aperture. Although without exception PST mods, the ERF considerations would be the same with a Lunt mod. I've tried external and internal ERF configurations and have had no observable problems to date from internal heating. On a good day the images are always good and on a bad day they are bad. I would certainly recommend the Baader D-ERF variety, not only are they of excellent quality but the reflective aspect reduces internal heat significantly. A 90mm D-ERF is perfectly adequate for mods up to 150mm aperture as they can be positioned further downstream without problem. I once used a 125mm Daystar 3.5mm thick non reflective ERF externally and it gave very satisfactory results.  

Pete do you think a full aperture external "low quality" filter would outperform the 90mm (80 ish clear) Baader internally mounted half way down the OTA. Also I read that the Daystar are configured for f/30

7 hours ago, Macavity said:

My apologies if there is something I'm not "getting" here, but an
external ERF doesn't have to MATCH the objective diameter... 
Better if it doesn't (probably). Stopping down may help things.

 

The omni xlt 120 is f8.33 which is a perfect match to the LS60. Stopping it down would lengthen the focal length not to mention wasting aperture. That would have defeated me letting my PST / TAL mod go if I'm going back to 100mm

 

[Macavity]

50 minutes ago, spaceboy said:

The omni xlt 120 is f8.33 which is a perfect match to the LS60. Stopping it down would lengthen the focal length not to mention wasting aperture. That would have defeated me letting my PST / TAL mod go if I'm going back to 100mm

Ah! I hadn't been aware which particular Lunt you were using in this. 
So you have a BF1200 blocking filter too (maybe)? Just for interest.

Aside: I have only noted one person mentioning the temperature
rise of an ERF. It went "off the clock" at 50 Deg C?!? Easy-ish to
calculate an order of magnitude maybe. I was reckoning on about
10 Watts heat for a typical sized solar scope. But not insubstantial. 

[spaceboy]

58 minutes ago, Macavity said:

Ah! I hadn't been aware which particular Lunt you were using in this. 
So you have a BF1200 blocking filter too (maybe)? Just for interest.

Sorry mate. I'd mentioned in the thread you were following over on solar chat I had the LS60tha so naturally assumed you'd already knew this.

And as for the B1200 .......

On ‎16‎/‎10‎/‎2017 at 13:59, spaceboy said:

Not only is an externally mounted Baader too expensive it is also pointless as I will only be using a B600 filter.

This is why I want to avoid an external filter as I hope to at a later date upgrade to a B1200 but want to keep the internally mounted erf as large as possible to keep it close to the objective to send as much energy back out of the ota as possible. The problem is while the Baader has a 90mm erf, once you drop it in a cell your lucky to get 80-85mm clear aperture so it pushes it even further down the ota. I'm assuming you also have to increase tilt further on the erf the closer it gets to the etalon to avoid reflections and ghosting ???

[Peter Drew]

@Spaceboy. I've had such good results with the Baader ERF's that I would prefer to use them internally or externally rather than another make option, particularly because of the reflectivity. It is the mica etalon in Daystar instruments that operate at F30 for best results, the ERF probably has little to do with it other than perhaps slightly less attenuation. Stopping down the donor telescope doesn't alter the focal length, just the focal ratio, the Lunt etalon will have been configured for around F7, significant deviation from this would likely result in lesser performance as the field angle changes. 

[JamesF]

2 hours ago, spaceboy said:

Sandwich came to mind earlier today and if I could get the same top baffle as is already in the scope I think I would go with that method. Some sponge type draft proofer to allow tilt on the ERF and flock to offer some sort of protection to the filter from the ota wall and baffles. I need only put the ERF in then slide a second baffle in and job done. Trouble is finding a baffle. We all know if astroboot ain't got it there's no getting it. That said I doubt even if spares were regularly available a baffle may still be a tall ask.

See if you can get a local engineering company to cut you one from aluminium and spray it with black bbq paint?

James

[Macavity]

11 hours ago, Peter Drew said:

the Lunt etalon will have been configured for around F7...

Or indeed f/8.3 [teasing]!  But to reiterate a great point Pete made:
It may be worthwhile trying NOT tilting the ERF first? Easier anyway!

I thought it might be interesting to show just how "bad" using my B600
blocking filter was my f=600mm ST120 Frankenscope was... Or not?
Neither image is particularly GOOD (Clouds... Bad focus / tuning. lol)

With the native *unmodified* f=350mm Lunt50, I have now centred 
the "sweet spot" quite well! (And not without some past efforts...)



Getting close to the "quoted limit" (f = 600mm) of the B600 gives this! 

I THINK it intriguing more than anything bad? I sense this is an example of the
oft-vaunted "Vignetting"? I suspect it (probably!) demonstrates why it becomes
necessary to Barlow my Frankenscope 2x to restore my nice symmetric image. 

To get a decent Full Disk I'd need as B1200 + a Bigger (obviously) Camera Chip!
The nice thing is that one gets "more for less", if not "something for nothing"?  

Note my deliberate caution in all the above. It's good to talk... Learn stuff etc. 

[Peter Drew]

Tilting the ERF is primarily for mitigating reflection effects. I don't think it has any other affect as the optical quality is high.   

[spaceboy]

7 hours ago, Macavity said:

Or indeed f/8.3 [teasing]!  But to reiterate a great point Pete made:
It may be worthwhile trying NOT tilting the ERF first? Easier anyway!

I thought it might be interesting to show just how "bad" using my B600
blocking filter was my f=600mm ST120 Frankenscope was... Or not?
Neither image is particularly GOOD (Clouds... Bad focus / tuning. lol)

With the native *unmodified* f=350mm Lunt50, I have now centred 
the "sweet spot" quite well! (And not without some past efforts...)



Getting close to the "quoted limit" (f = 600mm) of the B600 gives this! 

I THINK it intriguing more than anything bad? I sense this is an example of the
oft-vaunted "Vignetting"? I suspect it (probably!) demonstrates why it becomes
necessary to Barlow my Frankenscope 2x to restore my nice symmetric image. 

To get a decent Full Disk I'd need as B1200 + a Bigger (obviously) Camera Chip!
The nice thing is that one gets "more for less", if not "something for nothing"?  

Note my deliberate caution in all the above. It's good to talk... Learn stuff etc. 

Some good results there. I don't think I'll even bother with a B1200 if your getting that much on a sensor. In the PST mod I was lucky to get 60% of the disc at 24mm (x41). The lunt mod will be used at higher mags anyway and the sweet spot is created by the etalon so nothing to be gained by the larger BF for me visually. Full disc will still be available to the LS60 in it's native form.

 

19 hours ago, JamesF said:

See if you can get a local engineering company to cut you one from aluminium and spray it with black bbq paint?

James

Its all gone Pete Tong anyway James. I was so wrapped up in the idea of using a 110mm filter that I didn't stop to think the Baader is 5mm thick. While the filter could be sandwiched between two baffles square on, no sooner you introduce tilt (suggested 3mm) the filter will in fact become wider. With so little room already I doubt the scopes 113mm inside diameter will allow for it.

As I want to go with a baader this means either externally mounted 135mm at the same cost of a used Quark or a, 1/2 way down the ota internally mounted 90mm.

 

I think at least the "Solar mod ERF suggestions" question has been answered.  Thanks all

Now it's a case of either making or purchasing an affordable cell. Any suggestions .............???

[spaceboy]

I like the look of this  but the link from the thread to it is dead http://www.thorlabs....tNumber=LMR75/M

 

 

EDIT: found them https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1433 but sadly none for 90mm filters

[JamesF]

1 hour ago, spaceboy said:

Its all gone Pete Tong anyway James. I was so wrapped up in the idea of using a 110mm filter that I didn't stop to think the Baader is 5mm thick. While the filter could be sandwiched between two baffles square on, no sooner you introduce tilt (suggested 3mm) the filter will in fact become wider. With so little room already I doubt the scopes 113mm inside diameter will allow for it.

I think there's a possibility it will still work.  I don't think that tilting the filter can't increase the size by more than the length of the diagonal of the cross section of the filter taken at a diameter.  I hope that makes sense

That maximum size should therefore be the square root of the sum of the squares of the thickness of the filter and its diameter.  That is, the square root of ( 5 x 5 + 110 x 110 ), which is not very much more than the diameter anyhow (less than 110.2mm).

Either that or I have my maths very wrong   Given the day I've had, I'd not guarantee otherwise...

James

[spaceboy]

The trouble is a 110mm filter in a 113mm OTA gives me less than a 1mm gap around the circumference of the filter which I have to allow a little for expansion / play. When you add to this the tilt my guess is the filter is going to end up wedged in the OTA at some point. At the money these things are I have to make sure I get it right.

[JamesF]

Might be worth dropping Baader an email to see if they can tell you the coefficient of thermal expansion for the filter.  I'd guess that it's very small because any significant expansion would be likely to damage the coatings and/or ruin the optical properties of the filter.  Quite possibly it's down in the millionths per degree C range (pure quartz is about half a millionth per degree, standard glass seems to be just under ten millionths per degree.  Even at ten millionths per degree you'd still only get an increase in size of less than a quarter of a millimetre going from freezing to 200C (and at the same time the steel forming the tube would expand, allowing more room for the filter), so you'd still end up with a maximum dimension of 110.5mm.

Agreed you may need a little "wiggle room" to get it to fit.  You could try making a cradle from some thin material to see if it can be lowered into the position you're after or if it's too much pain?

Of course it's easy for me to sit here at my desk and experiment with your money   I'd hate for you to give up on your perceived optimal solution if it is genuinely feasible though.

James

[spaceboy]

I think getting the filter in is easy enough. Simply hold the filter with white gloves and get someone to lower the ota over my hand until it reaches the baffle then flip the ota over.

I think the best thing to do is try to find some cardboard the right thickness and cut out a 110mm circle and see how it fits the OTA. This will highlight any deviations in the OTA width along with giving me a more practical representation of how things will or won't work.

What ever the case I think it is safe to say the general consensus is to go with the Baader D-ERF no matter what size I go with, so that's that box ticked  Now it's a case of figuring what size will work and if I can find an afford way to have an adapter made for the lunt to attach to the XLT

[Peter Drew]

@Spaceboy. A 90mm Baader D-ERF really is the largest ERF needed for a mod up to 150mm aperture.   

[spaceboy]

I'm only planning on doing it once though Pete so saving the bit extra towards the larger ERF is ok as long as there are gains to be had...... and IT FITS that is??  I think the cells and adapters are my biggest gripe as far as potential cost goes as if my lathe was a tad bit bigger I could have done the majority of it myself, making any necessary changes along the way. Instead now I need to be better prepared and make sure everything is correct before I commit someone else time to it  

I appreciate a smaller filter will do the job but I also figured the larger 110mm will have everything back out the objective before any possibility of thermals or otherwise gathering and with it so close to the objective I shouldn't have any concerns of the lens getting any warmer than it need to do sitting in the midday sun.  It is my understanding the difference in views between an external filter over an internal one is an obvious one and my logic was, if internal was the only choice, the closer I get to the objective the better the views should hopefully be for it?? Unless I am missing something that is ???

[Peter Drew]

2 hours ago, spaceboy said:

I'm only planning on doing it once though Pete so saving the bit extra towards the larger ERF is ok as long as there are gains to be had...... and IT FITS that is??  I think the cells and adapters are my biggest gripe as far as potential cost goes as if my lathe was a tad bit bigger I could have done the majority of it myself, making any necessary changes along the way. Instead now I need to be better prepared and make sure everything is correct before I commit someone else time to it  

I appreciate a smaller filter will do the job but I also figured the larger 110mm will have everything back out the objective before any possibility of thermals or otherwise gathering and with it so close to the objective I shouldn't have any concerns of the lens getting any warmer than it need to do sitting in the midday sun.  It is my understanding the difference in views between an external filter over an internal one is an obvious one and my logic was, if internal was the only choice, the closer I get to the objective the better the views should hopefully be for it?? Unless I am missing something that is ???

I fully understand your concerns and would not have mentioned sub aperture ERF's if I hadn't already made several successful units. I have never noticed visually any difference in performance comparing external to internal ERF's, imaging might possibly be different. The main considerations seem to be seeing conditions and the quality of the individual donor items. 

[spaceboy]

OK so as long as people agree there would be no real and significant advantage in having the larger 110mm D-ERF I shall be taking Peter's knowledge in good measure and going with the 90mm filter now I have funds in place.

My next query is would an internal cell mount be ok in wood and what wood type would be best. I can get one made in ally when having the OTA / lunt adapter made but my thinking is wood can easily be painted matt black, it's softer material so more shock absorbing if the unthinkable might happen . My thought is that wood has a degree of friction too it so could (if within tolerances) in my mind at least be push fit with no need to fix it in place with a bolt. I had looked in to black Delrin as an alternative but this is stupidly expensive.

On another note. OTA have to be shorten for the solar scope to achieve the correct focal distance. Does this mean baffles have to be adjusted to suit ? I know the focuser end will likely need to be removed but what about the middle one or would you just remove that altogether and just use the celled ERF in it's place??