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Hi

I'm hoping a few here might have seen my problems in the past and know of suitable solutions.

I've been analysing my telescope woes and finally managed to identify the order of problems affecting me.  I'm measuring performance in imaging terms, so I don't just care about resolution, but also about ability to hold focus, refrain from dewing up suffering image shift etc.  Also my OTA is fairly unique, a Windowed Newtonian.  Whilst it has performed fairly well there are drawbacks which I'm trying to fix.

Problem No.1:  Dewing. 

The window is isolated from the tube wall by about 10mm of rubber.  A dew heater is going to have a jolly hard time so I've never bothered, preferring dew shields and hair dryers...  additionally a 14" diameter window is probably going to respond differently to dew heaters compared to say an 8" SCT.

Ideally I want to unseal the tube, pump filtered air through the whole thing, around the sides of the window, thus eliminating the hair dryer and maximising air temperature homogeneity.   To do that I need a flat plate across the bottom, with fan holes and so on, but that's extra weight so I'm thinking of replacing the rather weighty cell, then I can support the cell parts on a flat plate thus combining parts so saving weight.

I'm thinking something along these lines but not in wood due to water absorption (not mine, just a helpful pic I found on the net to illustrate):

cell.jpg

So QUESTION 1:  what could I make it out of, and where do I get that material?  (Mirror is 300mm x 50mm thick)..  the only thing I've seen commercially is the Orion Optics mirror cells but they're £280+vat..   PS whilst I'm pretty handy at making things, I don't have a milling machine or lathe, just hand tools and a pillar drill.

QUESTION 2:  Window Cell.  Given I'm venting the tube from the bottom I need to change the window cell to allow air past it, any ideas?..  just supporting the window on three blocks won't do as the tube will warp allowing the window to drop through, I've been thinking of using 6 supports in 3 pairs, each pair fixed to the tube by a mid point pivot, but that's going to restrict airflow along the length of each pair of supports, unless someone can suggest a way of getting the air past them

Problem No.2:  Holding Focus. 

Analysing my subs I'm getting roughly 100ppm tube expansion over an imaging session..I'm guessing that's over a 5C temperature change which is only 20ppm/C which is pretty good..   but not good enough (it spreads my stars over 3x the area they should be losing sensitivity)  The real solution is a carbon fibre tube but that's serious money.  An alternative would be some kind of autofocus solution, something that could check and correct focus between subs.  I have a JMI crayford with motor drive on the scope already, but it doesn't have a position counter of any sort..   so QUESTION 3: Is there a way to do autofocus as part of an imaging session?   is this Maxim territory and what other hardware am I looking at?..   again without breaking the bank.

Problem No.3:  Worm drive unevenness

I know I have a certain degree of high frequency thrashing going on with the worm, hardly a surprise with a 40 year old mount.   But it is a bronze worm wheel and stainless steel worm, any worth in polishing this (I'm assuming using something somewhat smoother than silvo as the polish)

I'm still working on the main problem I've had:   no obsy, and it's really taking all my time at the moment as I'm making something to last longer than I do:

here's a photo of the latest:

Frame.JPG

That's the south face, nearest beam is the sill.  This was just making sure all the joints would fit properly.   Foundations and brickwork are done.

anyway thanks in advance for any suggestions.

Derek

Edited by rfdesigner
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No takers?

hmm..  an update on my own efforts

Q1: Materials: I've looked at nylon and carbon fibre, both have drawbacks...  nylon is much easier to work with but will end up heavier and the last thing I need is extra weight, CF is a pain to fabricate.. I'm after lightweight but workable, any other ideas?

Q2: Venting: I've realised I have another option, keep it sealed, but install a fan to keep the air in the tube well mixed..  thus I accept the air could be somewhat hotter than ambient, but at least it's all the same. (within 1C)

Q3: Autofocus: the more look into this the more I realise I need a position sensor, a rotational encoder on the focusser knob looks like the way to go..   does no one do autofocus?

Derek

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Hi Derek,

I am sorry that you have not had any replies to your issues. Unfortunately I am not in a position to provide any assistance. Hopefully by me posting you topic will jump to the top of the pile and someone will pick it up give you the guidance you require.

Ian

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My current favourite program, Sequence Generator pro, will automate focusing during imaging runss. You can set it to re-focus every x number of subs, on filter changes or on temperature changes.

With regards to the worm, are you not auto-guiding? Auto-guiding will correct for periodic error in the worm, especially if it's nice and slow. You could also investigate PEC training and correction?

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You won't need a mill or a lathe to make a mirror cell. I built mine from aluminium and mild Steel :)

I could do an ally one, I was concerned about the weight though.. especially if I have a bottom plate that is solid except for the fan holes, I suppose I could use a thin frame then bolt on a thin sheet to seal it.

My current favourite program, Sequence Generator pro, will automate focusing during imaging runss. You can set it to re-focus every x number of subs, on filter changes or on temperature changes.

With regards to the worm, are you not auto-guiding? Auto-guiding will correct for periodic error in the worm, especially if it's nice and slow. You could also investigate PEC training and correction?

I'm auto guiding but the worm's a bit rough and my focus is drifting, so where I should be getting 2 arc sec FWHM round stars, I can get nearer 4"x3", which is about 3" of defocus and about 2" of drive "thrashing" (I have had 2"x2" but only at very high declinations, near the equator it's pretty bad.  I've tried programming in a PEC algorithum, but I only have a 4 minute main frequency, no faster ones as my steppers drive the worms directly, all the rest of the thrashing is worm/wheel roughness.

I've had a quick look at Sequence Generator Pro.. does it require a position sensor on the focuser?..  it seems to do pretty much everything else, nice program.

Derek

Edited by rfdesigner
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I could do an ally one, I was concerned about the weight though.. especially if I have a bottom plate that is solid except for the fan holes, I suppose I could use a thin frame then bolt on a thin sheet to seal it.

Derek

When you say you're concerned about weight, are you referring to not enough weight of too much? If too much weight you could remove considerable material from the back plate without losing strength Derek :)

I actually find the ally parts of my cell are pretty light, it's the mild steel that quickly adds considerable weight.

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I've had a quick look at Sequence Generator Pro.. does it require a position sensor on the focuser?..  it seems to do pretty much everything else, nice program.

Derek,

No, but a stepper motor is probably the best solution. it works by taking an image and measuring the Half Flux Radius of the stars in the whole image.

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When you say you're concerned about weight, are you referring to not enough weight of too much? If too much weight you could remove considerable material from the back plate without losing strength Derek :)

I actually find the ally parts of my cell are pretty light, it's the mild steel that quickly adds considerable weight.

I want to keep weight down.  The primary mirror is 50mm thick and the window is 18mm thick.. being a 12" scope that's a lot of glass.

I'll have to do some stress analysis and find out how much I can get away with before I see noticeable image shift as the scope flips.

Derek,

No, but a stepper motor is probably the best solution. it works by taking an image and measuring the Half Flux Radius of the stars in the whole image.

HFD..  ah.. is it based on the Focusmax software?

The focuser I have is a JMI crayford with the motofocus option.. swapping out the motor for a stepper is as much work as adding some kind of a rotary encoder (for me the mechanics is the hard bit, I find the electronics is relatively easy)

JMI do offer some kind of a position sensor setup, but again I can see the bank account suffering...........       thinking about it though, it'll still be cheaper than a Carbon Fibre tube.  hmmmm.

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Hi

I'm hoping a few here might have seen my problems in the past and know of suitable solutions.

I've been analysing my telescope woes and finally managed to identify the order of problems affecting me.  I'm measuring performance in imaging terms, so I don't just care about resolution, but also about ability to hold focus, refrain from dewing up suffering image shift etc.  Also my OTA is fairly unique, a Windowed Newtonian.  Whilst it has performed fairly well there are drawbacks which I'm trying to fix.

Problem No.1:  Dewing. 

The window is isolated from the tube wall by about 10mm of rubber.  A dew heater is going to have a jolly hard time so I've never bothered, preferring dew shields and hair dryers...  additionally a 14" diameter window is probably going to respond differently to dew heaters compared to say an 8" SCT.

Ideally I want to unseal the tube, pump filtered air through the whole thing, around the sides of the window, thus eliminating the hair dryer and maximising air temperature homogeneity.   To do that I need a flat plate across the bottom, with fan holes and so on, but that's extra weight so I'm thinking of replacing the rather weighty cell, then I can support the cell parts on a flat plate thus combining parts so saving weight.

I'm thinking something along these lines but not in wood due to water absorption (not mine, just a helpful pic I found on the net to illustrate):

cell.jpg

So QUESTION 1:  what could I make it out of, and where do I get that material?  (Mirror is 300mm x 50mm thick)..  the only thing I've seen commercially is the Orion Optics mirror cells but they're £280+vat..   PS whilst I'm pretty handy at making things, I don't have a milling machine or lathe, just hand tools and a pillar drill.

QUESTION 2:  Window Cell.  Given I'm venting the tube from the bottom I need to change the window cell to allow air past it, any ideas?..  just supporting the window on three blocks won't do as the tube will warp allowing the window to drop through, I've been thinking of using 6 supports in 3 pairs, each pair fixed to the tube by a mid point pivot, but that's going to restrict airflow along the length of each pair of supports, unless someone can suggest a way of getting the air past them

Problem No.2:  Holding Focus. 

Analysing my subs I'm getting roughly 100ppm tube expansion over an imaging session..I'm guessing that's over a 5C temperature change which is only 20ppm/C which is pretty good..   but not good enough (it spreads my stars over 3x the area they should be losing sensitivity)  The real solution is a carbon fibre tube but that's serious money.  An alternative would be some kind of autofocus solution, something that could check and correct focus between subs.  I have a JMI crayford with motor drive on the scope already, but it doesn't have a position counter of any sort..   so QUESTION 3: Is there a way to do autofocus as part of an imaging session?   is this Maxim territory and what other hardware am I looking at?..   again without breaking the bank.

Problem No.3:  Worm drive unevenness

I know I have a certain degree of high frequency thrashing going on with the worm, hardly a surprise with a 40 year old mount.   But it is a bronze worm wheel and stainless steel worm, any worth in polishing this (I'm assuming using something somewhat smoother than silvo as the polish)

I'm still working on the main problem I've had:   no obsy, and it's really taking all my time at the moment as I'm making something to last longer than I do:

here's a photo of the latest:

Frame.JPG

That's the south face, nearest beam is the sill.  This was just making sure all the joints would fit properly.   Foundations and brickwork are done.

anyway thanks in advance for any suggestions.

Derek

Hi Derek.

You may or may not know that a mirror cell is probably the most important individual component in a larger telescope apart from the optics! Your mirror as said is 300mm and 50mm thick. When in a telescope and supported think of it as blancmange! It is extremely difficult to design one for a larger mirror but can be done. You need to research something called PLOP if you haven't done so already. This will tell you the mathematically worked out points of support. If it were me I would buy one, - believe me, it would be cheaper!! 

I have just reground a 12" crown and worm wheel however with amazing results! I will do a post on this shortly as this is highly dooable.

Good luck Derek

Regards Harvey

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Harvey

Thanks.. in fact I have played with PLOP..   and with my mirror, due to the thickness I can just about get away with just a 3 point cell (which is just as well as that's what it has at the moment).. but I'll go for a 9 point if I'm making...  the key as I see it is to use enough material to avoid excessive flexure and to make it sufficiently accurately to get the pressure even.  I'm guessing half a dozen pixels of image shift from a meridian flip would be about the right level of stiffness to aim for.

Very good news on the worm and crown..   I will look forward to seeing your post.

Derek

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Hi derek

I have a 10 inch windowed newtonian with some interesting design solutions that address some of your problems ,i also have a 16 inch newtonian .naysmyth cassegrain  with a full thickness mirror ,i can send you some pics of the mirror cell if you like . 

If you give me your pm e-mail i can correspond off site if you wish. 

regards

clive

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