Better focusing - SCT

[lucid]

The knob that comes with the CPC Edge HD 8" (and i gather this is true of all vanilla knobs), is akin to trying to get the temperature right in the shower!

So I want an upgrade! 

Whats the opinion on whether that should be a replacement microfocusser or a crayford?

I'm looking at the moment at either this:

http://www.firstlightoptics.com/starlight-instruments-feather-touch-sct-microfocusers/microfocuser-for-celestron-cpc-800-6se-c8edge-8se.html

or this:

http://www.firstlightoptics.com/moonlite-focusers/mcs2-sfocuser-r.html

I understand there are considerations with weight re the crayford, but the trade off with the micro is image shift. Anything else I should know before jumping in?

This is likely going to be the first of a few upgrades over the coming year or so (aside from having already added a baader clickstop eyepiece).

Thanks for your help...

[Uplooker]

Hi Lucid,

Personally i would and did go for an external focuser, albeit a Revelation Rack and Pinion one, mostly to avoid image shift / mirror flop.

http://www.telescopehouse.com/acatalog/Revelation-Superfocus-2-inch-SCT-Rack-n-Pinion-Focuser.html

A lot of people will be along shortly to say how please they are with their microfocuser, I am sure they are, they just didn't float my boat. There might also be a slight element of being able to see more for my buck, the microfocusers are not inexpensive!

Ian

[parsecundo]

I have a Celestron 8" SCT  ( not the Edge model) and soon became frustrated with the focusser provided. I fitted the Microfocusser and found it to be well engineered and a vast improvement, in fact a pleasure to use. It is slightly heavier than the original but this hasn't been a problem so far. I can't speak for the Moonlite as I haven't used it.

[Peter Drew]

I have a Revelation dual speed and a WO dual speed Crayford on my SCTs, both work very well and cost less than £100 each secondhand in mint condition. A microfocuser will still be subject to image shift or mirror movement "lag" . 

[Tinker1947]

If you go the external route one of these should be near the top of the buy list.....

http://www.firstlightoptics.com/baader-steeltrack-focusers/baader-steeltrack-sct-focuser.html

[Mr Spock]

The Microfocuser is the best thing I've bought. So smooth and easy. I've never seen any lag, image shift, flop, or whatever else with my C9.25.

The only thing to bear in mind with external focusers is they increase the length of the light path. This means you have to move the primary closer to the secondary, with the possibility then of the primary's light cone being larger than the secondary

[alan potts]

I bought a Moonlite for mine which I have now removed and put in the barn, my Son found a centre spool from a reel of tape that fits nicely over the focuser and that works a treat. So just by making it larger it is easier to focus. Increase in focal length also makes a very long 3048mm even longer, so with a crayford and 2 inch diagonal I have no idea what it is but would think it is a least 200mm more.

Alan

[kerrylewis]

I got the Motofocus add on for my SCT . Best upgrade ever. Don't know how I managed without it. I think that they are expensive for what they are but if you use it all the time like I do I suppose it's good value.

[GuillermoBarrancos]

The OP created a similar topic here:

http://stargazerslounge.com/topic/215294-advice-on-multiple-upgrades-for-sct/

The CPC800 EdgeHD Deluxe has Mirror Lock, so Mirror flop / image shift should not be much of an issue compared to the regular SCT's.

So to improve focusing, you can just go for the micro focuser replacement, instead of putting a heavy focuser, like the moonlite on the visual back.

[PeterCPC]

As stated the Edge has mirror support knobs so mirror flop/image shift is almost a non-issue. Microfocuser takes up no extra space and adds no extra weight - well very, very minor.

Peter

[Peter Drew]

Celestrons seem to be better in respect of focuser issues, my 16" Meades have very heavy primary mirrors and a secondary focuser is obligatory. The distance the mirror of a SCT has to move to reach the new focus is minimal and will not result in light cone problems, it will slightly increase the effective focal length. 

[kerrylewis]

I think I may have missed the point of your question (not unusual!). The Motofocus unit obviously doesn't improve the focuser , but for visual use it makes the existing one much easier to use.

[lucid]

Thanks all, i may try the centre of the roll of tape trick, as suggested by alan potts, before moving on up.

[m.tweedy]

Thanks all, i may try the centre of the roll of tape trick, as suggested by alan potts, before moving on up.

Or as someone told me a peg clipped to the focuser does the same.

[coulthamst]

I have used a bath plug with the centre cut out to be a push fit on the focus knob. Five minutes careful work and I have a very acceptable finer focus system.

[knobby]

I may be talking rubbish :-). But there's been a couple of mentions of the mirror locks on the edge stopping mirror slop when focusing... Do not try adjusting focus while the mirror is locked! Unless you are adjusting with the add on crayford type.

[JamesF]

Cheap mods are a plastic jar lid, peg or other means to enlarge or add a lever to the existing focuser.  They're an improvement, certainly.  I used a peg on the focuser for my 127 Mak for some time.

If you're going the replacement focuser route then I'd say for visual use the microfocuser is a good bet.  For imaging personally I'd pretty much always use an external focuser and add a motor to it as long as the mount had sufficient clearance.

James

[lucid]

I may be talking rubbish :-). But there's been a couple of mentions of the mirror locks on the edge stopping mirror slop when focusing... Do not try adjusting focus while the mirror is locked! Unless you are adjusting with the add on crayford type.

This has confused me...

There are two other knobs, locks i assume, on the back of the EdgeHD OTA. I haven't fiddled with them yet. Should I be? I have been simply rotating the focus knob (orange surround) when I've taken the scope out for visual astro.

[dan_uk]

The two other knobs are the mirror lock. You would engage these i.e tighten them after achieving focus to reduce the chance of mirror flop. Although Celestron mention in their manual that you can still use the focus knob after locking these they still advise against it.

So set focus, then lock. Unlock if you need to adjust focus.

As mentioned above if you have a Crayford or R&P you could just lock the mirror and use these to focus.

Daniel

[PhotoGav]

I've been following this discussion with great interest, but now have a question for you...: how does a focus unit on the back of an SCT affect the back focus distance? Does it use up some of those precious mms?

Sent from my iPhone using Tapatalk

[JamesF]

I've been following this discussion with great interest, but now have a question for you...: how does a focus unit on the back of an SCT affect the back focus distance? Does it use up some of those precious mms?

The short answer is that it doesn't, because the focal plane of a Mak or SCT isn't in a set place.  A Mak or SCT focuses by moving the primary mirror, which effectively moves the focal plane to coincide with the eyepiece or camera.

The additional little wrinkle however is that with an optical system involving multiple components, the effective focal length changes depending on the distance(s) between the components.  By moving the primary mirror the distance between it and the secondary changes, thus changing the effective focal length.  That can make a meaningful change to the image scale at the focal plane depending on how much the primary has to be moved.  The addition of a secondary focuser may mean that by the time you've moved the primary far enough (compared with no secondary focuser) to get the focal plane to the camera or eyepiece you've actually increased the image scale quite a bit.

As an example, with my Canon 450D straight on the visual back of my 127 Mak, at focus the effective focal length is only about 1400mm, not the advertised 1500mm of the OTA.  Fortunately for me that's just enough to mean that the Moon and Sun (only) just fit onto the full sensor.  If I move the camera back 50mm or so and refocus the focal length increases to around 1500mm and the image no longer fits on the sensor because the image scale is too great.

James

[dan_uk]

Is that the case for EDGE OTAs? It was my understanding that if I used a Crayford plus Diagonal id be beyond the 146mm that Celestron recommend.

I should probably test this next time I'm out.

Daniel

[JamesF]

Is that the case for EDGE OTAs? It was my understanding that if I used a Crayford plus Diagonal id be beyond the 146mm that Celestron recommend.

I should probably test this next time I'm out.

Good point.  Not sure about the EDGE models.  They have a field-flattener in the baffle tube, don't they?

James

[Tinker1947]

There are some sizes in the OAG manual that will help with the spacing....

http://www.celestron.com/browse-shop/astronomy/astroimaging-accessories/guiders/off-axis-guider

[PhotoGav]

Thank you for the answers. I have the EdgeHD 800 and yes it has an internal flattener. I think that I will play safe and stick to the factory fitted focus mechanism! Though the micro focuser to replace it might be worth further investigation. Is anyone using one and can they recommend it?