Using how an Optec 2" Lepus 0.62X focal reducer

[callum_mc]

Hello

I'm pretty new to astrophotography and have only just bought a Canon EOS 600D DSLR to do some deep-sky imaging, after a period restricted to planetary stuff with a webcam.

I have a 10" Meade ACF telescope and understand the above reducer is suitable for a corrected telescope such as the Meade. I want to use my push-fit Vixen flip-mirror and attach this to the tube using a Baader click-lock adapter. At the other end I was intending to place the DSLR by means of a T-ring.

My naive question is, where in this chain would the reducer work and how would I need anything else to allow it to do so? I have heard a number of issues relating to back focus, though I don't even really understand what the term means!

Any advice would be greatly appreciated, since I don't want to spend what is a lot of money on the Optec Lepus reducer if it won't actually work.

Thanks in advance.

Callum

[Oddsocks]

Hi Callum,

Hopefully there will be another SGL member around that has made a similar set up to the one you intend but I can pass on a little information.

For all telescopes the term "back focus" is a measure of the distance from the rear surface of the focuser when fully wound in to the point of prime image focus outside the telescope.

You can determine the amount of back focus of any telescope by pointing the telescope at the moon, winding the focuser all the way in and then place a piece of white card over the focuser opening, now gradually move the card away from the body of the focuser while looking at the image of the moon projected on the white card, where the moon comes to focus is the prime focus point and the distance between the card and the rear surface of the focuser is the "back focus" distance.

For telecompressors and flatteners the term "back focus" means the amount of the telescopes available back focus distance that is used up by the device.

This is because the LEPUS focal reducer has it's own focal length and has to be placed so that it is the correct distance from the rear glass element (or mounting thread) of the corrector to the CCD chip surface inside the camera.

If the distance is either too long or too short then the corrector will not perform well and will introduce aberrations in the image such as colour fringing (CA), coma or dispersion (bloated stars and soft focus).

The LEPUS has a back focus of 100mm on-axis and has to be placed as near to that distance from the Canon CCD chip surface as possible or it will not perform well.

Once you start adding couplers and flip mirrors to the image chain you quickly find you have used all the back focus the telescope has spare and that you can not move the focuser far enough to focus the image on the camera chip.

The problems for you will be trying to use the LEPUS with your Vixen flip mirror as the rear thread of the LEPUS will not connect directly to the flip mirror, and the front thread is 48mm so if you fitted a 48mm to "T" adaptor and tried to use this behind the flip mirror then you will introduce vignetting.

the second issue is the longer the amount of tubing used between the back of the telescope and the camera then the greater will be chance for camera sag causing tilt off-axis for the CCD (unsharpness along one edge or side of the image) and vignetting or cut-off as the focussed cone of light is partially obscured by the walls of the various couplers, tubing extenders and flip mirror on it's way to the camera, and that is not even considering how the whole ungainly extension and camera will clear the telescopes mount!

The best way to use the LEPUS is to forget about the flip mirror and use the dedicated Optec camera mount adaptor with the distance tube for the Canon specified at the time of order.

The Lepus nose has a 2" barrel mount that will slide into the Baader click-lock and the Optec camera adaptor has the correct threaded adaptors to take it to the Canon "T" mount with the required 100mm back focus distance.

While the flip mirror can be useful for planetary work with the very small chip in a web-cam type camera I think it has little use for large format CCD work and though I have had a Meade flip mirror for nearly ten years it has only been used once!

The Optec Lepus has a 22mm image circle when used at the correct 100mm distance so I think there may be some slight vignetting and cut off at the edges of your images depending on the diagonal size of your Canon's CCD chip.

Here is a link to the Optec back focus dimensional diagram : http://www.optecinc....s_backfocus.pdf

I think this is the part number for the Optec camera adaptor but please don't rely on this, you should e-mail Optec to confirm: 19392-47C (19392-X)

Here is a link to the Optec page that describes the camera mount options: http://www.optecinc....epus_mounts.htm

Finally here is a link to the current Optec price list, the Canon Lepus camera adaptor is on page 8 and costs around $60: http://www.optecinc...._price_list.pdf

William.

[callum_mc]

Thank you very much William for your very clear and detailed reply to my problem. It is so easy to spend more and more money, just to find that the system you had in mind simply will not work.

So, you would consider that the best way to do this would be to get the appropriate adaptor to attach the Lepus focal reducer to the rear of the telescope and then buy another adaptor that attaches the reducer to the camera (or is there a spacer needed in between this)? Would you then simply use the Canon 'live view' to see where it is all pointing (in the absence of a flip-mirror).

Once again thank you for your excellent advice and sorry if my follow up questions seem a little naive......I am very green :-)

Callum

[Oddsocks]

That's ok Callum, I wasted hundreds of pounds when I started astro imaging more than ten years ago, there weren't any sites like SGL to seek help from.

To clarify, the Lepus includes a 2" barrel nose-piece for the telescope side, this will simply slide into your Baader click lock at the rear of the telescope.

(inside the nose-piece of the Lepus is a 48mm filter thread so you can fit a light pollution filter or Ha filter etc in the future if you need to)

The rear of the Lepus has a dovetail flange, no thread, so it needs the Optec camera coupler of the correct length to suit the Canon DSLR chip-to-T-mount distance.

So first you would need to order the Lepus reducer itself, #19407 at a whopping $199 cost!

You then have two choices:

First:

if you already have the Canon "T" adaptor then you order (for example) the Optec part No. #19392-X "T" thread mounting plate, the"X" in the part number is the length of the spacer tube needed to get the 100mm back-focus distance between the Lepus and the CCD chip in the camera.

In your case you would order 19392-47C at $60 cost, and this would be the camera coupler of 47mm length for a canon DSLR.

Second:

If you don't have the Canon "T" adaptor already then you order the Optec part No. #19397-X DSLR mounting plate at $90 cost, in this case I think the "X" suffix is "Canon EOS DSLR" but this is unclear in the Optec Catalogue.

And that is all you need to get connected.

#19407 Lepus reducer -PLUS- #19392-47C "T" thread mounting plate for Canon DSLR using existing Canon "T" adaptor -OR- #19397-CANON-EOS DSLR mounting plate including Canon bayonet fitting.

If you decide to go along this path then e-mail Optec first with your telescope model and camera model and they will confirm if the part numbers I quoted are correct, the tolerance on distance is only +/-1mm for best performance so this has to right.

When I had a Meade LX200 I began carrying out sky alignment by temporarily fitting a diagonal and eyepiece to the "visual back" connector and once aligned I would just remove the diagonal and visual back connector and screw on a Meade 6.3 reducer then fit the camera using a Meade camera coupler. After a while I realised I could get a good enough alignment just by using the regular Meade finder scope to centre on the alignment star (once I had the finder scope perfectly aligned to the tube axis), and so I fitted the camera right from the outset and didn't need to strain the gearboxes while trying to swap over between visual and camera once the clutches were locked and the mount was tracking.

I think this will work for you too, once you have the finder scope aligned you can carry out mount initialisation and star alignment just using the finder scope and you have the camera and reducer fitted before you begin.

Use the Meade object list or enter the coordinates manually of the object you want to image and let the telescope centre on the object then use Canon live view to focus and fine position adjust if necessary.

Finally though, and I don't want to discourage you, but the Meade 10" ACF has rather a long focal length of 2,500mm and even with the Lepus reducer you will still have an equivalent focal length of 1,500mm.

This means you will see an incredibly small area of sky on the camera, also at this focal length tracking accurately with very low periodic error is essential and this is not something the Meade mount is particularly good at.

If you look at many of the best astro photos on this site you will see they are taken with telescopes of around 400mm to 600mm focal length on very stable mounts.

I quickly found to my cost that my 1990's 10" LX200 was very poor at astrophotography in general although I did manage some reasonable planetary imaging with a Philips web-cam.

If you go ahead and buy the Lepus it will improve the performance of the Meade ACF and though not impossible you will find it incredibly difficult to obtain satisfactory results, you will certainly soon learn to master the art of the photomosaic!

To be serious though, you will be able to obtain astrophotos with the Meade ACF and the Lepus but it is not easy and you will struggle at first.

Unless you have an equatorial wedge for your Meade ACF then the maximum exposure time possible will be around 30 seconds even with the Lepus fitted, as the star field will rotate around the image centre causing star trails at the edges, this is not such a problem for DSLR photography as the camera is not cooled and you wouldn't normally take exposures that much longer anyway, just loads of single exposures and stacked afterwards in post processing.

If I could suggest another possibility for you I would consider for now just buying the Meade camera piggy back bracket for ~£35 and mounting your Canon directly on top of the telescope, there are loads of second hand Canon EOS lenses on e-bay and elsewhere, a lens of around 400mm focal length is ideal but the one that came with your camera will get you started without having to lay out a huge investment in cash.

Here is a link to the camera bracket:

http://www.green-wit...95#aMEA_2d07395

Hope that hasn't put you off too much?

William.

[callum_mc]

Hi William

That is very clear indeed. Thank you very much.

I bought the Meade ACF tube separately, at the same time also purchasing an NEQ6-PRO mount, because I had luckily been advised about the limitations of the fork mount. So, hopefully I might be in a better position to progress......fingers crossed :-)

For the first time I feel fairly sure of what I need to purchase. Unfortunately, I do not seem to be able to buy any of it in the UK but have communicated with a very helpful German supplier. So hopefully, with your advice, I should be able to find the equipment I require.

Once I manage to acquire some images, then all I have to learn is the very simple use of Iris and Photoshop, ha, ha ;-)

Thanks again William, you have been very helpful indeed.

Best wishes,

Callum