Field flattener and newtonian?

[Demon]

I get backfocus distance to camera, however this question is really about the other side and more so in relation to the secondary and focuser being where it needs to go???

Also I appreciate imaging newts have larger secondary mirrors for the larger chip size compared to pupil size outside the focuser.

So where do the field flateners need to go in relation to the secondary and focuser?

If a camera is positioned at the correct distance from the secondary without a field flattener, then if a field flattener (with no affect on focal ratio) is added, firstly would the camera stay in the same place in relation to secondary?

If a field flattened requiring 55mm backfocus was required it would need to go 55mm in front of camera, so would you need a secondary and focuser that allows the camera to be more than 55mm past the focuser?

Or is it possible with some focusers to mount field flattener on the secondary side of the focuser?

I appreciate other aspects including filter wheel etc but not getting into that yet as just want to understand first please.

I need to get a new secondary and want to make sure it's big enough but small as possible to take a flattener and all works with focuser.

I couldn't find what I was after searching, so would appreciate your expertise, hope this makes sense, thanks.

 

[Demon]

Realised part of it I think - so it typically screws on front of camera with spacers and looks to slide inside 2" focuser. So doesn't need extra space between focuser and camera.

However any filter wheel obviously won't fit inside focuser, so part of backfocus distance will have to stick out of focuser for filter wheel.

I'm guessing the field flattener doesn't change distance from secondary to camera if it doesn't change focal ratio.

So I need secondary mirror that gets camera enough past focuser for larger gear like filter wheel... Need to make sure I get simple field flattener that fits in as some look to have adjusters that won't fit...

Interesting to see different sizes of secondaries from various manufacturers, some use same size in 8/10/12" f4 versions presumably to reduce cost so 8" would likely get camera furthest out and 12" may struggle to fit large gear like filter wheel between...

Edited June 22, 2022 by Demon

[paul]

For Newts its coma correction you need. Most CCs have 55mm backfocus which most commercial camera+filterwheel combinations can accommodate - that may be a challenge for a DIY system.

 

The backfocus distance is set for the best star shapes. Some CC designs specify the optimal backfocus per scope focal length. Most CCs change (shorten) the distance of the camera to secondary. For my old 250mm newt I had to swap out the focuser to get a low profile one, I could not achieve focus otherwise. That was with the Skywatcher CC (x0.95) and the Lacerta KomakorrF4 CC( x1.0).

 

One thing to note - even though my 150mm newt was advertised as suitable for astro-imaging it wasn't perfect. At focus with a CC fitted the focus tube protruded into the light path by 18mm. I hacksawed that bit off to remove the D shaped stars.

 

The larger secondaries for dedicated imaging scopes allow for shorter tubes and to push the camera further away from the OTA to avoid both the issues I mentioned.

Edited June 23, 2022 by paul

[Demon]

Thanks Paul,

Not sure why your reply didn't pop up on my messages so apologies for late reply.

I think I've got my head around it, was mainly where the cc's fit and if they alter the optical distances. The main thing I've needed to do is draw it out in relation to my primary and secondary. I thought that my secondary would be small and I finally drew it and was right.

So I've been stopping down my scope by moving secondary to close to primary.

I just ordered a new secondary, both new so no dinks and going from 63 to 76.

The larger ones are a good bit more money but hoping to work with my small diagonal camera and moving focuser inside tube - should still miss optical path so fingers crossed.

I may try cc in future but for now getting good enough results at the moment.

I need to star test to see just how good my primary is - have no idea as second hand and diy...

Thanks again and appreciated.

 

JUST CALCULATED IT AND IF CORRECT I'VE BEEN STOPPING THE 11.5" F4.5 DOWN TO AN 8" AND EFFECTIVELY AN F6.6 🤐 DIDNT KNOW IT WAS THAT BAD...

Edited June 28, 2022 by Demon

[Demon]

Old 63mm and New 75mm 1/15. Looking forward to trying this - mounting ASAP. Checking calcs on exactly where it needs to go including offset. Also altering focuser inward at same time. Both spider and focuser currently off for work... 

Ps the old secondary had the name B. C. Watts on the back if anyone knows who that may be? 

Edited July 1, 2022 by Demon

[Demon]

Well I've got the new secondary siliconed and back on the scope, I also managed to move my whole focuser in about an inch and for the first time I can just see the whole of my primary. The distances I calculated all seam to work. I then collimated using the full process - making sure secondary is centred both under the focuser and then over the primary with appropriate offset - I think it's the first time I've ever truly managed to collimate it properly - not just using the laser but also initially with the cheshire. I got focus on the moon with approximately 30mm travel left on the focuser, so still a chance of fitting filter wheel. It was too windy, late and light to get on stars but I have a feeling I will now need a 55mm coma corrector. Pretty sure I know why I didn't much need a coma corrector before - was that I was stopping down to 8" and f6. 6 - will see but likely what I need to buy next. Also downloaded all the big narrower field plate solving index files to obsy mini pc. Well chuffed with how it's gone and going...can't wait for next time on galaxy...finger crossed...

[Demon]

So I'm not getting focus as I want...

So did I collimate it badly...am I expecting too much...bad seeing...bad optics...had thermals in tube...

Lots to start on next...need to try and diagnose...star testing???

Just to clarify I'm not talking specifically about stars around edge but all in centre also...

Might stick a Peltier on back of primary, change fans around...just when I thought it was getting there...arghhhhh

Edited July 10, 2022 by Demon

[paul]

Did you run out of focuser in travel? 

 

Collimating badly will affect the out of focus star shapes. Is the spider shadow  central to the out of focus star? it should be. Note that stars at the edge of view may not have cir ular out of focus shapes. Also bad collimation should not stop you achieving focus. 

 

You should at least get good focus in the centre, if the stars at the edge are not points then micro adjusting backfocus should make an improvement. 

 

If you achieved focus on the moon that should be good enough. 

[Demon]

Thanks Paul, I'm really pulling my hair out so help and suggestions greatly appreciated...

I've still got travel on focuser, no coma corrector so backfocus not part of equation. But as you say only thinking of centre stars at the moment.

I need to do out of focus tests however from what I've seen its just small raindrops like usual out of focus, I need to do proper testing, but the closest they come to pin points is more like small blobs. I have no real experience at this small field so worrying I'm just expecting too much and at limits of seeing.

It's also been very warm and also thinking mirrors are likely still too warm. Primary is 40mm and secondary is 20I'm thick. Although fans had been on for a few hours... But will order some temp sensors...

I am also worrying sharpcap and apt aren't playing ball and may be confusing all even more. Sharpcap was at top gain while apt was at other end...

How do you find heat effect your focus on your newt?

You've obviously been looking at collimating so it's sounds high on your list? Had many problems?

The other confusion with my rig is that it is fully adjustable, so focuser can be slid down truss, as can secondary seperately - Not shop bought where they are fixed in place. However first thing I did was to get sec under focuser...

Thanks and all suggestions greatly appreciated...

[paul]

What does an image of Vega look like, how are the spikes? Did you try a bahtinov mask?

Can you post an image?

 

Just talking in general terms: Stars only appear as single pixels when the star is under sampled - e.g. when the pixel size is bigger than the airy disc of the star - that's why short fl images look crunchy/sharp. At your focal length/aperture the pixel and star airy-disc are comparable.  Throw in guiding and atmosphere  you will always have stars covering several pixels.

 

The quality of the primary will obviously influence the outcome too.

[paul]

If you have used a Cheshire and the primary and secondaries are lined up then your collimation will be close to optimal. If out of focus stars in the centre still look like rain drops then it is some other bit of glass is out of collimation - but not the primary and secondary mirrors IMO. Do you have any filter fitted? 

[Demon]

I've been reading up on star testing so just waiting for an opportunity. I'm going to try and video focusing from one side to other side of focus or at least taking images at the different focuser positions. There's no other glass just the two mirrors. I definitely need to do it more methodically, I got a good pattern with my big bahtinov mask although thinking about it I cant remember after removing it getting any spikes from the spider... It kind of missed that bit out...which I believe they only show when close or at focus...

Also thinking after the other tests I can try putting my guide cam in just in case it tells me something.

I need to download some of the images I got and put them on But even after the raindrops got to their smallest, they were still big\many pixels compared to what I was expecting.

The seeing was pretty good although it's a 292mm primary and 9mm camera so a pretty small field...0.4 arcsec/pixel which I know is smaller than typ 1/pixel but stars still covering many pixels - will try and measure - images still in obsy...will retrieve them today...

I also need to check out the camera and software as something wasn't right there...

Your comments are greatly appreciated, I'm desperate to get this fixed/understand what I'm doing wrong. Thanks very much again.

[Demon]

Here's my star test... any comments greatly appreciated please.

I'm going to try and read up what this means, but looking again at how I did my collimation I think theres plenty of room for improvement - much more potential for errors on my scope. I'm going to stick a small lens on one of my cameras and follow astrosheds advanced newtonian collimation video using sharpcap and overlayed circles...similar I believe to Pauls PiCamCollimator principle...

I should also say this was a quick test early this evening with plenty of ambient light and the mirrors will have still been pretty warm from the day...

p.s. any ideas why I'd get light in the centre when out of focus? stray light getting through hole in material around truss...

 

Oops just rechecked video playback next day and it must have recoded badly, will change when I get a chance...

Edited July 14, 2022 by Demon

[paul]

TBH the video doesn't really show an issue of concern - it looks pretty good, but having the bright star more central when recording will allow you to better use the spider shadow to judge collimation.

 

Bright stars will always look like blobs rather than points when over exposed enough to see spikes in real time.

 

You have a lot or hot air circulating which for video is going to break up your star shapes frame by frame, that makes it impossible to critique. Integrating several frames or doing a 0.1 - 5 second DLSR exposure will show what the average star shape is and if there is any real distortion.

 

You seem to be close to good focus at 0.04 in the video, there is a hint that you have an obstruction in the light path positioned at about 7-8 'o'-clock which makes the star look slightly D shaped. But I would expect to see  its shadow in or out of focus so its probably just turbulence. 

 

If stars are really not circular at focus and you have accounted for obstructions - (for a long exposure image) then perhaps you have pinched optics (e.g primary mirror clips too tight or actual optical aberrations in the mirror figuring.

Optical aberrations: how to fix errors in your telescope | BBC Sky at Night Magazine

 

 

 

 

 

Edited July 14, 2022 by paul

[paul]

I have never seen a central spot before but I think that is a secondary star image resulting from reflection off the back of the secondary mirror (some light passing through the silvering and reflecting off the back surface) and by coincidence it is focused when the main image is not (as you might expect). It is significantly weaker signal and if it is a secondary reflection it will be out of focus normally. The fact that the spot is brighter on one side of the focus supports that theory.

 

 

Edited July 14, 2022 by paul

[Demon]

Thanks very much for your comments again Paul.

I'm slightly less upset that it all just didn't work and after reading up and seeing similar star tests on YouTube I'm also somewhat relieved that it doesn't look too far off and no glaring issues with the optics.

As you say I think it's mainly just a big thick mirror doing what it's supposed to, lots of light and even over short exposures plus some atmosphere, wobble and heat distortion on a narrow field but the stars are there.

All of those I can work on, getting ready for the longer cooler nights.

My guide cam talked to phd2 fine last night so hopefully that stays working.

I managed to find a little lens and c spacer and got it all rigged up last night - ready for collimating via video camera with circles on the display after work today.

P.s. I spotted theres a trial collimation feature in sharpcap for testing how well it has been done - may see what that does after it time.

I have a feeling apt/sharpcap may have also been stretching the images but not sure, will read up and check the settings.

There may be a little obstruction as I moved the focuser mounting plate onto the inside of the truss to get more focuser travel, however it's still just clear of the optics i.e. Oversized truss... I can live with that and if this all works get a better shorter focuser.

I've got some temp monitors coming so will get a better idea of what the temp differentials are.

Although not over yet, your help has been really appreciated thank you.

[Demon]

I got back to it late afternoon and this evening. I replaced my cheshire with the camera and lens and what a difference. I don't think I'll ever use either a laser or cheshire collimator again.

My gear is a country mile out, absolutely shocking, how it managed what looked like a good star test I have no idea. Nothing was centred and all was way off. Even rotating the camera shows me my adapter and focuser are not centering the camera by a significant amount. So even when I got the view in the centre of the secondary if I rotated the camera I could move off the entire secondary mirror. Seeing the circles in sharp cap and being able to zoom is fantastic, albeit scarely enlightening.

I eventually remembered a lens came with my asi178mm so my hope was to fit that fix the camera rotation and collimate with that - as this is my imaging camera so hopefully ignore rotation issues. I got the secondary adjusted centrally to the focuser, however when i adjusted the tilt of the secondary to align with the primary it got knocked out again. Instead of realigning the secondary to focuser I kept on the primary and adjusted so that the spider reflection was centred. When I got on a star it looked like a comet. Now I know what I'm doing I'll do it better next chance I get. This method using the camera is so much better and think I have a chance now.

Ps when I got on a star I did centre it precisely and zoom in.

Edited July 15, 2022 by Demon