F3.3 reducer - what's wrong?

[RobertI]

I use a Lodestar camera for Video Astronomy usually with my C8 and a F6.3 reducer.

I have seen many members here and on CN getting great results with the C8 and a F3.3 reducer used with a small chip camera like a Lodestar.

So I bought a Meade F3.3 reducer second hand. The recommended lens to chip distance was between 57mm and 61mm depending on what you read so I set up accordingly. My first outing was not good - severe radial elongation of stars in the outer half of the FOV. Tried various spacings but same results.

I tried again yesterday, but carefully collimated the scope first with an eyepiece plus diagonal, then using the camera in its straight through imaging position (reducer + 2" visual back and camera, no diagonal).

The results were the same, again regardless of spacing. I used a star on the edge of the field to analyse the problem - when the central star was in focus the outer star was radially elongated, but when I changed focus the outer star became circular then elogated tangientally (ie: 90 degrees different to before) presumably as it moved either side of focus.

So to address the possible causes:

- Scope not collimated - the scope was collimated ok, I'd be surprised if such severe results were casued by slight mis-collimation

- Lets to chip distance not right - i tried a variety of distances, measuring both from the shoulder of the FR and the centre of the FR with the same effect. And in any case the Meade F3.3 manual indicates that various FRs can be achieved with different spacings and there is no mention of increased distortion to the FOV as a result, so this aspect should not be an issue.

- Elements not orthogonal - i used a sturdy baader 2" to SCT adatptor and a 2" to 1.25" adaptor. It all seems extremely solid and again I'd be surprised if such severe effects were caused by this.

Given that others seem to be getting good results with the exact same setup, it seems the only thing left is a dodgy reducer. I have heard stories of problems, in one case someone said the middle element was reversed, although how someone could determine that I am not sure.

Can anyone HELP? Is there anything left to try?

Thanks

Rob

[michael.h.f.wilkinson]

These F/3.3 reducers were designed for the small chips of way back when. I would expect them to work on a lodestar, indeed. If the error is radially symmetric I would not assume that there is any problem with collimation, but some fault in the lens itself

[ollypenrice]

Yes, your best bet is going to be to try to beg or borrow another example of the reducer. Unless video astronomy has given them another lease of life they are more or less worthless on the market. I gave mine away, in fact. It's very likely that someone has one in the bottom of a box and would let you try it.

Olly

[nytecam]

Hi Rob - I've used my Meade f/3.3 (Japanese) FR successfully on the Lodestar 1/2" sensor with some coma in the extreme corners.

Results with the new SX Ultrastar 2/3" sensor less successful with coma extending around the whole frame edge to the point the use of this camera curtailed.

I too suspect that some element in your FR is miss-placed.

Nytecam

[ngwillym]

Hmm, I'm not sure those spacing are correct. Did your FR come with any other bits - namely a couple of spacer rings and some adapters?I have a Meade F3.3 reducer and the instruction sheet has an optical path that goes FR>T-adapter>spacer>camera. Using the 15mm spacer should give f5 and the 30mm spacer F3.3. So the distance from the FR to the chip is 30mm+depth of t adapter - which is a lot less than 57-30= 27mm? In fact - its nearer 15mm, so chip to FR distance is more like 45mm

[RobertI]

Thanks for the responses folks, very helpful.

Ngwillym - unfortunately the reducer came with no additional parts. I obtained the spacings from a variety of sources including this website which says the spacing should be 57mm. But I have seen the adapters as I have the F3.3 manual and it is confusing. To be honest I have tried pushing the camera much closer in and much further away than the 57mm with no improvement apart from changing FOV so not convinced this is the problem.

TBH I don't think I have anything to lose by reversing the middle element and seeing what happens. If no joy I will then beg for someone to lend me theirs to at least prove it does work with my scope!

I'll keep you posted.

Rob

[Dom543]

Rob,

Is Made in Japan written on it?

There has been documented issues with the non-Japan version.

If the reducer is the Made in Japan version and the elongation is only in one diagonal direction, then there may be an orthogonality issue. Try to rotate the camera. Also, this reducer is not very sensitive to a few millimeters of variations in spacing. If the elongation is in all radial directions, then you probably have the non-Japan version.

--Dom

[Peter Drew]

I have a spare F3.3 I will be happy to lend you if all else fails. 

[RobertI]

Peter - that is extremely generous of you, thank you so much.   I will of course cover expenses and return it to you unscathed. Let me try a few final tests and then I will PM you if convenient? 

Dom - Unfortunately my Reducer does not have Japan written on it - come to think of it, none of my equipment does! Thanks for the info though - chances of success with this reducer are looking increasingly slim.

Rob

[Astrojedi]

Rob,

I think the problem is with the FR. But I am curious...

You say you are experiencing coma at the edges at all distances and different FR.

What are the different focal ratios you have tested this FR with I.e. Do you see the same amount of coma at F3.3 vs. F6? Also are you using single exposures or stacks to examine coma?

Can you post images showing the coma at different focal ratios. I want see the focal plane issues of the FR.

[RobertI]

Rob,

I think the problem is with the FR. But I am curious...

You say you are experiencing coma at the edges at all distances and different FR.

What are the different focal ratios you have tested this FR with I.e. Do you see the same amount of coma at F3.3 vs. F6? Also are you using single exposures or stacks to examine coma?

Can you post images showing the coma at different focal ratios. I want see the focal plane issues of the FR.

Hi Hiten,

I have to confess my testing became more unscientific as my disappointment grew! I wasn't stacking and was viewing 1 second exposures of Polaris using an un-driven mount. I could try again and be more methodical this time - use a driven mount on a brighter star, trying various measurements and resulting F-ratios, stacking the results, etc.

To help me understand whether this would be a worthwhile exercise, what sorts of conclusions might you draw and would you be able to help me to interpret the results?  

I don't know whether the people who are having success with this reducer just 'plug and play' and it works, or whether there are weeks of fine-adjusting involved to get it right? If the latter then perhaps more effort is needed on my part!

Many Thanks

Rob

[DoctorD]

Hi Olly

Your F3.3 is still being used regularly with my Lodestar C.

Thanks!!!!

Paul

[aparker]

Hi Rob,

Sounds frustrating!  All I can offer is my experience with a generic 0.5X reducer and my C8.  I first set it up with the lens-to-chip distance set at what I had read was correct on the seller's website.  I got strong coma around the edges of the image as you report.  A quick check of images via an on-line plate solver demonstrated that I had much more than 0.5X reduction (smaller than expected FOV).  I moved the camera (Lodestar) in until I got the expected FOV for 1000mm FL, and the coma mostly went away.  If you have already checked that you are getting the expected FOV, then I'm not sure what to suggest (except maybe a less aggressive FR).

Good luck

Alex

[DoctorD]

Hi Rob

Here's a picture of my set-up as used on my C8:

My reducer (courtesy of Olly) is a Meade made in China model.

Works out about F3 on my C8 and exhibits coma or something similar around the edges.

Have a look in my gallery -  http://stargazerslounge.com/gallery/member/11951-doctord/

I use the same set-up with my 102MAK for most of my EAA/Video Astronomy.

HTH

Paul

[Astrojedi]

Hard to help when I don't know what I am looking at. How far out of the axis does the coma start? Is it symmetric around the axis? etc. Looking at your problem visually could provide clues to what the issue is if I can relate it to my past experience with using a number of FRs.

Also 57mm - 61mm spacing seems high. With my Japan Meade 3.3 at 57.5mm spacing from the edge of the reducer (30mm spacer + 15mm Spacer + 12.5mm sensor depth in camera) I get to f2.8 which produces coma even with my ASI224 which is a 1/3 sensor. With a 1/2 sensor like the lodestar you need to be closer to f3.3 or f3.5 in order to minimize vignetting and coma. I would say no more than 45mm distance which includes 12.5mm sensor depth (hence 30mm + 5mm spacers).

The Japanese reducers tend to perform better at high focal reduction but if you have a Chinese one you may have to step down to f3.5 - f3.7 (you need to keep in mind that these FRs were designed for 1/3 sensors).

MeadeFRBack.pdf

MeadeFRFront.pdf

[Dave In Vermont]

I remember the Meade 3.3FR and the why they were pulled off the market - people tried to use them visually and assumed, incorrectly, that they were terrible and caused nasty optical defects. So Meade pulled them off the market, rather than explain they were only intended for use in astrophotography and/or video. I swear Meade must eat lead-paint chips sometimes.....

I have one and have learned how to use them for video-AP from MallinCam. Glad I learned about them before I tossed mine out. The key seems to be in the spacing, as this document will show you:

http://www.mallincam.net/uploads/2/6/9/1/26913006/focal_reduction_for_dummies.pdf

While it's written around the venerable MallinCam-family, the principles outlined are universal. Certainly worth a read.

Light, camera, action -

Dave

[RobertI]

Thanks for the helpful comments Hiten, Alex, Dave and Paul.

Just to confirm I disassembled the reducer last night  and all the components were set up as described in the manual (same as posted by Hiten - thanks for including these), so we can eliminate that as a problem.

Paul - thanks for the image of your setup. Am I right in thinking that the sensor is positioned at the 'join' between the two pieces of the Lodestar barrel, hence, your FR to chip distance is slightly more than 63mm? Your images are very good and coma not as bad as mine which is helpful.

I have included an image from last year below - this is really just to demo the FOV - I wouldn't take this as a definitive exhibit of the problem as there is star trailing and the scope may not have been collimated correctly at that time. But I estimate from the FOV that the FR is about 3.7. I believe I used a FR to Chip distance of around 57mm though I cannot be 100% sure.

Hiten - I can see that capturing some examples with my current set up to share with you would be a great benefit so I shall attempt to do this next time out. 

Thanks again

Rob

[DoctorD]

Hi Rob

Yes, the sensor is at the join line, 12.5mm back from the front of the camera.

I experience similar distortion to that in your image but not as severe.

I'm going to try playing with the spacing next time I have the opportunity.

Clear skies

Paul

[Astrojedi]

Rob,

I ran your image through my scripts and also just looking at it visually I am seeing a combination of 3 issues here focus, mount/tracking and coma.

The image seems to be at ~f3.7 or FL ~750 (assuming you are using the original lodestar)

In the image all the stars are vertically elongated including center to the edges - even the ones exhibiting coma which is making the coma look worse. This is due to tracking errors.

Also your FWHM seems high even taking typical seeing into account (the tracking errors are also throwing my script off a little bit)

In summary, I think your FR just barely meets its original specs i.e. running at 3.3 with a 1/3 sensor (unlike most Japanese ones which were designed and manufactured beyond specs).

Also note that at higher focal reduction the focus needs to be very precise. If you are not already using a Bhatinov mask I would highly recommend it.

I think you will need to step down to at least f4 or f4.5 with this reducer to minimize coma with the Lodestar. If you have a camera with a smaller 1/3 sensor I would also try that (with the same scope) to see how it performs.

Hiten

[RobertI]

Rob,

I ran your image through my scripts and also just looking at it visually I am seeing a combination of 3 issues here focus, mount/tracking and coma.

The image seems to be at ~f3.7 or FL ~750 (assuming you are using the original lodestar)

In the image all the stars are vertically elongated including center to the edges - even the ones exhibiting coma which is making the coma look worse. This is due to tracking errors.

Also your FWHM seems high even taking typical seeing into account (the tracking errors are also throwing my script off a little bit)

In summary, I think your FR just barely meets its original specs i.e. running at 3.3 with a 1/3 sensor (unlike most Japanese ones which were designed and manufactured beyond specs).

Also note that at higher focal reduction the focus needs to be very precise. If you are not already using a Bhatinov mask I would highly recommend it.

I think you will need to step down to at least f4 or f4.5 with this reducer to minimize coma with the Lodestar. If you have a camera with a smaller 1/3 sensor I would also try that (with the same scope) to see how it performs.

Hiten

Thanks for taking the time to do this Hiten, much appreciated. I didn't have a Bahtinov mask when this image was taken but I do now and have found it invaluable. I think I shall have another 'proper' attempt on the next clear night (both at F3.3 and 4.5 as per your suggestion) to see if I can get results I am happy with and post the results for comment. Unfortunately I do not have a camera with a 1/3 sensor.

One final thought, how low do you think I could go with an F6.3 reducer? Could I get to close to F4.5 with better results than the F3.3 at F4.5??

Thanks

Rob

[Astrojedi]

I would try running the Meade 3.3 at 4.5 first. The 6.3 reducers are designed a little differently.

[Dom543]

You may want to look at this new thread about focal reducers for 825 cameras on CN

http://www.cloudynights.com/topic/517610-another-focal-reducer-thread-for-825-cameras/#entry6876031

--Dom

[RobertI]

You may want to look at this new thread about focal reducers for 825 cameras on CN

http://www.cloudynights.com/topic/517610-another-focal-reducer-thread-for-825-cameras/#entry6876031

--Dom

Thanks Dom, very useful.